Provided by: mksh_46-2ubuntu3_amd64 bug

NAME

     mksh, sh — MirBSD Korn shell

SYNOPSIS

     mksh [-+abCefhiklmnprUuvXx] [-T /dev/ttyCn | -] [-+o option] [-c string | -s | file
          [argument ...]]
     builtin-name [argument ...]

DESCRIPTION

     mksh is a command interpreter intended for both interactive and shell script use.  Its
     command language is a superset of the sh(C) shell language and largely compatible to the
     original Korn shell.

   I'm an Android user, so what's mksh?
     mksh is a UNIX shell / command interpreter, similar to COMMAND.COM or CMD.EXE, which has
     been included with Android Open Source Project for a while now.  Basically, it's a program
     that runs in a terminal (console window), takes user input and runs commands or scripts,
     which it can also be asked to do by other programs, even in the background.  Any privilege
     pop-ups you might be encountering are thus not mksh issues but questions by some other
     program utilising it.

   Invocation
     Most builtins can be called directly, for example if a link points from its name to the
     shell; not all make sense, have been tested or work at all though.

     The options are as follows:

     -c string  mksh will execute the command(s) contained in string.

     -i         Interactive shell.  A shell is “interactive” if this option is used or if both
                standard input and standard error are attached to a tty(4).  An interactive shell
                has job control enabled, ignores the SIGINT, SIGQUIT, and SIGTERM signals, and
                prints prompts before reading input (see the PS1 and PS2 parameters).  It also
                processes the ENV parameter or the mkshrc file (see below).  For non-interactive
                shells, the trackall option is on by default (see the set command below).

     -l         Login shell.  If the basename the shell is called with (i.e. argv[0]) starts with
                ‘-’ or if this option is used, the shell is assumed to be a login shell; see
                Startup files below.

     -p         Privileged shell.  A shell is “privileged” if this option is used or if the real
                user ID or group ID does not match the effective user ID or group ID (see
                getuid(2) and getgid(2)).  Clearing the privileged option causes the shell to set
                its effective user ID (group ID) to its real user ID (group ID).  For further
                implications, see Startup files.

     -r         Restricted shell.  A shell is “restricted” if this option is used.  The following
                restrictions come into effect after the shell processes any profile and ENV
                files:

                ·   The cd (and chdir) command is disabled.
                ·   The SHELL, ENV, and PATH parameters cannot be changed.
                ·   Command names can't be specified with absolute or relative paths.
                ·   The -p option of the built-in command command can't be used.
                ·   Redirections that create files can't be used (i.e. ‘>’, ‘>|’, ‘>>’, ‘<>’).

     -s         The shell reads commands from standard input; all non-option arguments are
                positional parameters.

     -T tty     Spawn mksh on the tty(4) device given.  Superuser only.  If tty is a dash, detach
                from controlling terminal (daemonise) instead.

     In addition to the above, the options described in the set built-in command can also be used
     on the command line: both [-+abCefhkmnuvXx] and [-+o option] can be used for single letter
     or long options, respectively.

     If neither the -c nor the -s option is specified, the first non-option argument specifies
     the name of a file the shell reads commands from.  If there are no non-option arguments, the
     shell reads commands from the standard input.  The name of the shell (i.e. the contents of
     $0) is determined as follows: if the -c option is used and there is a non-option argument,
     it is used as the name; if commands are being read from a file, the file is used as the
     name; otherwise, the basename the shell was called with (i.e. argv[0]) is used.

     The exit status of the shell is 127 if the command file specified on the command line could
     not be opened, or non-zero if a fatal syntax error occurred during the execution of a
     script.  In the absence of fatal errors, the exit status is that of the last command
     executed, or zero, if no command is executed.

   Startup files
     For the actual location of these files, see FILES.  A login shell processes the system
     profile first.  A privileged shell then processes the suid profile.  A non-privileged login
     shell processes the user profile next.  A non-privileged interactive shell checks the value
     of the ENV parameter after subjecting it to parameter, command, arithmetic and tilde (‘~’)
     substitution; if unset or empty, the user mkshrc profile is processed; otherwise, if a file
     whose name is the substitution result exists, it is processed; non-existence is silently
     ignored.

   Command syntax
     The shell begins parsing its input by removing any backslash-newline combinations, then
     breaking it into words.  Words (which are sequences of characters) are delimited by unquoted
     whitespace characters (space, tab, and newline) or meta-characters (‘<’, ‘>’, ‘|’, ‘;’, ‘(’,
     ‘)’, and ‘&’).  Aside from delimiting words, spaces and tabs are ignored, while newlines
     usually delimit commands.  The meta-characters are used in building the following tokens:
     ‘<’, ‘<&’, ‘<<’, ‘<<<’, ‘>’, ‘>&’, ‘>>’, ‘&>’, etc. are used to specify redirections (see
     Input/output redirection below); ‘|’ is used to create pipelines; ‘|&’ is used to create co-
     processes (see Co-processes below); ‘;’ is used to separate commands; ‘&’ is used to create
     asynchronous pipelines; ‘&&’ and ‘||’ are used to specify conditional execution; ‘;;’, ‘;&’
     and ‘;|’ are used in case statements; ‘(( .. ))’ is used in arithmetic expressions; and
     lastly, ‘( .. )’ is used to create subshells.

     Whitespace and meta-characters can be quoted individually using a backslash (‘\’), or in
     groups using double (‘"’) or single (‘'’) quotes.  Note that the following characters are
     also treated specially by the shell and must be quoted if they are to represent themselves:
     ‘\’, ‘"’, ‘'’, ‘#’, ‘$’, ‘`’, ‘~’, ‘{’, ‘}’, ‘*’, ‘?’, and ‘[’.  The first three of these
     are the above mentioned quoting characters (see Quoting below); ‘#’, if used at the
     beginning of a word, introduces a comment – everything after the ‘#’ up to the nearest
     newline is ignored; ‘$’ is used to introduce parameter, command, and arithmetic
     substitutions (see Substitution below); ‘`’ introduces an old-style command substitution
     (see Substitution below); ‘~’ begins a directory expansion (see Tilde expansion below); ‘{’
     and ‘}’ delimit csh(1)-style alterations (see Brace expansion below); and finally, ‘*’, ‘?’,
     and ‘[’ are used in file name generation (see File name patterns below).

     As words and tokens are parsed, the shell builds commands, of which there are two basic
     types: simple-commands, typically programmes that are executed, and compound-commands, such
     as for and if statements, grouping constructs, and function definitions.

     A simple-command consists of some combination of parameter assignments (see Parameters
     below), input/output redirections (see Input/output redirections below), and command words;
     the only restriction is that parameter assignments come before any command words.  The
     command words, if any, define the command that is to be executed and its arguments.  The
     command may be a shell built-in command, a function, or an external command (i.e. a separate
     executable file that is located using the PATH parameter; see Command execution below).
     Note that all command constructs have an exit status: for external commands, this is related
     to the status returned by wait(2) (if the command could not be found, the exit status is
     127; if it could not be executed, the exit status is 126); the exit status of other command
     constructs (built-in commands, functions, compound-commands, pipelines, lists, etc.) are all
     well-defined and are described where the construct is described.  The exit status of a
     command consisting only of parameter assignments is that of the last command substitution
     performed during the parameter assignment or 0 if there were no command substitutions.

     Commands can be chained together using the ‘|’ token to form pipelines, in which the
     standard output of each command but the last is piped (see pipe(2)) to the standard input of
     the following command.  The exit status of a pipeline is that of its last command, unless
     the pipefail option is set (see there).  All commands of a pipeline are executed in separate
     subshells; this is allowed by POSIX but differs from both variants of AT&T UNIX ksh, where
     all but the last command were executed in subshells; see the read builtin's description for
     implications and workarounds.  A pipeline may be prefixed by the ‘!’ reserved word which
     causes the exit status of the pipeline to be logically complemented: if the original status
     was 0, the complemented status will be 1; if the original status was not 0, the complemented
     status will be 0.

     Lists of commands can be created by separating pipelines by any of the following tokens:
     ‘&&’, ‘||’, ‘&’, ‘|&’, and ‘;’.  The first two are for conditional execution: “cmd1 && cmd2”
     executes cmd2 only if the exit status of cmd1 is zero; ‘||’ is the opposite – cmd2 is
     executed only if the exit status of cmd1 is non-zero.  ‘&&’ and ‘||’ have equal precedence
     which is higher than that of ‘&’, ‘|&’, and ‘;’, which also have equal precedence.  Note
     that the ‘&&’ and ‘||’ operators are "left-associative".  For example, both of these
     commands will print only "bar":

           $ false && echo foo || echo bar
           $ true || echo foo && echo bar

     The ‘&’ token causes the preceding command to be executed asynchronously; that is, the shell
     starts the command but does not wait for it to complete (the shell does keep track of the
     status of asynchronous commands; see Job control below).  When an asynchronous command is
     started when job control is disabled (i.e. in most scripts), the command is started with
     signals SIGINT and SIGQUIT ignored and with input redirected from /dev/null (however,
     redirections specified in the asynchronous command have precedence).  The ‘|&’ operator
     starts a co-process which is a special kind of asynchronous process (see Co-processes
     below).  Note that a command must follow the ‘&&’ and ‘||’ operators, while it need not
     follow ‘&’, ‘|&’, or ‘;’.  The exit status of a list is that of the last command executed,
     with the exception of asynchronous lists, for which the exit status is 0.

     Compound commands are created using the following reserved words.  These words are only
     recognised if they are unquoted and if they are used as the first word of a command (i.e.
     they can't be preceded by parameter assignments or redirections):

           case     else     function     then      !       (
           do       esac     if           time      [[      ((
           done     fi       in           until     {
           elif     for      select       while     }

     In the following compound command descriptions, command lists (denoted as list) that are
     followed by reserved words must end with a semicolon, a newline, or a (syntactically
     correct) reserved word.  For example, the following are all valid:

           $ { echo foo; echo bar; }
           $ { echo foo; echo bar<newline>}
           $ { { echo foo; echo bar; } }

     This is not valid:

           $ { echo foo; echo bar }

     (list)
           Execute list in a subshell.  There is no implicit way to pass environment changes from
           a subshell back to its parent.

     { list; }
           Compound construct; list is executed, but not in a subshell.  Note that ‘{’ and ‘}’
           are reserved words, not meta-characters.

     case word in [[(] pattern [| pat] ...) list [;; | ;& | ;| ]] ... esac
           The case statement attempts to match word against a specified pattern; the list
           associated with the first successfully matched pattern is executed.  Patterns used in
           case statements are the same as those used for file name patterns except that the
           restrictions regarding ‘.’ and ‘/’ are dropped.  Note that any unquoted space before
           and after a pattern is stripped; any space within a pattern must be quoted.  Both the
           word and the patterns are subject to parameter, command, and arithmetic substitution,
           as well as tilde substitution.

           For historical reasons, open and close braces may be used instead of in and esac e.g.
           case $foo { *) echo bar;; }.

           The list terminators are:

           ‘;;’  Terminate after the list.

           ‘;&’  Fall through into the next list.

           ‘;|’  Evaluate the remaining pattern-list tuples.

           The exit status of a case statement is that of the executed list; if no list is
           executed, the exit status is zero.

     for name [in word ...]; do list; done
           For each word in the specified word list, the parameter name is set to the word and
           list is executed.  If in is not used to specify a word list, the positional parameters
           ($1, $2, etc.) are used instead.  For historical reasons, open and close braces may be
           used instead of do and done e.g. for i; { echo $i; }.  The exit status of a for
           statement is the last exit status of list; if list is never executed, the exit status
           is zero.

     if list; then list; [elif list; then list;] ... [else list;] fi
           If the exit status of the first list is zero, the second list is executed; otherwise,
           the list following the elif, if any, is executed with similar consequences.  If all
           the lists following the if and elifs fail (i.e. exit with non-zero status), the list
           following the else is executed.  The exit status of an if statement is that of non-
           conditional list that is executed; if no non-conditional list is executed, the exit
           status is zero.

     select name [in word ...]; do list; done
           The select statement provides an automatic method of presenting the user with a menu
           and selecting from it.  An enumerated list of the specified word(s) is printed on
           standard error, followed by a prompt (PS3: normally ‘#? ’).  A number corresponding to
           one of the enumerated words is then read from standard input, name is set to the
           selected word (or unset if the selection is not valid), REPLY is set to what was read
           (leading/trailing space is stripped), and list is executed.  If a blank line (i.e.
           zero or more IFS octets) is entered, the menu is reprinted without executing list.

           When list completes, the enumerated list is printed if REPLY is NULL, the prompt is
           printed, and so on.  This process continues until an end-of-file is read, an interrupt
           is received, or a break statement is executed inside the loop.  If “in word ...” is
           omitted, the positional parameters are used (i.e. $1, $2, etc.).  For historical
           reasons, open and close braces may be used instead of do and done e.g. select i; {
           echo $i; }.  The exit status of a select statement is zero if a break statement is
           used to exit the loop, non-zero otherwise.

     until list; do list; done
           This works like while, except that the body is executed only while the exit status of
           the first list is non-zero.

     while list; do list; done
           A while is a pre-checked loop.  Its body is executed as often as the exit status of
           the first list is zero.  The exit status of a while statement is the last exit status
           of the list in the body of the loop; if the body is not executed, the exit status is
           zero.

     function name { list; }
           Defines the function name (see Functions below).  Note that redirections specified
           after a function definition are performed whenever the function is executed, not when
           the function definition is executed.

     name() command
           Mostly the same as function (see Functions below).  Whitespace (space or tab) after
           name will be ignored most of the time.

     function name() { list; }
           The same as name() (bashism).  The function keyword is ignored.

     time [-p] [pipeline]
           The Command execution section describes the time reserved word.

     (( expression ))
           The arithmetic expression expression is evaluated; equivalent to “let expression” (see
           Arithmetic expressions and the let command, below).

     [[ expression ]]
           Similar to the test and [ ... ] commands (described later), with the following
           exceptions:

           ·   Field splitting and file name generation are not performed on arguments.

           ·   The -a (AND) and -o (OR) operators are replaced with ‘&&’ and ‘||’, respectively.

           ·   Operators (e.g. ‘-f’, ‘=’, ‘!’) must be unquoted.

           ·   Parameter, command, and arithmetic substitutions are performed as expressions are
               evaluated and lazy expression evaluation is used for the ‘&&’ and ‘||’ operators.
               This means that in the following statement, $(<foo) is evaluated if and only if
               the file foo exists and is readable:

                     $ [[ -r foo && $(<foo) = b*r ]]

           ·   The second operand of the ‘!=’ and ‘=’ expressions are patterns (e.g. the
               comparison [[ foobar = f*r ]] succeeds).  This even works indirectly:

                     $ bar=foobar; baz='f*r'
                     $ [[ $bar = $baz ]]; echo $?
                     $ [[ $bar = "$baz" ]]; echo $?

               Perhaps surprisingly, the first comparison succeeds, whereas the second doesn't.

   Quoting
     Quoting is used to prevent the shell from treating characters or words specially.  There are
     three methods of quoting.  First, ‘\’ quotes the following character, unless it is at the
     end of a line, in which case both the ‘\’ and the newline are stripped.  Second, a single
     quote (‘'’) quotes everything up to the next single quote (this may span lines).  Third, a
     double quote (‘"’) quotes all characters, except ‘$’, ‘`’ and ‘\’, up to the next unquoted
     double quote.  ‘$’ and ‘`’ inside double quotes have their usual meaning (i.e. parameter,
     command, or arithmetic substitution) except no field splitting is carried out on the results
     of double-quoted substitutions.  If a ‘\’ inside a double-quoted string is followed by ‘\’,
     ‘$’, ‘`’, or ‘"’, it is replaced by the second character; if it is followed by a newline,
     both the ‘\’ and the newline are stripped; otherwise, both the ‘\’ and the character
     following are unchanged.

     If a single-quoted string is preceded by an unquoted ‘$’, C style backslash expansion (see
     below) is applied (even single quote characters inside can be escaped and do not terminate
     the string then); the expanded result is treated as any other single-quoted string.  If a
     double-quoted string is preceded by an unquoted ‘$’, the latter is ignored.

   Backslash expansion
     In places where backslashes are expanded, certain C and AT&T UNIX ksh or GNU bash style
     escapes are translated.  These include ‘\a’, ‘\b’, ‘\f’, ‘\n’, ‘\r’, ‘\t’, ‘\U########’,
     ‘\u####’, and ‘\v’.  For ‘\U########’ and ‘\u####’, “#” means a hexadecimal digit, of thich
     there may be none up to four or eight; these escapes translate a Unicode codepoint to UTF-8.
     Furthermore, ‘\E’ and ‘\e’ expand to the escape character.

     In the print builtin mode, ‘\"’, ‘\'’, and ‘\?’ are explicitly excluded; octal sequences
     must have the none up to three octal digits “#” prefixed with the digit zero (‘\0###’);
     hexadecimal sequences ‘\x##’ are limited to none up to two hexadecimal digits “#”; both
     octal and hexadecimal sequences convert to raw octets; ‘\#’, where # is none of the above,
     translates to \# (backslashes are retained).

     Backslash expansion in the C style mode slightly differs: octal sequences ‘\###’ must have
     no digit zero prefixing the one up to three octal digits “#” and yield raw octets;
     hexadecimal sequences ‘\x#*’ greedily eat up as many hexadecimal digits “#” as they can and
     terminate with the first non-hexadecimal digit; these translate a Unicode codepoint to
     UTF-8.  The sequence ‘\c#’, where “#” is any octet, translates to Ctrl-# (which basically
     means, ‘\c?’ becomes DEL, everything else is bitwise ANDed with 0x1F).  Finally, ‘\#’, where
     # is none of the above, translates to # (has the backslash trimmed), even if it is a
     newline.

   Aliases
     There are two types of aliases: normal command aliases and tracked aliases.  Command aliases
     are normally used as a short hand for a long or often used command.  The shell expands
     command aliases (i.e. substitutes the alias name for its value) when it reads the first word
     of a command.  An expanded alias is re-processed to check for more aliases.  If a command
     alias ends in a space or tab, the following word is also checked for alias expansion.  The
     alias expansion process stops when a word that is not an alias is found, when a quoted word
     is found, or when an alias word that is currently being expanded is found.  Aliases are
     specifically an interactive feature: while they do happen to work in scripts and on the
     command line in some cases, aliases are expanded during lexing, so their use must be in a
     separate command tree from their definition; otherwise, the alias will not be found.
     Noticeably, command lists (separated by semicolon, in command substitutions also by newline)
     may be one same parse tree.

     The following command aliases are defined automatically by the shell:

           autoload='typeset -fu'
           functions='typeset -f'
           hash='alias -t'
           history='fc -l'
           integer='typeset -i'
           local='typeset'
           login='exec login'
           nameref='typeset -n'
           nohup='nohup '
           r='fc -e -'
           stop='kill -STOP'
           suspend='kill -STOP $$'
           type='whence -v'

     Tracked aliases allow the shell to remember where it found a particular command.  The first
     time the shell does a path search for a command that is marked as a tracked alias, it saves
     the full path of the command.  The next time the command is executed, the shell checks the
     saved path to see that it is still valid, and if so, avoids repeating the path search.
     Tracked aliases can be listed and created using alias -t.  Note that changing the PATH
     parameter clears the saved paths for all tracked aliases.  If the trackall option is set
     (i.e. set -o trackall or set -h), the shell tracks all commands.  This option is set
     automatically for non-interactive shells.  For interactive shells, only the following
     commands are automatically tracked: cat(1), cc(1), chmod(1), cp(1), date(1), ed(1),
     emacs(1), grep(1), ls(1), make(1), mv(1), pr(1), rm(1), sed(1), sh(1), vi(1), and who(1).

   Substitution
     The first step the shell takes in executing a simple-command is to perform substitutions on
     the words of the command.  There are three kinds of substitution: parameter, command, and
     arithmetic.  Parameter substitutions, which are described in detail in the next section,
     take the form $name or ${...}; command substitutions take the form $(command) or
     (deprecated) `command` or (executed in the current environment) ${ command;} and strip
     trailing newlines; and arithmetic substitutions take the form $((expression)).  Parsing the
     current-environment command substitution requires a space, tab or newline after the opening
     brace and that the closing brace be recognised as a keyword (i.e. is preceded by a newline
     or semicolon).  They are also called funsubs (function substitutions) and behave like
     functions in that local and return work, and in that exit terminates the parent shell.

     Another variant of substitution are the valsubs (value substitutions) ${|command;} which are
     also executed in the current environment, like funsubs, but share their I/O with the parent;
     instead, they evaluate to whatever the, initially empty, expression-local variable REPLY is
     set to within the commands.

     If a substitution appears outside of double quotes, the results of the substitution are
     generally subject to word or field splitting according to the current value of the IFS
     parameter.  The IFS parameter specifies a list of octets which are used to break a string up
     into several words; any octets from the set space, tab, and newline that appear in the IFS
     octets are called “IFS whitespace”.  Sequences of one or more IFS whitespace octets, in
     combination with zero or one non-IFS whitespace octets, delimit a field.  As a special case,
     leading and trailing IFS whitespace and trailing IFS non-whitespace are stripped (i.e. no
     leading or trailing empty field is created by it); leading non-IFS whitespace does create an
     empty field.

     Example: If IFS is set to “<space>:”, and VAR is set to
     “<space>A<space>:<space><space>B::D”, the substitution for $VAR results in four fields: ‘A’,
     ‘B’, ‘’ (an empty field), and ‘D’.  Note that if the IFS parameter is set to the NULL
     string, no field splitting is done; if the parameter is unset, the default value of space,
     tab, and newline is used.

     Also, note that the field splitting applies only to the immediate result of the
     substitution.  Using the previous example, the substitution for $VAR:E results in the
     fields: ‘A’, ‘B’, ‘’, and ‘D:E’, not ‘A’, ‘B’, ‘’, ‘D’, and ‘E’.  This behavior is POSIX
     compliant, but incompatible with some other shell implementations which do field splitting
     on the word which contained the substitution or use IFS as a general whitespace delimiter.

     The results of substitution are, unless otherwise specified, also subject to brace expansion
     and file name expansion (see the relevant sections below).

     A command substitution is replaced by the output generated by the specified command which is
     run in a subshell.  For $(command) and ${ command;} substitutions, normal quoting rules are
     used when command is parsed; however, for the deprecated `command` form, a ‘\’ followed by
     any of ‘$’, ‘`’, or ‘\’ is stripped (a ‘\’ followed by any other character is unchanged).
     As a special case in command substitutions, a command of the form <file is interpreted to
     mean substitute the contents of file.  Note that $(<foo) has the same effect as $(cat foo).

     Note that some shells do not use a recursive parser for command substitutions, leading to
     failure for certain constructs; to be portable, use as workaround ‘x=$(cat) <<"EOF"’ (or the
     newline-keeping ‘x=<<"EOF"’ extension) instead to merely slurp the string.  IEEE Std 1003.1
     (“POSIX.1”) recommends to use case statements of the form ‘x=$(case $foo in (bar) echo $bar
     ;; (*) echo $baz ;; esac)’ instead, which would work but not serve as example for this
     portability issue.

           x=$(case $foo in bar) echo $bar ;; *) echo $baz ;; esac)
           # above fails to parse on old shells; below is the workaround
           x=$(eval $(cat)) <<"EOF"
           case $foo in bar) echo $bar ;; *) echo $baz ;; esac
           EOF

     Arithmetic substitutions are replaced by the value of the specified expression.  For
     example, the command print $((2+3*4)) displays 14.  See Arithmetic expressions for a
     description of an expression.

   Parameters
     Parameters are shell variables; they can be assigned values and their values can be accessed
     using a parameter substitution.  A parameter name is either one of the special single
     punctuation or digit character parameters described below, or a letter followed by zero or
     more letters or digits (‘_’ counts as a letter).  The latter form can be treated as arrays
     by appending an array index of the form [expr] where expr is an arithmetic expression.
     Array indices in mksh are limited to the range 0 through 4294967295, inclusive.  That is,
     they are a 32-bit unsigned integer.

     Parameter substitutions take the form $name, ${name}, or ${name[expr]} where name is a
     parameter name.  Substitution of all array elements with ${name[*]} and ${name[@]} works
     equivalent to $* and $@ for positional parameters.  If substitution is performed on a
     parameter (or an array parameter element) that is not set, a null string is substituted
     unless the nounset option (set -o nounset or set -u) is set, in which case an error occurs.

     Parameters can be assigned values in a number of ways.  First, the shell implicitly sets
     some parameters like ‘#’, ‘PWD’, and ‘$’; this is the only way the special single character
     parameters are set.  Second, parameters are imported from the shell's environment at
     startup.  Third, parameters can be assigned values on the command line: for example, FOO=bar
     sets the parameter “FOO” to “bar”; multiple parameter assignments can be given on a single
     command line and they can be followed by a simple-command, in which case the assignments are
     in effect only for the duration of the command (such assignments are also exported; see
     below for the implications of this).  Note that both the parameter name and the ‘=’ must be
     unquoted for the shell to recognise a parameter assignment.  The construct FOO+=baz is also
     recognised; the old and new values are immediately concatenated.  The fourth way of setting
     a parameter is with the export, global, readonly, and typeset commands; see their
     descriptions in the Command execution section.  Fifth, for and select loops set parameters
     as well as the getopts, read, and set -A commands.  Lastly, parameters can be assigned
     values using assignment operators inside arithmetic expressions (see Arithmetic expressions
     below) or using the ${name=value} form of the parameter substitution (see below).

     Parameters with the export attribute (set using the export or typeset -x commands, or by
     parameter assignments followed by simple commands) are put in the environment (see
     environ(7)) of commands run by the shell as name=value pairs.  The order in which parameters
     appear in the environment of a command is unspecified.  When the shell starts up, it
     extracts parameters and their values from its environment and automatically sets the export
     attribute for those parameters.

     Modifiers can be applied to the ${name} form of parameter substitution:

     ${name:-word}
             If name is set and not NULL, it is substituted; otherwise, word is substituted.

     ${name:+word}
             If name is set and not NULL, word is substituted; otherwise, nothing is substituted.

     ${name:=word}
             If name is set and not NULL, it is substituted; otherwise, it is assigned word and
             the resulting value of name is substituted.

     ${name:?word}
             If name is set and not NULL, it is substituted; otherwise, word is printed on
             standard error (preceded by name:) and an error occurs (normally causing termination
             of a shell script, function, or script sourced using the ‘.’ built-in).  If word is
             omitted, the string “parameter null or not set” is used instead.  Currently a bug,
             if word is a variable which expands to the null string, the error message is also
             printed.

     Note that, for all of the above, word is actually considered quoted, and special parsing
     rules apply.  The parsing rules also differ on whether the expression is double-quoted: word
     then uses double-quoting rules, except for the double quote itself (‘"’) and the closing
     brace, which, if backslash escaped, gets quote removal applied.

     In the above modifiers, the ‘:’ can be omitted, in which case the conditions only depend on
     name being set (as opposed to set and not NULL).  If word is needed, parameter, command,
     arithmetic, and tilde substitution are performed on it; if word is not needed, it is not
     evaluated.

     The following forms of parameter substitution can also be used (if name is an array, its
     element #0 will be substituted in a scalar context):

     ${#name}
             The number of positional parameters if name is ‘*’, ‘@’, or not specified; otherwise
             the length (in characters) of the string value of parameter name.

     ${#name[*]}
     ${#name[@]}
             The number of elements in the array name.

     ${%name}
             The width (in screen columns) of the string value of parameter name, or -1 if
             ${name} contains a control character.

     ${!name}
             The name of the variable referred to by name.  This will be name except when name is
             a name reference (bound variable), created by the nameref command (which is an alias
             for typeset -n).

     ${!name[*]}
     ${!name[@]}
             The names of indices (keys) in the array name.

     ${name#pattern}
     ${name##pattern}
             If pattern matches the beginning of the value of parameter name, the matched text is
             deleted from the result of substitution.  A single ‘#’ results in the shortest
             match, and two of them result in the longest match.  Cannot be applied to a vector
             (${*} or ${@} or ${array[*]} or ${array[@]}).

     ${name%pattern}
     ${name%%pattern}
             Like ${..#..} substitution, but it deletes from the end of the value.  Cannot be
             applied to a vector.

     ${name/pattern/string}
     ${name//pattern/string}
             Like ${..#..} substitution, but it replaces the longest match of pattern, anchored
             anywhere in the value, with string.  If pattern begins with ‘#’, it is anchored at
             the beginning of the value; if it begins with ‘%’, it is anchored at the end.
             Patterns that are empty or consist only of wildcards are invalid.  A single ‘/’
             replaces the first occurence of the search pattern, and two of them replace all
             occurences.  If /string is omitted, the pattern is replaced by the empty string,
             i.e. deleted.  Cannot be applied to a vector.  Inefficiently implemented, may be
             slow.

     ${name:pos:len}
             The first len characters of name, starting at position pos, are substituted.  Both
             pos and :len are optional.  If pos is negative, counting starts at the end of the
             string; if it is omitted, it defaults to 0.  If len is omitted or greater than the
             length of the remaining string, all of it is substituted.  Both pos and len are
             evaluated as arithmetic expressions.  Currently, pos must start with a space,
             opening parenthesis or digit to be recognised.  Cannot be applied to a vector.

     ${name @#[seed]}
             The internal hash of the expansion of name, with an optional (defaulting to zero)
             [seed].  At the moment, this is NZAAT (a 32-bit hash based on Bob Jenkins' one-at-a-
             time hash), but this is not set.  This is the hash the shell uses internally for its
             associative arrays.

     ${name@Q}
             A quoted expression safe for re-entry, whose value is the value of the name
             parameter, is substituted.

     Note that pattern may need extended globbing pattern (@(...)), single ('...') or double
     ("...") quote escaping unless -o sh is set.

     The following special parameters are implicitly set by the shell and cannot be set directly
     using assignments:

     !       Process ID of the last background process started.  If no background processes have
             been started, the parameter is not set.

     #       The number of positional parameters ($1, $2, etc.).

     $       The PID of the shell, or the PID of the original shell if it is a subshell.  Do NOT
             use this mechanism for generating temporary file names; see mktemp(1) instead.

     -       The concatenation of the current single letter options (see the set command below
             for a list of options).

     ?       The exit status of the last non-asynchronous command executed.  If the last command
             was killed by a signal, $? is set to 128 plus the signal number.

     0       The name of the shell, determined as follows: the first argument to mksh if it was
             invoked with the -c option and arguments were given; otherwise the file argument, if
             it was supplied; or else the basename the shell was invoked with (i.e. argv[0]).  $0
             is also set to the name of the current script or the name of the current function,
             if it was defined with the function keyword (i.e. a Korn shell style function).

     1 .. 9  The first nine positional parameters that were supplied to the shell, function, or
             script sourced using the ‘.’ built-in.  Further positional parameters may be
             accessed using ${number}.

     *       All positional parameters (except 0), i.e. $1, $2, $3, ...
             If used outside of double quotes, parameters are separate words (which are subjected
             to word splitting); if used within double quotes, parameters are separated by the
             first character of the IFS parameter (or the empty string if IFS is NULL).

     @       Same as $*, unless it is used inside double quotes, in which case a separate word is
             generated for each positional parameter.  If there are no positional parameters, no
             word is generated.  $@ can be used to access arguments, verbatim, without losing
             NULL arguments or splitting arguments with spaces.

     The following parameters are set and/or used by the shell:

     _            (underscore) When an external command is executed by the shell, this parameter
                  is set in the environment of the new process to the path of the executed
                  command.  In interactive use, this parameter is also set in the parent shell to
                  the last word of the previous command.

     BASHPID      The PID of the shell or subshell.

     CDPATH       Search path for the cd built-in command.  It works the same way as PATH for
                  those directories not beginning with ‘/’ in cd commands.  Note that if CDPATH
                  is set and does not contain ‘.’ or contains an empty path, the current
                  directory is not searched.  Also, the cd built-in command will display the
                  resulting directory when a match is found in any search path other than the
                  empty path.

     COLUMNS      Set to the number of columns on the terminal or window.  Always set, defaults
                  to 80, unless the value as reported by stty(1) is non-zero and sane enough
                  (minimum is 12x3); similar for LINES.  This parameter is used by the
                  interactive line editing modes, and by the select, set -o, and kill -l commands
                  to format information columns.  Importing from the environment or unsetting
                  this parameter removes the binding to the actual terminal size in favour of the
                  provided value.

     ENV          If this parameter is found to be set after any profile files are executed, the
                  expanded value is used as a shell startup file.  It typically contains function
                  and alias definitions.

     ERRNO        Integer value of the shell's errno variable.  It indicates the reason the last
                  system call failed.  Not yet implemented.

     EXECSHELL    If set, this parameter is assumed to contain the shell that is to be used to
                  execute commands that execve(2) fails to execute and which do not start with a
                  “#!shell” sequence.

     FCEDIT       The editor used by the fc command (see below).

     FPATH        Like PATH, but used when an undefined function is executed to locate the file
                  defining the function.  It is also searched when a command can't be found using
                  PATH.  See Functions below for more information.

     HISTFILE     The name of the file used to store command history.  When assigned to, history
                  is loaded from the specified file.  Also, several invocations of the shell will
                  share history if their HISTFILE parameters all point to the same file.

                  Note: If HISTFILE isn't set, no history file is used.  This is different from
                  AT&T UNIX ksh.

     HISTSIZE     The number of commands normally stored for history.  The default is 2047.

     HOME         The default directory for the cd command and the value substituted for an
                  unqualified ~ (see Tilde expansion below).

     IFS          Internal field separator, used during substitution and by the read command, to
                  split values into distinct arguments; normally set to space, tab, and newline.
                  See Substitution above for details.

                  Note: This parameter is not imported from the environment when the shell is
                  started.

     KSHEGID      The effective group id of the shell.

     KSHGID       The real group id of the shell.

     KSHUID       The real user id of the shell.

     KSH_VERSION  The name and version of the shell (read-only).  See also the version commands
                  in Emacs editing mode and Vi editing mode sections, below.

     LINENO       The line number of the function or shell script that is currently being
                  executed.

     LINES        Set to the number of lines on the terminal or window.  Always set, defaults to
                  24.  See COLUMNS.

     EPOCHREALTIME
                  Time since the epoch, as returned by gettimeofday(2), formatted as decimal
                  tv_sec followed by a dot (‘.’) and tv_usec padded to exactly six decimal
                  digits.

     OLDPWD       The previous working directory.  Unset if cd has not successfully changed
                  directories since the shell started, or if the shell doesn't know where it is.

     OPTARG       When using getopts, it contains the argument for a parsed option, if it
                  requires one.

     OPTIND       The index of the next argument to be processed when using getopts.  Assigning 1
                  to this parameter causes getopts to process arguments from the beginning the
                  next time it is invoked.

     PATH         A colon separated list of directories that are searched when looking for
                  commands and files sourced using the ‘.’ command (see below).  An empty string
                  resulting from a leading or trailing colon, or two adjacent colons, is treated
                  as a ‘.’ (the current directory).

     PGRP         The process ID of the shell's process group leader.

     PIPESTATUS   An array containing the errorlevel (exit status) codes, one by one, of the last
                  pipeline run in the foreground.

     PPID         The process ID of the shell's parent.

     PS1          The primary prompt for interactive shells.  Parameter, command, and arithmetic
                  substitutions are performed, and ‘!’ is replaced with the current command
                  number (see the fc command below).  A literal ‘!’ can be put in the prompt by
                  placing ‘!!’ in PS1.

                  The default prompt is ‘$ ’ for non-root users, ‘# ’ for root.  If mksh is
                  invoked by root and PS1 does not contain a ‘#’ character, the default value
                  will be used even if PS1 already exists in the environment.

                  The mksh distribution comes with a sample dot.mkshrc containing a sophisticated
                  example, but you might like the following one (note that
                  ${HOSTNAME:=$(hostname)} and the root-vs-user distinguishing clause are (in
                  this example) executed at PS1 assignment time, while the $USER and $PWD are
                  escaped and thus will be evaluated each time a prompt is displayed):

                  PS1='${USER:=$(id -un)}'"@${HOSTNAME:=$(hostname)}:\$PWD $(
                          if (( USER_ID )); then print \$; else print \#; fi) "

                  Note that since the command-line editors try to figure out how long the prompt
                  is (so they know how far it is to the edge of the screen), escape codes in the
                  prompt tend to mess things up.  You can tell the shell not to count certain
                  sequences (such as escape codes) by prefixing your prompt with a character
                  (such as Ctrl-A) followed by a carriage return and then delimiting the escape
                  codes with this character.  Any occurences of that character in the prompt are
                  not printed.  By the way, don't blame me for this hack; it's derived from the
                  original ksh88(1), which did print the delimiter character so you were out of
                  luck if you did not have any non-printing characters.

                  Since Backslashes and other special characters may be interpreted by the shell,
                  to set PS1 either escape the backslash itself, or use double quotes.  The
                  latter is more practical.  This is a more complex example, avoiding to directly
                  enter special characters (for example with ^V in the emacs editing mode), which
                  embeds the current working directory, in reverse video (colour would work,
                  too), in the prompt string:

                        x=$(print \\001)
                        PS1="$x$(print \\r)$x$(tput smso)$x\$PWD$x$(tput rmso)$x> "

                  Due to a strong suggestion from David G. Korn, mksh now also supports the
                  following form:

                        PS1=$'\1\r\1\e[7m\1$PWD\1\e[0m\1> '

     PS2          Secondary prompt string, by default ‘> ’, used when more input is needed to
                  complete a command.

     PS3          Prompt used by the select statement when reading a menu selection.  The default
                  is ‘#? ’.

     PS4          Used to prefix commands that are printed during execution tracing (see the set
                  -x command below).  Parameter, command, and arithmetic substitutions are
                  performed before it is printed.  The default is ‘+ ’.  You may want to set it
                  to ‘[$EPOCHREALTIME] ’ instead, to include timestamps.

     PWD          The current working directory.  May be unset or NULL if the shell doesn't know
                  where it is.

     RANDOM       Each time RANDOM is referenced, it is assigned a number between 0 and 32767
                  from a Linear Congruential PRNG first.

     REPLY        Default parameter for the read command if no names are given.  Also used in
                  select loops to store the value that is read from standard input.

     SECONDS      The number of seconds since the shell started or, if the parameter has been
                  assigned an integer value, the number of seconds since the assignment plus the
                  value that was assigned.

     TMOUT        If set to a positive integer in an interactive shell, it specifies the maximum
                  number of seconds the shell will wait for input after printing the primary
                  prompt (PS1).  If the time is exceeded, the shell exits.

     TMPDIR       The directory temporary shell files are created in.  If this parameter is not
                  set, or does not contain the absolute path of a writable directory, temporary
                  files are created in /tmp.

     USER_ID      The effective user id of the shell.

   Tilde expansion
     Tilde expansion which is done in parallel with parameter substitution, is done on words
     starting with an unquoted ‘~’.  The characters following the tilde, up to the first ‘/’, if
     any, are assumed to be a login name.  If the login name is empty, ‘+’, or ‘-’, the value of
     the HOME, PWD, or OLDPWD parameter is substituted, respectively.  Otherwise, the password
     file is searched for the login name, and the tilde expression is substituted with the user's
     home directory.  If the login name is not found in the password file or if any quoting or
     parameter substitution occurs in the login name, no substitution is performed.

     In parameter assignments (such as those preceding a simple-command or those occurring in the
     arguments of alias, export, global, readonly, and typeset), tilde expansion is done after
     any assignment (i.e. after the equals sign) or after an unquoted colon (‘:’); login names
     are also delimited by colons.

     The home directory of previously expanded login names are cached and re-used.  The alias -d
     command may be used to list, change, and add to this cache (e.g. alias -d
     fac=/usr/local/facilities; cd ~fac/bin).

   Brace expansion (alteration)
     Brace expressions take the following form:

           prefix{str1,...,strN}suffix

     The expressions are expanded to N words, each of which is the concatenation of prefix, stri,
     and suffix (e.g. “a{c,b{X,Y},d}e” expands to four words: “ace”, “abXe”, “abYe”, and “ade”).
     As noted in the example, brace expressions can be nested and the resulting words are not
     sorted.  Brace expressions must contain an unquoted comma (‘,’) for expansion to occur (e.g.
     {} and {foo} are not expanded).  Brace expansion is carried out after parameter substitution
     and before file name generation.

   File name patterns
     A file name pattern is a word containing one or more unquoted ‘?’, ‘*’, ‘+’, ‘@’, or ‘!’
     characters or “[..]” sequences.  Once brace expansion has been performed, the shell replaces
     file name patterns with the sorted names of all the files that match the pattern (if no
     files match, the word is left unchanged).  The pattern elements have the following meaning:

     ?       Matches any single character.

     *       Matches any sequence of octets.

     [..]    Matches any of the octets inside the brackets.  Ranges of octets can be specified by
             separating two octets by a ‘-’ (e.g. “[a0-9]” matches the letter ‘a’ or any digit).
             In order to represent itself, a ‘-’ must either be quoted or the first or last octet
             in the octet list.  Similarly, a ‘]’ must be quoted or the first octet in the list
             if it is to represent itself instead of the end of the list.  Also, a ‘!’ appearing
             at the start of the list has special meaning (see below), so to represent itself it
             must be quoted or appear later in the list.

     [!..]   Like [..], except it matches any octet not inside the brackets.

     *(pattern|...|pattern)
             Matches any string of octets that matches zero or more occurrences of the specified
             patterns.  Example: The pattern *(foo|bar) matches the strings “”, “foo”, “bar”,
             “foobarfoo”, etc.

     +(pattern|...|pattern)
             Matches any string of octets that matches one or more occurrences of the specified
             patterns.  Example: The pattern +(foo|bar) matches the strings “foo”, “bar”,
             “foobar”, etc.

     ?(pattern|...|pattern)
             Matches the empty string or a string that matches one of the specified patterns.
             Example: The pattern ?(foo|bar) only matches the strings “”, “foo”, and “bar”.

     @(pattern|...|pattern)
             Matches a string that matches one of the specified patterns.  Example: The pattern
             @(foo|bar) only matches the strings “foo” and “bar”.

     !(pattern|...|pattern)
             Matches any string that does not match one of the specified patterns.  Examples: The
             pattern !(foo|bar) matches all strings except “foo” and “bar”; the pattern !(*)
             matches no strings; the pattern !(?)* matches all strings (think about it).

     Note that complicated globbing, especially with alternatives, is slow; using separate
     comparisons may (or may not) be faster.

     Note that mksh (and pdksh) never matches ‘.’ and ‘..’, but AT&T UNIX ksh, Bourne sh, and GNU
     bash do.

     Note that none of the above pattern elements match either a period (‘.’) at the start of a
     file name or a slash (‘/’), even if they are explicitly used in a [..] sequence; also, the
     names ‘.’ and ‘..’ are never matched, even by the pattern ‘.*’.

     If the markdirs option is set, any directories that result from file name generation are
     marked with a trailing ‘/’.

   Input/output redirection
     When a command is executed, its standard input, standard output, and standard error (file
     descriptors 0, 1, and 2, respectively) are normally inherited from the shell.  Three
     exceptions to this are commands in pipelines, for which standard input and/or standard
     output are those set up by the pipeline, asynchronous commands created when job control is
     disabled, for which standard input is initially set to be from /dev/null, and commands for
     which any of the following redirections have been specified:

     > file      Standard output is redirected to file.  If file does not exist, it is created;
                 if it does exist, is a regular file, and the noclobber option is set, an error
                 occurs; otherwise, the file is truncated.  Note that this means the command cmd
                 <foo >foo will open foo for reading and then truncate it when it opens it for
                 writing, before cmd gets a chance to actually read foo.

     >| file     Same as >, except the file is truncated, even if the noclobber option is set.

     >> file     Same as >, except if file exists it is appended to instead of being truncated.
                 Also, the file is opened in append mode, so writes always go to the end of the
                 file (see open(2)).

     < file      Standard input is redirected from file, which is opened for reading.

     <> file     Same as <, except the file is opened for reading and writing.

     << marker   After reading the command line containing this kind of redirection (called a
                 “here document”), the shell copies lines from the command source into a
                 temporary file until a line matching marker is read.  When the command is
                 executed, standard input is redirected from the temporary file.  If marker
                 contains no quoted characters, the contents of the temporary file are processed
                 as if enclosed in double quotes each time the command is executed, so parameter,
                 command, and arithmetic substitutions are performed, along with backslash (‘\’)
                 escapes for ‘$’, ‘`’, ‘\’, and ‘\newline’, but not for ‘"’.  If multiple here
                 documents are used on the same command line, they are saved in order.

                 If no marker is given, the here document ends at the next << and substitution
                 will be performed.  If marker is only a set of either single “''” or double ‘""’
                 quotes with nothing in between, the here document ends at the next empty line
                 and substitution will not be performed.

     <<- marker  Same as <<, except leading tabs are stripped from lines in the here document.

     <<< word    Same as <<, except that word is the here document.  This is called a here
                 string.

     <& fd       Standard input is duplicated from file descriptor fd.  fd can be a number,
                 indicating the number of an existing file descriptor; the letter ‘p’, indicating
                 the file descriptor associated with the output of the current co-process; or the
                 character ‘-’, indicating standard input is to be closed.  Note that fd is
                 limited to a single digit in most shell implementations.

     >& fd       Same as <&, except the operation is done on standard output.

     &> file     Same as > file 2>&1.  This is a GNU bash extension supported by mksh which also
                 supports the preceding explicit fd number, for example, 3&> file is the same as
                 3> file 2>&3 in mksh but a syntax error in GNU bash.  Setting the -o posix or -o
                 sh shell options disable parsing of this redirection; it's a compatibility
                 feature to legacy scripts, to not be used when writing new shell code.

     &>| file, &>> file, &>& fd
                 Same as >| file, >> file, or >& fd, followed by 2>&1, as above.  These are mksh
                 extensions.

     In any of the above redirections, the file descriptor that is redirected (i.e. standard
     input or standard output) can be explicitly given by preceding the redirection with a number
     (portably, only a single digit).  Parameter, command, and arithmetic substitutions, tilde
     substitutions, and (if the shell is interactive) file name generation are all performed on
     the file, marker, and fd arguments of redirections.  Note, however, that the results of any
     file name generation are only used if a single file is matched; if multiple files match, the
     word with the expanded file name generation characters is used.  Note that in restricted
     shells, redirections which can create files cannot be used.

     For simple-commands, redirections may appear anywhere in the command; for compound-commands
     (if statements, etc.), any redirections must appear at the end.  Redirections are processed
     after pipelines are created and in the order they are given, so the following will print an
     error with a line number prepended to it:

           $ cat /foo/bar 2>&1 >/dev/null | pr -n -t

     File descriptors created by input/output redirections are private to the Korn shell, but
     passed to sub-processes if -o posix or -o sh is set.

   Arithmetic expressions
     Integer arithmetic expressions can be used with the let command, inside $((..)) expressions,
     inside array references (e.g. name[expr]), as numeric arguments to the test command, and as
     the value of an assignment to an integer parameter.

     Expressions are calculated using signed arithmetic and the mksh_ari_t type (a 32-bit signed
     integer), unless they begin with a sole ‘#’ character, in which case they use mksh_uari_t (a
     32-bit unsigned integer).

     Expressions may contain alpha-numeric parameter identifiers, array references, and integer
     constants and may be combined with the following C operators (listed and grouped in
     increasing order of precedence):

     Unary operators:

           + - ! ~ ++ --

     Binary operators:

           ,
           = += -= *= /= %= <<<= >>>= <<= >>= &= ^= |=
           ||
           &&
           |
           ^
           &
           == !=
           < <= > >=
           <<< >>> << >>
           + -
           * / %

     Ternary operators:

           ?: (precedence is immediately higher than assignment)

     Grouping operators:

           ( )

     Integer constants and expressions are calculated using an exactly 32-bit wide, signed or
     unsigned, type with silent wraparound on integer overflow.  Integer constants may be
     specified with arbitrary bases using the notation base#number, where base is a decimal
     integer specifying the base, and number is a number in the specified base.  Additionally,
     base-16 integers may be specified by prefixing them with ‘0x’ (case-insensitive) in all
     forms of arithmetic expressions, except as numeric arguments to the test built-in command.
     Prefixing numbers with a sole digit zero (‘0’) leads to the shell interpreting it as base-8
     integer in posix mode only; historically, (pd)ksh has never done so either anyway, and it's
     unsafe to do that, but POSIX demands it nowadays.  As a special mksh extension, numbers to
     the base of one are treated as either (8-bit transparent) ASCII or Unicode codepoints,
     depending on the shell's utf8-mode flag (current setting).  The AT&T UNIX ksh93 syntax of
     “'x'” instead of “1#x” is also supported.  Note that NUL bytes (integral value of zero)
     cannot be used.  An unset or empty parameter evaluates to 0 in integer context.  In Unicode
     mode, raw octets are mapped into the range EF80..EFFF as in OPTU-8, which is in the PUA and
     has been assigned by CSUR for this use.  If more than one octet in ASCII mode, or a sequence
     of more than one octet not forming a valid and minimal CESU-8 sequence is passed, the
     behaviour is undefined (usually, the shell aborts with a parse error, but rarely, it
     succeeds, e.g. on the sequence C2 20).  That's why you should always use ASCII mode unless
     you know that the input is well-formed UTF-8 in the range of 0000..FFFD.

     The operators are evaluated as follows:

           unary +
                   Result is the argument (included for completeness).

           unary -
                   Negation.

           !       Logical NOT; the result is 1 if argument is zero, 0 if not.

           ~       Arithmetic (bit-wise) NOT.

           ++      Increment; must be applied to a parameter (not a literal or other expression).
                   The parameter is incremented by 1.  When used as a prefix operator, the result
                   is the incremented value of the parameter; when used as a postfix operator,
                   the result is the original value of the parameter.

           --      Similar to ++, except the parameter is decremented by 1.

           ,       Separates two arithmetic expressions; the left-hand side is evaluated first,
                   then the right.  The result is the value of the expression on the right-hand
                   side.

           =       Assignment; the variable on the left is set to the value on the right.

           += -= *= /= %= <<<= >>>= <<= >>= &= ^= |=
                   Assignment operators.  <var><op>=<expr> is the same as <var>=<var><op><expr>,
                   with any operator precedence in <expr> preserved.  For example, “var1 *= 5 +
                   3” is the same as specifying “var1 = var1 * (5 + 3)”.

           ||      Logical OR; the result is 1 if either argument is non-zero, 0 if not.  The
                   right argument is evaluated only if the left argument is zero.

           &&      Logical AND; the result is 1 if both arguments are non-zero, 0 if not.  The
                   right argument is evaluated only if the left argument is non-zero.

           |       Arithmetic (bit-wise) OR.

           ^       Arithmetic (bit-wise) XOR (exclusive-OR).

           &       Arithmetic (bit-wise) AND.

           ==      Equal; the result is 1 if both arguments are equal, 0 if not.

           !=      Not equal; the result is 0 if both arguments are equal, 1 if not.

           <       Less than; the result is 1 if the left argument is less than the right, 0 if
                   not.

           <= > >=
                   Less than or equal, greater than or equal, greater than.  See <.

           <<< >>>
                   Rotate left (right); the result is similar to shift (see <<) except that the
                   bits shifted out at one end are shifted in at the other end, instead of zero
                   or sign bits.

           << >>   Shift left (right); the result is the left argument with its bits shifted left
                   (right) by the amount given in the right argument.

           + - * /
                   Addition, subtraction, multiplication, and division.

           %       Remainder; the result is the remainder of the division of the left argument by
                   the right.

           <arg1>?<arg2>:<arg3>
                   If <arg1> is non-zero, the result is <arg2>; otherwise the result is <arg3>.
                   The non-result argument is not evaluated.

   Co-processes
     A co-process (which is a pipeline created with the ‘|&’ operator) is an asynchronous process
     that the shell can both write to (using print -p) and read from (using read -p).  The input
     and output of the co-process can also be manipulated using >&p and <&p redirections,
     respectively.  Once a co-process has been started, another can't be started until the co-
     process exits, or until the co-process's input has been redirected using an exec n>&p
     redirection.  If a co-process's input is redirected in this way, the next co-process to be
     started will share the output with the first co-process, unless the output of the initial
     co-process has been redirected using an exec n<&p redirection.

     Some notes concerning co-processes:

     ·   The only way to close the co-process's input (so the co-process reads an end-of-file) is
         to redirect the input to a numbered file descriptor and then close that file descriptor:
         exec 3>&p; exec 3>&-

     ·   In order for co-processes to share a common output, the shell must keep the write
         portion of the output pipe open.  This means that end-of-file will not be detected until
         all co-processes sharing the co-process's output have exited (when they all exit, the
         shell closes its copy of the pipe).  This can be avoided by redirecting the output to a
         numbered file descriptor (as this also causes the shell to close its copy).  Note that
         this behaviour is slightly different from the original Korn shell which closes its copy
         of the write portion of the co-process output when the most recently started co-process
         (instead of when all sharing co-processes) exits.

     ·   print -p will ignore SIGPIPE signals during writes if the signal is not being trapped or
         ignored; the same is true if the co-process input has been duplicated to another file
         descriptor and print -un is used.

   Functions
     Functions are defined using either Korn shell function function-name syntax or the
     Bourne/POSIX shell function-name() syntax (see below for the difference between the two
     forms).  Functions are like .‐scripts (i.e. scripts sourced using the ‘.’ built-in) in that
     they are executed in the current environment.  However, unlike .‐scripts, shell arguments
     (i.e. positional parameters $1, $2, etc.) are never visible inside them.  When the shell is
     determining the location of a command, functions are searched after special built-in
     commands, before builtins and the PATH is searched.

     An existing function may be deleted using unset -f function-name.  A list of functions can
     be obtained using typeset +f and the function definitions can be listed using typeset -f.
     The autoload command (which is an alias for typeset -fu) may be used to create undefined
     functions: when an undefined function is executed, the shell searches the path specified in
     the FPATH parameter for a file with the same name as the function which, if found, is read
     and executed.  If after executing the file the named function is found to be defined, the
     function is executed; otherwise, the normal command search is continued (i.e. the shell
     searches the regular built-in command table and PATH).  Note that if a command is not found
     using PATH, an attempt is made to autoload a function using FPATH (this is an undocumented
     feature of the original Korn shell).

     Functions can have two attributes, “trace” and “export”, which can be set with typeset -ft
     and typeset -fx, respectively.  When a traced function is executed, the shell's xtrace
     option is turned on for the function's duration.  The “export” attribute of functions is
     currently not used.  In the original Korn shell, exported functions are visible to shell
     scripts that are executed.

     Since functions are executed in the current shell environment, parameter assignments made
     inside functions are visible after the function completes.  If this is not the desired
     effect, the typeset command can be used inside a function to create a local parameter.  Note
     that AT&T UNIX ksh93 uses static scoping (one global scope, one local scope per function)
     and allows local variables only on Korn style functions, whereas mksh uses dynamic scoping
     (nested scopes of varying locality).  Note that special parameters (e.g. $$, $!) can't be
     scoped in this way.

     The exit status of a function is that of the last command executed in the function.  A
     function can be made to finish immediately using the return command; this may also be used
     to explicitly specify the exit status.

     Functions defined with the function reserved word are treated differently in the following
     ways from functions defined with the () notation:

     ·   The $0 parameter is set to the name of the function (Bourne-style functions leave $0
         untouched).

     ·   Parameter assignments preceding function calls are not kept in the shell environment
         (executing Bourne-style functions will keep assignments).

     ·   OPTIND is saved/reset and restored on entry and exit from the function so getopts can be
         used properly both inside and outside the function (Bourne-style functions leave OPTIND
         untouched, so using getopts inside a function interferes with using getopts outside the
         function).

     ·   Bourne-style function definitions take precedence over alias dereferences and remove
         alias definitions upon encounter, while aliases take precedence over Korn-style
         functions.

     In the future, the following differences may also be added:

     ·   A separate trap/signal environment will be used during the execution of functions.  This
         will mean that traps set inside a function will not affect the shell's traps and signals
         that are not ignored in the shell (but may be trapped) will have their default effect in
         a function.

     ·   The EXIT trap, if set in a function, will be executed after the function returns.

   Command execution
     After evaluation of command-line arguments, redirections, and parameter assignments, the
     type of command is determined: a special built-in command, a function, a normal builtin, or
     the name of a file to execute found using the PATH parameter.  The checks are made in the
     above order.  Special built-in commands differ from other commands in that the PATH
     parameter is not used to find them, an error during their execution can cause a non-
     interactive shell to exit, and parameter assignments that are specified before the command
     are kept after the command completes.  Regular built-in commands are different only in that
     the PATH parameter is not used to find them.

     The original ksh and POSIX differ somewhat in which commands are considered special or
     regular.

     POSIX special built-in utilities:

     ., :, break, continue, eval, exec, exit, export, readonly, return, set, shift, times, trap,
     unset

     Additional mksh commands keeping assignments:

     builtin, global, typeset, wait

     Builtins that are not special:

     [, alias, bg, bind, cat, cd, command, echo, false, fc, fg, getopts, jobs, kill, let, mknod,
     print, pwd, read, realpath, rename, sleep, test, true, ulimit, umask, unalias, whence

     Once the type of command has been determined, any command-line parameter assignments are
     performed and exported for the duration of the command.

     The following describes the special and regular built-in commands:

     . file [arg ...]
            This is called the “dot” command.  Execute the commands in file in the current
            environment.  The file is searched for in the directories of PATH.  If arguments are
            given, the positional parameters may be used to access them while file is being
            executed.  If no arguments are given, the positional parameters are those of the
            environment the command is used in.

     : [...]
            The null command.  Exit status is set to zero.

     [ expression ]
            See test.

     alias [-d | -t [-r] | +-x] [-p] [+] [name [=value] ...]
            Without arguments, alias lists all aliases.  For any name without a value, the
            existing alias is listed.  Any name with a value defines an alias (see Aliases
            above).

            When listing aliases, one of two formats is used.  Normally, aliases are listed as
            name=value, where value is quoted.  If options were preceded with ‘+’, or a lone ‘+’
            is given on the command line, only name is printed.

            The -d option causes directory aliases which are used in tilde expansion to be listed
            or set (see Tilde expansion above).

            If the -p option is used, each alias is prefixed with the string “alias ”.

            The -t option indicates that tracked aliases are to be listed/set (values specified
            on the command line are ignored for tracked aliases).  The -r option indicates that
            all tracked aliases are to be reset.

            The -x option sets (+x clears) the export attribute of an alias, or, if no names are
            given, lists the aliases with the export attribute (exporting an alias has no
            effect).

     bg [job ...]
            Resume the specified stopped job(s) in the background.  If no jobs are specified, %+
            is assumed.  See Job control below for more information.

     bind [-l]
            The current bindings are listed.  If the -l flag is given, bind instead lists the
            names of the functions to which keys may be bound.  See Emacs editing mode for more
            information.

     bind [-m] string=[substitute] ...
     bind string=[editing-command] ...
            The specified editing command is bound to the given string, which should consist of a
            control character optionally preceded by one of the two prefix characters and
            optionally succeded by a tilde character.  Future input of the string will cause the
            editing command to be immediately invoked.  If the -m flag is given, the specified
            input string will afterwards be immediately replaced by the given substitute string
            which may contain editing commands but not other macros.  If a tilde postfix is
            given, a tilde trailing the one or two prefices and the control character is ignored,
            any other trailing character will be processed afterwards.

            Control characters may be written using caret notation i.e. ^X represents Ctrl-X.
            Note that although only two prefix characters (usually ESC and ^X) are supported,
            some multi-character sequences can be supported.

            The following default bindings show how the arrow keys, the home, end and delete key
            on a BSD wsvt25, xterm-xfree86 or GNU screen terminal are bound (of course some
            escape sequences won't work out quite this nicely):

                  bind '^X'=prefix-2
                  bind '^[['=prefix-2
                  bind '^XA'=up-history
                  bind '^XB'=down-history
                  bind '^XC'=forward-char
                  bind '^XD'=backward-char
                  bind '^X1~'=beginning-of-line
                  bind '^X7~'=beginning-of-line
                  bind '^XH'=beginning-of-line
                  bind '^X4~'=end-of-line
                  bind '^X8~'=end-of-line
                  bind '^XF'=end-of-line
                  bind '^X3~'=delete-char-forward

     break [level]
            Exit the levelth inner-most for, select, until, or while loop.  level defaults to 1.

     builtin [--] command [arg ...]
            Execute the built-in command command.

     cat [-u] [file ...]
            Read files sequentially, in command line order, and write them to standard output.
            If a file is a single dash (‘-’) or absent, read from standard input.  Unless
            compiled with MKSH_NO_EXTERNAL_CAT, if any options are given, an external cat(1)
            utility is invoked instead if called from the shell.  For direct builtin calls, the
            POSIX -u option is supported as a no-op.

     cd [-L] [dir]
     cd -P [-e] [dir]
     chdir [-eLP] [dir]
            Set the working directory to dir.  If the parameter CDPATH is set, it lists the
            search path for the directory containing dir.  A NULL path means the current
            directory.  If dir is found in any component of the CDPATH search path other than the
            NULL path, the name of the new working directory will be written to standard output.
            If dir is missing, the home directory HOME is used.  If dir is ‘-’, the previous
            working directory is used (see the OLDPWD parameter).

            If the -L option (logical path) is used or if the physical option isn't set (see the
            set command below), references to ‘..’ in dir are relative to the path used to get to
            the directory.  If the -P option (physical path) is used or if the physical option is
            set, ‘..’ is relative to the filesystem directory tree.  The PWD and OLDPWD
            parameters are updated to reflect the current and old working directory,
            respectively.  If the -e option is set for physical filesystem traversal, and PWD
            could not be set, the exit code is 1; greater than 1 if an error occurred, 0
            otherwise.

     cd [-eLP] old new
     chdir [-eLP] old new
            The string new is substituted for old in the current directory, and the shell
            attempts to change to the new directory.

     command [-pVv] cmd [arg ...]
            If neither the -v nor -V option is given, cmd is executed exactly as if command had
            not been specified, with two exceptions: firstly, cmd cannot be a shell function; and
            secondly, special built-in commands lose their specialness (i.e. redirection and
            utility errors do not cause the shell to exit, and command assignments are not
            permanent).

            If the -p option is given, a default search path is used instead of the current value
            of PATH, the actual value of which is system dependent.

            If the -v option is given, instead of executing cmd, information about what would be
            executed is given (and the same is done for arg ...).  For special and regular built-
            in commands and functions, their names are simply printed; for aliases, a command
            that defines them is printed; and for commands found by searching the PATH parameter,
            the full path of the command is printed.  If no command is found (i.e. the path
            search fails), nothing is printed and command exits with a non-zero status.  The -V
            option is like the -v option, except it is more verbose.

     continue [level]
            Jumps to the beginning of the levelth inner-most for, select, until, or while loop.
            level defaults to 1.

     echo [-Een] [arg ...]
            Warning: this utility is not portable; use the Korn shell builtin print instead.

            Prints its arguments (separated by spaces) followed by a newline, to the standard
            output.  The newline is suppressed if any of the arguments contain the backslash
            sequence ‘\c’.  See the print command below for a list of other backslash sequences
            that are recognised.

            The options are provided for compatibility with BSD shell scripts.  The -n option
            suppresses the trailing newline, -e enables backslash interpretation (a no-op, since
            this is normally done), and -E suppresses backslash interpretation.

            If the posix or sh option is set or this is a direct builtin call, only the first
            argument is treated as an option, and only if it is exactly “-n”.  Backslash
            interpretation is disabled.

     eval command ...
            The arguments are concatenated (with spaces between them) to form a single string
            which the shell then parses and executes in the current environment.

     exec [command [arg ...]]
            The command is executed without forking, replacing the shell process.

            If no command is given except for I/O redirection, the I/O redirection is permanent
            and the shell is not replaced.  Any file descriptors greater than 2 which are opened
            or dup(2)'d in this way are not made available to other executed commands (i.e.
            commands that are not built-in to the shell).  Note that the Bourne shell differs
            here; it does pass these file descriptors on.

     exit [status]
            The shell exits with the specified exit status.  If status is not specified, the exit
            status is the current value of the $? parameter.

     export [-p] [parameter[=value]]
            Sets the export attribute of the named parameters.  Exported parameters are passed in
            the environment to executed commands.  If values are specified, the named parameters
            are also assigned.

            If no parameters are specified, all parameters with the export attribute set are
            printed one per line; either their names, or, if a ‘-’ with no option letter is
            specified, name=value pairs, or, with -p, export commands suitable for re-entry.

     false  A command that exits with a non-zero status.

     fc [-e editor | -l [-n]] [-r] [first [last]]
            first and last select commands from the history.  Commands can be selected by history
            number or a string specifying the most recent command starting with that string.  The
            -l option lists the command on standard output, and -n inhibits the default command
            numbers.  The -r option reverses the order of the list.  Without -l, the selected
            commands are edited by the editor specified with the -e option, or if no -e is
            specified, the editor specified by the FCEDIT parameter (if this parameter is not
            set, /bin/ed is used), and then executed by the shell.

     fc -e - | -s [-g] [old=new] [prefix]
            Re-execute the selected command (the previous command by default) after performing
            the optional substitution of old with new.  If -g is specified, all occurrences of
            old are replaced with new.  The meaning of -e - and -s is identical: re-execute the
            selected command without invoking an editor.  This command is usually accessed with
            the predefined: alias r='fc -e -'

     fg [job ...]
            Resume the specified job(s) in the foreground.  If no jobs are specified, %+ is
            assumed.  See Job control below for more information.

     getopts optstring name [arg ...]
            Used by shell procedures to parse the specified arguments (or positional parameters,
            if no arguments are given) and to check for legal options.  optstring contains the
            option letters that getopts is to recognise.  If a letter is followed by a colon, the
            option is expected to have an argument.  Options that do not take arguments may be
            grouped in a single argument.  If an option takes an argument and the option
            character is not the last character of the argument it is found in, the remainder of
            the argument is taken to be the option's argument; otherwise, the next argument is
            the option's argument.

            Each time getopts is invoked, it places the next option in the shell parameter name
            and the index of the argument to be processed by the next call to getopts in the
            shell parameter OPTIND.  If the option was introduced with a ‘+’, the option placed
            in name is prefixed with a ‘+’.  When an option requires an argument, getopts places
            it in the shell parameter OPTARG.

            When an illegal option or a missing option argument is encountered, a question mark
            or a colon is placed in name (indicating an illegal option or missing argument,
            respectively) and OPTARG is set to the option character that caused the problem.
            Furthermore, if optstring does not begin with a colon, a question mark is placed in
            name, OPTARG is unset, and an error message is printed to standard error.

            When the end of the options is encountered, getopts exits with a non-zero exit
            status.  Options end at the first (non-option argument) argument that does not start
            with a ‘-’, or when a ‘--’ argument is encountered.

            Option parsing can be reset by setting OPTIND to 1 (this is done automatically
            whenever the shell or a shell procedure is invoked).

            Warning: Changing the value of the shell parameter OPTIND to a value other than 1, or
            parsing different sets of arguments without resetting OPTIND, may lead to unexpected
            results.

     global ...
            See typeset.

     hash [-r] [name ...]
            Without arguments, any hashed executable command pathnames are listed.  The -r option
            causes all hashed commands to be removed from the hash table.  Each name is searched
            as if it were a command name and added to the hash table if it is an executable
            command.

     jobs [-lnp] [job ...]
            Display information about the specified job(s); if no jobs are specified, all jobs
            are displayed.  The -n option causes information to be displayed only for jobs that
            have changed state since the last notification.  If the -l option is used, the
            process ID of each process in a job is also listed.  The -p option causes only the
            process group of each job to be printed.  See Job control below for the format of job
            and the displayed job.

     kill [-s signame | -signum | -signame] { job | pid | pgrp } ...
            Send the specified signal to the specified jobs, process IDs, or process groups.  If
            no signal is specified, the TERM signal is sent.  If a job is specified, the signal
            is sent to the job's process group.  See Job control below for the format of job.

     kill -l [exit-status ...]
            Print the signal name corresponding to exit-status.  If no arguments are specified, a
            list of all the signals, their numbers, and a short description of them are printed.

     let [expression ...]
            Each expression is evaluated (see Arithmetic expressions above).  If all expressions
            are successfully evaluated, the exit status is 0 (1) if the last expression evaluated
            to non-zero (zero).  If an error occurs during the parsing or evaluation of an
            expression, the exit status is greater than 1.  Since expressions may need to be
            quoted, (( expr )) is syntactic sugar for let "expr".

     let]   Internally used alias for let.

     mknod [-m mode] name b|c major minor
     mknod [-m mode] name p
            Create a device special file.  The file type may be b (block type device), c
            (character type device), or p (named pipe, FIFO).  The file created may be modified
            according to its mode (via the -m option), major (major device number), and minor
            (minor device number).

            See mknod(8) for further information.

     print [-nprsu[n] | -R [-en]] [argument ...]
            print prints its arguments on the standard output, separated by spaces and terminated
            with a newline.  The -n option suppresses the newline.  By default, certain C escapes
            are translated.  These include these mentioned in Backslash expansion above, as well
            as ‘\c’, which is equivalent to using the -n option.  Backslash expansion may be
            inhibited with the -r option.  The -s option prints to the history file instead of
            standard output; the -u option prints to file descriptor n (n defaults to 1 if
            omitted); and the -p option prints to the co-process (see Co-processes above).

            The -R option is used to emulate, to some degree, the BSD echo(1) command which does
            not process ‘\’ sequences unless the -e option is given.  As above, the -n option
            suppresses the trailing newline.

     printf format [arguments ...]
            Formatted output.  Approximately the same as the printf(1), utility, except it uses
            the same Backslash expansion and I/O code and does hot handle floating point as the
            rest of mksh.  This is not normally part of mksh; however, distributors may have
            added this as builtin as a speed hack.  Do not use in new code.

     pwd [-LP]
            Print the present working directory.  If the -L option is used or if the physical
            option isn't set (see the set command below), the logical path is printed (i.e. the
            path used to cd to the current directory).  If the -P option (physical path) is used
            or if the physical option is set, the path determined from the filesystem (by
            following ‘..’ directories to the root directory) is printed.

     read [-A | -a] [-d x] [-N z | -n z] [-p | -u[n]] [-t n] [-rs] [p ...]
            Reads a line of input, separates the input into fields using the IFS parameter (see
            Substitution above), and assigns each field to the specified parameters p.  If no
            parameters are specified, the REPLY parameter is used to store the result.  With the
            -A and -a options, only no or one parameter is accepted.  If there are more
            parameters than fields, the extra parameters are set to the empty string or 0; if
            there are more fields than parameters, the last parameter is assigned the remaining
            fields (including the word separators).

            The options are as follows:

            -A     Store the result into the parameter p (or REPLY) as array of words.

            -a     Store the result without word splitting into the parameter p (or REPLY) as
                   array of characters (wide characters if the utf8-mode option is enacted,
                   octets otherwise).

            -d x   Use the first byte of x, NUL if empty, instead of the ASCII newline character
                   as input line delimiter.

            -N z   Instead of reading till end-of-line, read exactly z bytes; less if EOF or a
                   timeout occurs.

            -n z   Instead of reading till end-of-line, read up to z bytes but return as soon as
                   any bytes are read, e.g. from a slow terminal device, or if EOF or a timeout
                   occurs.

            -p     Read from the currently active co-process, see Co-processes above for details
                   on this.

            -u[n]  Read from the file descriptor n (defaults to 0, i.e. standard input).  The
                   argument must immediately follow the option character.

            -t n   Interrupt reading after n seconds (specified as positive decimal value with an
                   optional fractional part).

            -r     Normally, the ASCII backslash character escapes the special meaning of the
                   following character and is stripped from the input; read does not stop when
                   encountering a backslash-newline sequence and does not store that newline in
                   the result.  This option enables raw mode, in which backslashes are not
                   processed.

            -s     The input line is saved to the history.

            If the input is a terminal, both the -N and -n options set it into raw mode; they
            read an entire file if -1 is passed as z argument.

            The first parameter may have a question mark and a string appended to it, in which
            case the string is used as a prompt (printed to standard error before any input is
            read) if the input is a tty(4) (e.g. read nfoo?'number of foos: ').

            If no input is read or a timeout occurred, read exits with a non-zero status.

            Another handy set of tricks: If read is run in a loop such as while read foo; do ...;
            done then leading whitespace will be removed (IFS) and backslashes processed.  You
            might want to use while IFS= read -r foo; do ...; done for pristine I/O.  Similarily,
            when using the -a option, use of the -r option might be prudent; the same applies
            for:

                  find . -type f -print0 | \
                      while IFS= read -d '' -r filename; do
                          print -r -- "found <${filename#./}>"
                  done

            The inner loop will be executed in a subshell and variable changes cannot be
            propagated if executed in a pipeline:

                  bar | baz | while read foo; do ...; done

            Use co-processes instead:

                  bar | baz |&
                  while read -p foo; do ...; done
                  exec 3>&p; exec 3>&-

     readonly [-p] [parameter [=value] ...]
            Sets the read-only attribute of the named parameters.  If values are given,
            parameters are set to them before setting the attribute.  Once a parameter is made
            read-only, it cannot be unset and its value cannot be changed.

            If no parameters are specified, the names of all parameters with the read-only
            attribute are printed one per line, unless the -p option is used, in which case
            readonly commands defining all read-only parameters, including their values, are
            printed.

     realpath [--] name
            Prints the resolved absolute pathname corresponding to name.  If name ends with a
            slash (‘/’), it's also checked for existence and whether it is a directory;
            otherwise, realpath returns 0 if the pathname either exists or can be created
            immediately, i.e. all but the last component exist and are directories.

     rename [--] from to
            Renames the file from to to.  Both must be complete pathnames and on the same device.
            This builtin is intended for emergency situations where /bin/mv becomes unusable, and
            directly calls rename(2).

     return [status]
            Returns from a function or . script, with exit status status.  If no status is given,
            the exit status of the last executed command is used.  If used outside of a function
            or . script, it has the same effect as exit.  Note that mksh treats both profile and
            ENV files as . scripts, while the original Korn shell only treats profiles as .
            scripts.

     set [+-abCefhiklmnprsUuvXx] [+-o option] [+-A name] [--] [arg ...]
            The set command can be used to set (-) or clear (+) shell options, set the positional
            parameters, or set an array parameter.  Options can be changed using the +-o option
            syntax, where option is the long name of an option, or using the +-letter syntax,
            where letter is the option's single letter name (not all options have a single letter
            name).  The following table lists both option letters (if they exist) and long names
            along with a description of what the option does:

            -A name
                 Sets the elements of the array parameter name to arg ... If -A is used, the
                 array is reset (i.e. emptied) first; if +A is used, the first N elements are set
                 (where N is the number of arguments); the rest are left untouched.

                 An alternative syntax for the command set -A foo -- a b c which is compatible to
                 GNU bash and also supported by AT&T UNIX ksh93 is: foo=(a b c); foo+=(d e)

                 Another AT&T UNIX ksh93 and GNU bash extension allows specifying the indices
                 used for arg ... (from the above example, a b c) like this: set -A foo -- [0]=a
                 [1]=b [2]=c or foo=([0]=a [1]=b [2]=c) which can also be written foo=([0]=a b c)
                 because indices are incremented automatically.

            -a | -o allexport
                 All new parameters are created with the export attribute.

            -b | -o notify
                 Print job notification messages asynchronously, instead of just before the
                 prompt.  Only used if job control is enabled (-m).

            -C | -o noclobber
                 Prevent > redirection from overwriting existing files.  Instead, >| must be used
                 to force an overwrite.

            -e | -o errexit
                 Exit (after executing the ERR trap) as soon as an error occurs or a command
                 fails (i.e. exits with a non-zero status).  This does not apply to commands
                 whose exit status is explicitly tested by a shell construct such as if, until,
                 while, &&, ||, or ! statements.

            -f | -o noglob
                 Do not expand file name patterns.

            -h | -o trackall
                 Create tracked aliases for all executed commands (see Aliases above).  Enabled
                 by default for non-interactive shells.

            -i | -o interactive
                 The shell is an interactive shell.  This option can only be used when the shell
                 is invoked.  See above for a description of what this means.

            -k | -o keyword
                 Parameter assignments are recognised anywhere in a command.

            -l | -o login
                 The shell is a login shell.  This option can only be used when the shell is
                 invoked.  See above for a description of what this means.

            -m | -o monitor
                 Enable job control (default for interactive shells).

            -n | -o noexec
                 Do not execute any commands.  Useful for checking the syntax of scripts (ignored
                 if interactive).

            -p | -o privileged
                 The shell is a privileged shell.  It is set automatically if, when the shell
                 starts, the real UID or GID does not match the effective UID (EUID) or GID
                 (EGID), respectively.  See above for a description of what this means.

            -r | -o restricted
                 The shell is a restricted shell.  This option can only be used when the shell is
                 invoked.  See above for a description of what this means.

            -s | -o stdin
                 If used when the shell is invoked, commands are read from standard input.  Set
                 automatically if the shell is invoked with no arguments.

                 When -s is used with the set command it causes the specified arguments to be
                 sorted before assigning them to the positional parameters (or to array name, if
                 -A is used).

            -U | -o utf8-mode
                 Enable UTF-8 support in the Emacs editing mode and internal string handling
                 functions.  This flag is disabled by default, but can be enabled by setting it
                 on the shell command line; is enabled automatically for interactive shells if
                 requested at compile time, your system supports setlocale(LC_CTYPE, "") and
                 optionally nl_langinfo(CODESET), or the LC_ALL, LC_CTYPE, or LANG environment
                 variables, and at least one of these returns something that matches “UTF-8” or
                 “utf8” case-insensitively; for direct builtin calls depending on the
                 aforementioned environment variables; or for stdin or scripts, if the input
                 begins with a UTF-8 Byte Order Mark.

            -u | -o nounset
                 Referencing of an unset parameter, other than “$@” or “$*”, is treated as an
                 error, unless one of the ‘-’, ‘+’, or ‘=’ modifiers is used.

            -v | -o verbose
                 Write shell input to standard error as it is read.

            -X | -o markdirs
                 Mark directories with a trailing ‘/’ during file name generation.

            -x | -o xtrace
                 Print command trees when they are executed, preceded by the value of PS4.

            -o bgnice
                 Background jobs are run with lower priority.

            -o braceexpand
                 Enable brace expansion (a.k.a. alternation).  This is enabled by default.  If
                 disabled, tilde expansion after an equals sign is disabled as a side effect.

            -o emacs
                 Enable BRL emacs-like command-line editing (interactive shells only); see Emacs
                 editing mode.

            -o gmacs
                 Enable gmacs-like command-line editing (interactive shells only).  Currently
                 identical to emacs editing except that transpose-chars (^T) acts slightly
                 differently.

            -o ignoreeof
                 The shell will not (easily) exit when end-of-file is read; exit must be used.
                 To avoid infinite loops, the shell will exit if EOF is read 13 times in a row.

            -o nohup
                 Do not kill running jobs with a SIGHUP signal when a login shell exits.
                 Currently set by default, but this may change in the future to be compatible
                 with AT&T UNIX ksh, which doesn't have this option, but does send the SIGHUP
                 signal.

            -o nolog
                 No effect.  In the original Korn shell, this prevents function definitions from
                 being stored in the history file.

            -o physical
                 Causes the cd and pwd commands to use “physical” (i.e. the filesystem's) ‘..’
                 directories instead of “logical” directories (i.e. the shell handles ‘..’, which
                 allows the user to be oblivious of symbolic links to directories).  Clear by
                 default.  Note that setting this option does not affect the current value of the
                 PWD parameter; only the cd command changes PWD.  See the cd and pwd commands
                 above for more details.

            -o pipefail
                 Make the exit status of a pipeline (before logically complementing) the
                 rightmost non-zero errorlevel, or zero if all commands exited with zero.

            -o posix
                 Enable a somewhat more POSIXish mode.  As a side effect, setting this flag turns
                 off braceexpand mode, which can be turned back on manually, and sh mode.

            -o sh
                 Enable /bin/sh (kludge) mode.  Automatically enabled if the basename of the
                 shell invocation begins with “sh” and this autodetection feature is compiled in
                 (not in MirBSD).  As a side effect, setting this flag turns off braceexpand
                 mode, which can be turned back on manually, and posix mode.

            -o vi
                 Enable vi(1)-like command-line editing (interactive shells only).

            -o vi-esccomplete
                 In vi command-line editing, do command and file name completion when escape (^[)
                 is entered in command mode.

            -o vi-tabcomplete
                 In vi command-line editing, do command and file name completion when tab (^I) is
                 entered in insert mode.  This is the default.

            -o viraw
                 No effect.  In the original Korn shell, unless viraw was set, the vi command-
                 line mode would let the tty(4) driver do the work until ESC (^[) was entered.
                 mksh is always in viraw mode.

            These options can also be used upon invocation of the shell.  The current set of
            options (with single letter names) can be found in the parameter ‘$-’.  set -o with
            no option name will list all the options and whether each is on or off; set +o will
            print the long names of all options that are currently on.

            Remaining arguments, if any, are positional parameters and are assigned, in order, to
            the positional parameters (i.e. $1, $2, etc.).  If options end with ‘--’ and there
            are no remaining arguments, all positional parameters are cleared.  If no options or
            arguments are given, the values of all names are printed.  For unknown historical
            reasons, a lone ‘-’ option is treated specially – it clears both the -v and -x
            options.

     shift [number]
            The positional parameters number+1, number+2, etc. are renamed to ‘1’, ‘2’, etc.
            number defaults to 1.

     sleep seconds
            Suspends execution for a minimum of the seconds specified as positive decimal value
            with an optional fractional part.  Signal delivery may continue execution earlier.

     source file [arg ...]
            Like . (“dot”), except that the current working directory is appended to the PATH in
            GNU bash and mksh.  In ksh93 and mksh, this is implemented as a shell alias instead
            of a builtin.

     test expression
     [ expression ]
            test evaluates the expression and returns zero status if true, 1 if false, or greater
            than 1 if there was an error.  It is normally used as the condition command of if and
            while statements.  Symbolic links are followed for all file expressions except -h and
            -L.

            The following basic expressions are available:

            -a file            file exists.

            -b file            file is a block special device.

            -c file            file is a character special device.

            -d file            file is a directory.

            -e file            file exists.

            -f file            file is a regular file.

            -G file            file's group is the shell's effective group ID.

            -g file            file's mode has the setgid bit set.

            -H file            file is a context dependent directory (only useful on HP-UX).

            -h file            file is a symbolic link.

            -k file            file's mode has the sticky(8) bit set.

            -L file            file is a symbolic link.

            -O file            file's owner is the shell's effective user ID.

            -o option          Shell option is set (see the set command above for a list of
                               options).  As a non-standard extension, if the option starts with
                               a ‘!’, the test is negated; the test always fails if option
                               doesn't exist (so [ -o foo -o -o !foo ] returns true if and only
                               if option foo exists).  The same can be achieved with [ -o ?foo ]
                               like in AT&T UNIX ksh93.  option can also be the short flag led by
                               either ‘-’ or ‘+’ (no logical negation), for example ‘-x’ or ‘+x’
                               instead of ‘xtrace’.

            -p file            file is a named pipe (FIFO).

            -r file            file exists and is readable.

            -S file            file is a unix(4)-domain socket.

            -s file            file is not empty.

            -t fd              File descriptor fd is a tty(4) device.

            -u file            file's mode has the setuid bit set.

            -w file            file exists and is writable.

            -x file            file exists and is executable.

            file1 -nt file2    file1 is newer than file2 or file1 exists and file2 does not.

            file1 -ot file2    file1 is older than file2 or file2 exists and file1 does not.

            file1 -ef file2    file1 is the same file as file2.

            string             string has non-zero length.

            -n string          string is not empty.

            -z string          string is empty.

            string = string    Strings are equal.

            string == string   Strings are equal.

            string > string    First string operand is greater than second string operand.

            string < string    First string operand is less than second string operand.

            string != string   Strings are not equal.

            number -eq number  Numbers compare equal.

            number -ne number  Numbers compare not equal.

            number -ge number  Numbers compare greater than or equal.

            number -gt number  Numbers compare greater than.

            number -le number  Numbers compare less than or equal.

            number -lt number  Numbers compare less than.

            The above basic expressions, in which unary operators have precedence over binary
            operators, may be combined with the following operators (listed in increasing order
            of precedence):

                  expr -o expr            Logical OR.
                  expr -a expr            Logical AND.
                  ! expr                  Logical NOT.
                  ( expr )                Grouping.

            Note that a number actually may be an arithmetic expression, such as a mathematical
            term or the name of an integer variable:

                  x=1; [ "x" -eq 1 ]      evaluates to true

            Note that some special rules are applied (courtesy of POSIX) if the number of
            arguments to test or inside the brackets [ ... ] is less than five: if leading ‘!’
            arguments can be stripped such that only one to three arguments remain, then the
            lowered comparison is executed; (thanks to XSI) parentheses \( ... \) lower four- and
            three-argument forms to two- and one-argument forms, respectively; three-argument
            forms ultimately prefer binary operations, followed by negation and parenthesis
            lowering; two- and four-argument forms prefer negation followed by parenthesis; the
            one-argument form always implies -n.

            Note: A common mistake is to use “if [ $foo = bar ]” which fails if parameter “foo”
            is NULL or unset, if it has embedded spaces (i.e. IFS octets), or if it is a unary
            operator like ‘!’ or ‘-n’.  Use tests like “if [ x"$foo" = x"bar" ]” instead, or the
            double-bracket operator “if [[ $foo = bar ]]” or, to avoid pattern matching (see [[
            above): “if [[ $foo = "$bar" ]]”

            The [[ ... ]] construct is not only more secure to use but also often faster.

     time [-p] [pipeline]
            If a pipeline is given, the times used to execute the pipeline are reported.  If no
            pipeline is given, then the user and system time used by the shell itself, and all
            the commands it has run since it was started, are reported.  The times reported are
            the real time (elapsed time from start to finish), the user CPU time (time spent
            running in user mode), and the system CPU time (time spent running in kernel mode).
            Times are reported to standard error; the format of the output is:

                  0m0.00s real     0m0.00s user     0m0.00s system

            If the -p option is given the output is slightly longer:

                  real     0.00
                  user     0.00
                  sys      0.00

            It is an error to specify the -p option unless pipeline is a simple command.

            Simple redirections of standard error do not affect the output of the time command:

                  $ time sleep 1 2>afile
                  $ { time sleep 1; } 2>afile

            Times for the first command do not go to “afile”, but those of the second command do.

     times  Print the accumulated user and system times used both by the shell and by processes
            that the shell started which have exited.  The format of the output is:

                  0m0.00s 0m0.00s
                  0m0.00s 0m0.00s

     trap [handler signal ...]
            Sets a trap handler that is to be executed when any of the specified signals are
            received.  handler is either a NULL string, indicating the signals are to be ignored,
            a minus sign (‘-’), indicating that the default action is to be taken for the signals
            (see signal(3)), or a string containing shell commands to be evaluated and executed
            at the first opportunity (i.e. when the current command completes, or before printing
            the next PS1 prompt) after receipt of one of the signals.  signal is the name of a
            signal (e.g. PIPE or ALRM) or the number of the signal (see the kill -l command
            above).

            There are two special signals: EXIT (also known as 0) which is executed when the
            shell is about to exit, and ERR, which is executed after an error occurs (an error is
            something that would cause the shell to exit if the -e or errexit option were set –
            see the set command above).  EXIT handlers are executed in the environment of the
            last executed command.  Note that for non-interactive shells, the trap handler cannot
            be changed for signals that were ignored when the shell started.

            With no arguments, trap lists, as a series of trap commands, the current state of the
            traps that have been set since the shell started.  Note that the output of trap
            cannot be usefully piped to another process (an artifact of the fact that traps are
            cleared when subprocesses are created).

            The original Korn shell's DEBUG trap and the handling of ERR and EXIT traps in
            functions are not yet implemented.

     true   A command that exits with a zero value.

     global [[+-alpnrtUux] [-L[n]] [-R[n]] [-Z[n]] [-i[n]] | -f [-tux]] [name [=value] ...]
     typeset [[+-alpnrtUux] [-LRZ[n]] [-i[n]] | -f [-tux]] [name [=value] ...]
            Display or set parameter attributes.  With no name arguments, parameter attributes
            are displayed; if no options are used, the current attributes of all parameters are
            printed as typeset commands; if an option is given (or ‘-’ with no option letter),
            all parameters and their values with the specified attributes are printed; if options
            are introduced with ‘+’, parameter values are not printed.

            If name arguments are given, the attributes of the named parameters are set (-) or
            cleared (+).  Values for parameters may optionally be specified.  For name[*], the
            change affects the entire array, and no value may be specified.

            If typeset is used inside a function, any parameters specified are localised.  This
            is not done by the otherwise identical global.  Note: This means that mksh 's global
            command is not equivalent to other programming languages' as it does not allow a
            function called from another function to access a parameter at truly global scope,
            but only prevents putting an accessed one into local scope.

            When -f is used, typeset operates on the attributes of functions.  As with
            parameters, if no name arguments are given, functions are listed with their values
            (i.e. definitions) unless options are introduced with ‘+’, in which case only the
            function names are reported.

            -a      Indexed array attribute.

            -f      Function mode.  Display or set functions and their attributes, instead of
                    parameters.

            -i[n]   Integer attribute.  n specifies the base to use when displaying the integer
                    (if not specified, the base given in the first assignment is used).
                    Parameters with this attribute may be assigned values containing arithmetic
                    expressions.

            -L[n]   Left justify attribute.  n specifies the field width.  If n is not specified,
                    the current width of a parameter (or the width of its first assigned value)
                    is used.  Leading whitespace (and zeros, if used with the -Z option) is
                    stripped.  If necessary, values are either truncated or space padded to fit
                    the field width.

            -l      Lower case attribute.  All upper case characters in values are converted to
                    lower case.  (In the original Korn shell, this parameter meant “long integer”
                    when used with the -i option.)

            -n      Create a bound variable (name reference): any access to the variable name
                    will access the variable value in the current scope (this is different from
                    AT&T UNIX ksh93!)  instead.  Also different from AT&T UNIX ksh93 is that
                    value is lazily evaluated at the time name is accessed.  This can be used by
                    functions to access variables whose names are passed as parametres, instead
                    of using eval.

            -p      Print complete typeset commands that can be used to re-create the attributes
                    and values of parameters.

            -R[n]   Right justify attribute.  n specifies the field width.  If n is not
                    specified, the current width of a parameter (or the width of its first
                    assigned value) is used.  Trailing whitespace is stripped.  If necessary,
                    values are either stripped of leading characters or space padded to make them
                    fit the field width.

            -r      Read-only attribute.  Parameters with this attribute may not be assigned to
                    or unset.  Once this attribute is set, it cannot be turned off.

            -t      Tag attribute.  Has no meaning to the shell; provided for application use.

                    For functions, -t is the trace attribute.  When functions with the trace
                    attribute are executed, the xtrace (-x) shell option is temporarily turned
                    on.

            -U      Unsigned integer attribute.  Integers are printed as unsigned values (combine
                    with the -i option).  This option is not in the original Korn shell.

            -u      Upper case attribute.  All lower case characters in values are converted to
                    upper case.  (In the original Korn shell, this parameter meant “unsigned
                    integer” when used with the -i option which meant upper case letters would
                    never be used for bases greater than 10.  See the -U option.)

                    For functions, -u is the undefined attribute.  See Functions above for the
                    implications of this.

            -x      Export attribute.  Parameters (or functions) are placed in the environment of
                    any executed commands.  Exported functions are not yet implemented.

            -Z[n]   Zero fill attribute.  If not combined with -L, this is the same as -R, except
                    zero padding is used instead of space padding.  For integers, the number
                    instead of the base is padded.

            If any of the -i, -L, -l, -R, -U, -u, or -Z options are changed, all others from this
            set are cleared, unless they are also given on the same command line.

     ulimit [-aBCcdefHiLlMmnOPpqrSsTtVvw] [value]
            Display or set process limits.  If no options are used, the file size limit (-f) is
            assumed.  value, if specified, may be either an arithmetic expression or the word
            “unlimited”.  The limits affect the shell and any processes created by the shell
            after a limit is imposed.  Note that some systems may not allow limits to be
            increased once they are set.  Also note that the types of limits available are system
            dependent – some systems have only the -f limit.

            -a     Display all limits; unless -H is used, soft limits are displayed.

            -B n   Set the socket buffer size to n kibibytes.

            -C n   Set the number of cached threads to n.

            -c n   Impose a size limit of n blocks on the size of core dumps.

            -d n   Impose a size limit of n kibibytes on the size of the data area.

            -e n   Set the maximum niceness to n.

            -f n   Impose a size limit of n blocks on files written by the shell and its child
                   processes (files of any size may be read).

            -H     Set the hard limit only (the default is to set both hard and soft limits).

            -i n   Set the number of pending signals to n.

            -L n   Control flocks; documentation is missing.

            -l n   Impose a limit of n kibibytes on the amount of locked (wired) physical memory.

            -M n   Set the AIO locked memory to n kibibytes.

            -m n   Impose a limit of n kibibytes on the amount of physical memory used.

            -n n   Impose a limit of n file descriptors that can be open at once.

            -O n   Set the number of AIO operations to n.

            -P n   Limit the number of threads per process to n.

            -p n   Impose a limit of n processes that can be run by the user at any one time.

            -q n   Limit the size of POSIX message queues to n bytes.

            -r n   Set the maximum real-time priority to n.

            -S     Set the soft limit only (the default is to set both hard and soft limits).

            -s n   Impose a size limit of n kibibytes on the size of the stack area.

            -T n   Impose a time limit of n real seconds to be used by each process.

            -t n   Impose a time limit of n CPU seconds spent in user mode to be used by each
                   process.

            -V n   Set the number of vnode monitors on Haiku to n.

            -v n   Impose a limit of n kibibytes on the amount of virtual memory (address space)
                   used.

            -w n   Impose a limit of n kibibytes on the amount of swap space used.

            As far as ulimit is concerned, a block is 512 bytes.

     umask [-S] [mask]
            Display or set the file permission creation mask, or umask (see umask(2)).  If the -S
            option is used, the mask displayed or set is symbolic; otherwise, it is an octal
            number.

            Symbolic masks are like those used by chmod(1).  When used, they describe what
            permissions may be made available (as opposed to octal masks in which a set bit means
            the corresponding bit is to be cleared).  For example, “ug=rwx,o=” sets the mask so
            files will not be readable, writable, or executable by “others”, and is equivalent
            (on most systems) to the octal mask “007”.

     unalias [-adt] [name ...]
            The aliases for the given names are removed.  If the -a option is used, all aliases
            are removed.  If the -t or -d options are used, the indicated operations are carried
            out on tracked or directory aliases, respectively.

     unset [-fv] parameter ...
            Unset the named parameters (-v, the default) or functions (-f).  With parameter[*],
            attributes are kept, only values are unset.

            The exit status is non-zero if any of the parameters have the read-only attribute
            set, zero otherwise.

     wait [job ...]
            Wait for the specified job(s) to finish.  The exit status of wait is that of the last
            specified job; if the last job is killed by a signal, the exit status is 128 + the
            number of the signal (see kill -l exit-status above); if the last specified job can't
            be found (because it never existed, or had already finished), the exit status of wait
            is 127.  See Job control below for the format of job.  wait will return if a signal
            for which a trap has been set is received, or if a SIGHUP, SIGINT, or SIGQUIT signal
            is received.

            If no jobs are specified, wait waits for all currently running jobs (if any) to
            finish and exits with a zero status.  If job monitoring is enabled, the completion
            status of jobs is printed (this is not the case when jobs are explicitly specified).

     whence [-pv] [name ...]
            For each name, the type of command is listed (reserved word, built-in, alias,
            function, tracked alias, or executable).  If the -p option is used, a path search is
            performed even if name is a reserved word, alias, etc.  Without the -v option, whence
            is similar to command -v except that whence will find reserved words and won't print
            aliases as alias commands.  With the -v option, whence is the same as command -V.
            Note that for whence, the -p option does not affect the search path used, as it does
            for command.  If the type of one or more of the names could not be determined, the
            exit status is non-zero.

   Job control
     Job control refers to the shell's ability to monitor and control jobs which are processes or
     groups of processes created for commands or pipelines.  At a minimum, the shell keeps track
     of the status of the background (i.e. asynchronous) jobs that currently exist; this
     information can be displayed using the jobs commands.  If job control is fully enabled
     (using set -m or set -o monitor), as it is for interactive shells, the processes of a job
     are placed in their own process group.  Foreground jobs can be stopped by typing the suspend
     character from the terminal (normally ^Z), jobs can be restarted in either the foreground or
     background using the fg and bg commands, and the state of the terminal is saved or restored
     when a foreground job is stopped or restarted, respectively.

     Note that only commands that create processes (e.g. asynchronous commands, subshell
     commands, and non-built-in, non-function commands) can be stopped; commands like read cannot
     be.

     When a job is created, it is assigned a job number.  For interactive shells, this number is
     printed inside “[..]”, followed by the process IDs of the processes in the job when an
     asynchronous command is run.  A job may be referred to in the bg, fg, jobs, kill, and wait
     commands either by the process ID of the last process in the command pipeline (as stored in
     the $! parameter) or by prefixing the job number with a percent sign (‘%’).  Other percent
     sequences can also be used to refer to jobs:

     %+ | %% | %    The most recently stopped job, or, if there are no stopped jobs, the oldest
                    running job.

     %-             The job that would be the %+ job if the latter did not exist.

     %n             The job with job number n.

     %?string       The job with its command containing the string string (an error occurs if
                    multiple jobs are matched).

     %string        The job with its command starting with the string string (an error occurs if
                    multiple jobs are matched).

     When a job changes state (e.g. a background job finishes or foreground job is stopped), the
     shell prints the following status information:

           [number] flag status command

     where...

     number   is the job number of the job;

     flag     is the ‘+’ or ‘-’ character if the job is the %+ or %- job, respectively, or space
              if it is neither;

     status   indicates the current state of the job and can be:

              Done [number]
                         The job exited.  number is the exit status of the job which is omitted
                         if the status is zero.

              Running    The job has neither stopped nor exited (note that running does not
                         necessarily mean consuming CPU time – the process could be blocked
                         waiting for some event).

              Stopped [signal]
                         The job was stopped by the indicated signal (if no signal is given, the
                         job was stopped by SIGTSTP).

              signal-description [“core dumped”]
                         The job was killed by a signal (e.g. memory fault, hangup); use kill -l
                         for a list of signal descriptions.  The “core dumped” message indicates
                         the process created a core file.

     command  is the command that created the process.  If there are multiple processes in the
              job, each process will have a line showing its command and possibly its status, if
              it is different from the status of the previous process.

     When an attempt is made to exit the shell while there are jobs in the stopped state, the
     shell warns the user that there are stopped jobs and does not exit.  If another attempt is
     immediately made to exit the shell, the stopped jobs are sent a SIGHUP signal and the shell
     exits.  Similarly, if the nohup option is not set and there are running jobs when an attempt
     is made to exit a login shell, the shell warns the user and does not exit.  If another
     attempt is immediately made to exit the shell, the running jobs are sent a SIGHUP signal and
     the shell exits.

   Interactive input line editing
     The shell supports three modes of reading command lines from a tty(4) in an interactive
     session, controlled by the emacs, gmacs, and vi options (at most one of these can be set at
     once).  The default is emacs.  Editing modes can be set explicitly using the set built-in.
     If none of these options are enabled, the shell simply reads lines using the normal tty(4)
     driver.  If the emacs or gmacs option is set, the shell allows emacs-like editing of the
     command; similarly, if the vi option is set, the shell allows vi-like editing of the
     command.  These modes are described in detail in the following sections.

     In these editing modes, if a line is longer than the screen width (see the COLUMNS
     parameter), a ‘>’, ‘+’, or ‘<’ character is displayed in the last column indicating that
     there are more characters after, before and after, or before the current position,
     respectively.  The line is scrolled horizontally as necessary.

     Completed lines are pushed into the history, unless they begin with an IFS octet or IFS
     white space, or are the same as the previous line.

   Emacs editing mode
     When the emacs option is set, interactive input line editing is enabled.  Warning: This mode
     is slightly different from the emacs mode in the original Korn shell.  In this mode, various
     editing commands (typically bound to one or more control characters) cause immediate actions
     without waiting for a newline.  Several editing commands are bound to particular control
     characters when the shell is invoked; these bindings can be changed using the bind command.

     The following is a list of available editing commands.  Each description starts with the
     name of the command, suffixed with a colon; an [n] (if the command can be prefixed with a
     count); and any keys the command is bound to by default, written using caret notation e.g.
     the ASCII ESC character is written as ^[.  These control sequences are not case sensitive.
     A count prefix for a command is entered using the sequence ^[n, where n is a sequence of 1
     or more digits.  Unless otherwise specified, if a count is omitted, it defaults to 1.

     Note that editing command names are used only with the bind command.  Furthermore, many
     editing commands are useful only on terminals with a visible cursor.  The default bindings
     were chosen to resemble corresponding Emacs key bindings.  The user's tty(4) characters
     (e.g. ERASE) are bound to reasonable substitutes and override the default bindings.

     abort: ^C, ^G
             Abort the current command, empty the line buffer and set the exit state to
             interrupted.

     auto-insert: [n]
             Simply causes the character to appear as literal input.  Most ordinary characters
             are bound to this.

     backward-char: [n] ^B, ^XD, ANSI-CurLeft
             Moves the cursor backward n characters.

     backward-word: [n] ^[b, ANSI-Ctrl-CurLeft, ANSI-Alt-CurLeft
             Moves the cursor backward to the beginning of the word; words consist of
             alphanumerics, underscore (‘_’), and dollar sign (‘$’) characters.

     beginning-of-history: ^[<
             Moves to the beginning of the history.

     beginning-of-line: ^A, ANSI-Home
             Moves the cursor to the beginning of the edited input line.

     capitalise-word: [n] ^[C, ^[c
             Uppercase the first character in the next n words, leaving the cursor past the end
             of the last word.

     clear-screen: ^[^L
             Prints a compile-time configurable sequence to clear the screen and home the cursor,
             redraws the entire prompt and the currently edited input line.  The default sequence
             works for almost all standard terminals.

     comment: ^[#
             If the current line does not begin with a comment character, one is added at the
             beginning of the line and the line is entered (as if return had been pressed);
             otherwise, the existing comment characters are removed and the cursor is placed at
             the beginning of the line.

     complete: ^[^[
             Automatically completes as much as is unique of the command name or the file name
             containing the cursor.  If the entire remaining command or file name is unique, a
             space is printed after its completion, unless it is a directory name in which case
             ‘/’ is appended.  If there is no command or file name with the current partial word
             as its prefix, a bell character is output (usually causing a beep to be sounded).

     complete-command: ^X^[
             Automatically completes as much as is unique of the command name having the partial
             word up to the cursor as its prefix, as in the complete command above.

     complete-file: ^[^X
             Automatically completes as much as is unique of the file name having the partial
             word up to the cursor as its prefix, as in the complete command described above.

     complete-list: ^I, ^[=
             Complete as much as is possible of the current word, and list the possible
             completions for it.  If only one completion is possible, match as in the complete
             command above.  Note that ^I is usually generated by the TAB (tabulator) key.

     delete-char-backward: [n] ERASE, ^?, ^H
             Deletes n characters before the cursor.

     delete-char-forward: [n] ANSI-Del
             Deletes n characters after the cursor.

     delete-word-backward: [n] WERASE, ^[^?, ^[^H, ^[h
             Deletes n words before the cursor.

     delete-word-forward: [n] ^[d
             Deletes characters after the cursor up to the end of n words.

     down-history: [n] ^N, ^XB, ANSI-CurDown
             Scrolls the history buffer forward n lines (later).  Each input line originally
             starts just after the last entry in the history buffer, so down-history is not
             useful until either search-history, search-history-up or up-history has been
             performed.

     downcase-word: [n] ^[L, ^[l
             Lowercases the next n words.

     edit-line: [n] ^Xe
             Edit line n or the current line, if not specified, interactively.  The actual
             command executed is fc -e ${VISUAL:-${EDITOR:-vi}} n.

     end-of-history: ^[>
             Moves to the end of the history.

     end-of-line: ^E, ANSI-End
             Moves the cursor to the end of the input line.

     eot: ^_
             Acts as an end-of-file; this is useful because edit-mode input disables normal
             terminal input canonicalization.

     eot-or-delete: [n] ^D
             Acts as eot if alone on a line; otherwise acts as delete-char-forward.

     error: (not bound)
             Error (ring the bell).

     exchange-point-and-mark: ^X^X
             Places the cursor where the mark is and sets the mark to where the cursor was.

     expand-file: ^[*
             Appends a ‘*’ to the current word and replaces the word with the result of
             performing file globbing on the word.  If no files match the pattern, the bell is
             rung.

     forward-char: [n] ^F, ^XC, ANSI-CurRight
             Moves the cursor forward n characters.

     forward-word: [n] ^[f, ANSI-Ctrl-CurRight, ANSI-Alt-CurRight
             Moves the cursor forward to the end of the nth word.

     goto-history: [n] ^[g
             Goes to history number n.

     kill-line: KILL
             Deletes the entire input line.

     kill-region: ^W
             Deletes the input between the cursor and the mark.

     kill-to-eol: [n] ^K
             Deletes the input from the cursor to the end of the line if n is not specified;
             otherwise deletes characters between the cursor and column n.

     list: ^[?
             Prints a sorted, columnated list of command names or file names (if any) that can
             complete the partial word containing the cursor.  Directory names have ‘/’ appended
             to them.

     list-command: ^X?
             Prints a sorted, columnated list of command names (if any) that can complete the
             partial word containing the cursor.

     list-file: ^X^Y
             Prints a sorted, columnated list of file names (if any) that can complete the
             partial word containing the cursor.  File type indicators are appended as described
             under list above.

     newline: ^J, ^M
             Causes the current input line to be processed by the shell.  The current cursor
             position may be anywhere on the line.

     newline-and-next: ^O
             Causes the current input line to be processed by the shell, and the next line from
             history becomes the current line.  This is only useful after an up-history,
             search-history or search-history-up.

     no-op: QUIT
             This does nothing.

     prefix-1: ^[
             Introduces a 2-character command sequence.

     prefix-2: ^X, ^[[, ^[O
             Introduces a 2-character command sequence.

     prev-hist-word: [n] ^[., ^[_
             The last word, or, if given, the nth word (zero-based) of the previous (on repeated
             execution, second-last, third-last, etc.) command is inserted at the cursor.  Use of
             this editing command trashes the mark.

     quote: ^^, ^V
             The following character is taken literally rather than as an editing command.

     redraw: ^L
             Reprints the last line of the prompt string and the current input line on a new
             line.

     search-character-backward: [n] ^[^]
             Search backward in the current line for the nth occurrence of the next character
             typed.

     search-character-forward: [n] ^]
             Search forward in the current line for the nth occurrence of the next character
             typed.

     search-history: ^R
             Enter incremental search mode.  The internal history list is searched backwards for
             commands matching the input.  An initial ‘^’ in the search string anchors the
             search.  The escape key will leave search mode.  Other commands, including sequences
             of escape as prefix-1 followed by a prefix-1 or prefix-2 key will be executed after
             leaving search mode.  The abort (^G) command will restore the input line before
             search started.  Successive search-history commands continue searching backward to
             the next previous occurrence of the pattern.  The history buffer retains only a
             finite number of lines; the oldest are discarded as necessary.

     search-history-up: ANSI-PgUp
             Search backwards through the history buffer for commands whose beginning match the
             portion of the input line before the cursor.  When used on an empty line, this has
             the same effect as up-history.

     search-history-down: ANSI-PgDn
             Search forwards through the history buffer for commands whose beginning match the
             portion of the input line before the cursor.  When used on an empty line, this has
             the same effect as down-history.  This is only useful after an up-history,
             search-history or search-history-up.

     set-mark-command: ^[<space>
             Set the mark at the cursor position.

     transpose-chars: ^T
             If at the end of line, or if the gmacs option is set, this exchanges the two
             previous characters; otherwise, it exchanges the previous and current characters and
             moves the cursor one character to the right.

     up-history: [n] ^P, ^XA, ANSI-CurUp
             Scrolls the history buffer backward n lines (earlier).

     upcase-word: [n] ^[U, ^[u
             Uppercase the next n words.

     version: ^[^V
             Display the version of mksh.  The current edit buffer is restored as soon as a key
             is pressed.  The restoring keypress is processed, unless it is a space.

     yank: ^Y
             Inserts the most recently killed text string at the current cursor position.

     yank-pop: ^[y
             Immediately after a yank, replaces the inserted text string with the next previously
             killed text string.

   Vi editing mode
     Note: The vi command-line editing mode is orphaned, yet still functional.

     The vi command-line editor in mksh has basically the same commands as the vi(1) editor with
     the following exceptions:

     ·   You start out in insert mode.

     ·   There are file name and command completion commands: =, \, *, ^X, ^E, ^F, and,
         optionally, <tab> and <esc>.

     ·   The _ command is different (in mksh, it is the last argument command; in vi(1) it goes
         to the start of the current line).

     ·   The / and G commands move in the opposite direction to the j command.

     ·   Commands which don't make sense in a single line editor are not available (e.g. screen
         movement commands and ex(1)-style colon (:) commands).

     Like vi(1), there are two modes: “insert” mode and “command” mode.  In insert mode, most
     characters are simply put in the buffer at the current cursor position as they are typed;
     however, some characters are treated specially.  In particular, the following characters are
     taken from current tty(4) settings (see stty(1)) and have their usual meaning (normal values
     are in parentheses): kill (^U), erase (^?), werase (^W), eof (^D), intr (^C), and quit (^\).
     In addition to the above, the following characters are also treated specially in insert
     mode:

     ^E       Command and file name enumeration (see below).

     ^F       Command and file name completion (see below).  If used twice in a row, the list of
              possible completions is displayed; if used a third time, the completion is undone.

     ^H       Erases previous character.

     ^J | ^M  End of line.  The current line is read, parsed, and executed by the shell.

     ^V       Literal next.  The next character typed is not treated specially (can be used to
              insert the characters being described here).

     ^X       Command and file name expansion (see below).

     <esc>    Puts the editor in command mode (see below).

     <tab>    Optional file name and command completion (see ^F above), enabled with set -o
              vi-tabcomplete.

     In command mode, each character is interpreted as a command.  Characters that don't
     correspond to commands, are illegal combinations of commands, or are commands that can't be
     carried out, all cause beeps.  In the following command descriptions, an [n] indicates the
     command may be prefixed by a number (e.g. 10l moves right 10 characters); if no number
     prefix is used, n is assumed to be 1 unless otherwise specified.  The term “current
     position” refers to the position between the cursor and the character preceding the cursor.
     A “word” is a sequence of letters, digits, and underscore characters or a sequence of non-
     letter, non-digit, non-underscore, and non-whitespace characters (e.g. “ab2*&^” contains two
     words) and a “big-word” is a sequence of non-whitespace characters.

     Special mksh vi commands:

     The following commands are not in, or are different from, the normal vi file editor:

     [n]_        Insert a space followed by the nth big-word from the last command in the history
                 at the current position and enter insert mode; if n is not specified, the last
                 word is inserted.

     #           Insert the comment character (‘#’) at the start of the current line and return
                 the line to the shell (equivalent to I#^J).

     [n]g        Like G, except if n is not specified, it goes to the most recent remembered
                 line.

     [n]v        Edit line n using the vi(1) editor; if n is not specified, the current line is
                 edited.  The actual command executed is fc -e ${VISUAL:-${EDITOR:-vi}} n.

     * and ^X    Command or file name expansion is applied to the current big-word (with an
                 appended ‘*’ if the word contains no file globbing characters) – the big-word is
                 replaced with the resulting words.  If the current big-word is the first on the
                 line or follows one of the characters ‘;’, ‘|’, ‘&’, ‘(’, or ‘)’, and does not
                 contain a slash (‘/’), then command expansion is done; otherwise file name
                 expansion is done.  Command expansion will match the big-word against all
                 aliases, functions, and built-in commands as well as any executable files found
                 by searching the directories in the PATH parameter.  File name expansion matches
                 the big-word against the files in the current directory.  After expansion, the
                 cursor is placed just past the last word and the editor is in insert mode.

     [n]\, [n]^F, [n]<tab>, and [n]<esc>
                 Command/file name completion.  Replace the current big-word with the longest
                 unique match obtained after performing command and file name expansion.  <tab>
                 is only recognised if the vi-tabcomplete option is set, while <esc> is only
                 recognised if the vi-esccomplete option is set (see set -o).  If n is specified,
                 the nth possible completion is selected (as reported by the command/file name
                 enumeration command).

     = and ^E    Command/file name enumeration.  List all the commands or files that match the
                 current big-word.

     ^V          Display the version of mksh.  The current edit buffer is restored as soon as a
                 key is pressed.  The restoring keypress is ignored.

     @c          Macro expansion.  Execute the commands found in the alias c.

     Intra-line movement commands:

     [n]h and [n]^H
             Move left n characters.

     [n]l and [n]<space>
             Move right n characters.

     0       Move to column 0.

     ^       Move to the first non-whitespace character.

     [n]|    Move to column n.

     $       Move to the last character.

     [n]b    Move back n words.

     [n]B    Move back n big-words.

     [n]e    Move forward to the end of the word, n times.

     [n]E    Move forward to the end of the big-word, n times.

     [n]w    Move forward n words.

     [n]W    Move forward n big-words.

     %       Find match.  The editor looks forward for the nearest parenthesis, bracket, or brace
             and then moves the cursor to the matching parenthesis, bracket, or brace.

     [n]fc   Move forward to the nth occurrence of the character c.

     [n]Fc   Move backward to the nth occurrence of the character c.

     [n]tc   Move forward to just before the nth occurrence of the character c.

     [n]Tc   Move backward to just before the nth occurrence of the character c.

     [n];    Repeats the last f, F, t, or T command.

     [n],    Repeats the last f, F, t, or T command, but moves in the opposite direction.

     Inter-line movement commands:

     [n]j, [n]+, and [n]^N
             Move to the nth next line in the history.

     [n]k, [n]-, and [n]^P
             Move to the nth previous line in the history.

     [n]G    Move to line n in the history; if n is not specified, the number of the first
             remembered line is used.

     [n]g    Like G, except if n is not specified, it goes to the most recent remembered line.

     [n]/string
             Search backward through the history for the nth line containing string; if string
             starts with ‘^’, the remainder of the string must appear at the start of the history
             line for it to match.

     [n]?string
             Same as /, except it searches forward through the history.

     [n]n    Search for the nth occurrence of the last search string; the direction of the search
             is the same as the last search.

     [n]N    Search for the nth occurrence of the last search string; the direction of the search
             is the opposite of the last search.

     ANSI-CurUp
             Take the characters from the beginning of the line to the current cursor position as
             search string and do a backwards history search for lines beginning with this
             string; keep the cursor position.  This works only in insert mode and keeps it
             enabled.

     Edit commands

     [n]a    Append text n times; goes into insert mode just after the current position.  The
             append is only replicated if command mode is re-entered i.e. <esc> is used.

     [n]A    Same as a, except it appends at the end of the line.

     [n]i    Insert text n times; goes into insert mode at the current position.  The insertion
             is only replicated if command mode is re-entered i.e. <esc> is used.

     [n]I    Same as i, except the insertion is done just before the first non-blank character.

     [n]s    Substitute the next n characters (i.e. delete the characters and go into insert
             mode).

     S       Substitute whole line.  All characters from the first non-blank character to the end
             of the line are deleted and insert mode is entered.

     [n]cmove-cmd
             Change from the current position to the position resulting from n move-cmds (i.e.
             delete the indicated region and go into insert mode); if move-cmd is c, the line
             starting from the first non-blank character is changed.

     C       Change from the current position to the end of the line (i.e. delete to the end of
             the line and go into insert mode).

     [n]x    Delete the next n characters.

     [n]X    Delete the previous n characters.

     D       Delete to the end of the line.

     [n]dmove-cmd
             Delete from the current position to the position resulting from n move-cmds;
             move-cmd is a movement command (see above) or d, in which case the current line is
             deleted.

     [n]rc   Replace the next n characters with the character c.

     [n]R    Replace.  Enter insert mode but overwrite existing characters instead of inserting
             before existing characters.  The replacement is repeated n times.

     [n]~    Change the case of the next n characters.

     [n]ymove-cmd
             Yank from the current position to the position resulting from n move-cmds into the
             yank buffer; if move-cmd is y, the whole line is yanked.

     Y       Yank from the current position to the end of the line.

     [n]p    Paste the contents of the yank buffer just after the current position, n times.

     [n]P    Same as p, except the buffer is pasted at the current position.

     Miscellaneous vi commands

     ^J and ^M
             The current line is read, parsed, and executed by the shell.

     ^L and ^R
             Redraw the current line.

     [n].    Redo the last edit command n times.

     u       Undo the last edit command.

     U       Undo all changes that have been made to the current line.

     intr and quit
             The interrupt and quit terminal characters cause the current line to be deleted and
             a new prompt to be printed.

FILES

     ~/.mkshrc          User mkshrc profile (non-privileged interactive shells); see Startup
                        files. The location can be changed at compile time (for embedded
                        systems); AOSP Android builds use /system/etc/mkshrc.
     ~/.profile         User profile (non-privileged login shells); see Startup files near the
                        top of this manual.
     /etc/profile       System profile (login shells); see Startup files.
     /etc/shells        Shell database.
     /etc/suid_profile  Suid profile (privileged shells); see Startup files.

     Note: On Android, /system/etc/ contains the system and suid profile.

SEE ALSO

     awk(1), cat(1), ed(1), getopt(1), sed(1), sh(1), stty(1), dup(2), execve(2), getgid(2),
     getuid(2), mknod(2), mkfifo(2), open(2), pipe(2), rename(2), wait(2), getopt(3),
     nl_langinfo(3), setlocale(3), signal(3), system(3), tty(4), shells(5), environ(7),
     script(7), utf-8(7), mknod(8)

     http://docsrv.sco.com:507/en/man/html.C/sh.C.html

     https://www.mirbsd.org/ksh-chan.htm

     Morris Bolsky, The KornShell Command and Programming Language, Prentice Hall PTR, xvi + 356
     pages, 1989, ISBN 978-0-13-516972-8 (0-13-516972-0).

     Morris I. Bolsky and David G. Korn, The New KornShell Command and Programming Language (2nd
     Edition), Prentice Hall PTR, xvi + 400 pages, 1995, ISBN 978-0-13-182700-4 (0-13-182700-6).

     Stephen G. Kochan and Patrick H. Wood, UNIX Shell Programming, Hayden, Revised Edition,
     xi + 490 pages, 1990, ISBN 978-0-672-48448-3 (0-672-48448-X).

     IEEE Inc., IEEE Standard for Information Technology  Portable Operating System Interface
     (POSIX), IEEE Press, Part 2: Shell and Utilities, xvii + 1195 pages, 1993, ISBN
     978-1-55937-255-8 (1-55937-255-9).

     Bill Rosenblatt, Learning the Korn Shell, O'Reilly, 360 pages, 1993, ISBN 978-1-56592-054-5
     (1-56592-054-6).

     Bill Rosenblatt and Arnold Robbins, Learning the Korn Shell, Second Edition, O'Reilly, 432
     pages, 2002, ISBN 978-0-596-00195-7 (0-596-00195-9).

     Barry Rosenberg, KornShell Programming Tutorial, Addison-Wesley Professional, xxi + 324
     pages, 1991, ISBN 978-0-201-56324-5 (0-201-56324-X).

AUTHORS

     The MirBSD Korn Shell is developed by Thorsten Glaser <tg@mirbsd.org> and currently
     maintained as part of The MirOS Project.  This shell is based upon the Public Domain Korn
     SHell.  The developer of mksh recognises the efforts of the pdksh authors, who had dedicated
     their work into Public Domain, our users, and all contributors, such as the Debian and
     OpenBSD projects.  See the documentation, CVS, and web site for details.

CAVEATS

     mksh only supports the Unicode BMP (Basic Multilingual Plane).

     mksh has a different scope model from AT&T UNIX ksh, which leads to subtile differences in
     semantics for identical builtins.  This can cause issues with a nameref to suddenly point to
     a local variable by accident; fixing this is hard.

     The parts of a pipeline, like below, are executed in subshells.  Thus, variable assignments
     inside them fail.  Use co-processes instead.

           foo | bar | read baz            # will not change $baz
           foo | bar |& read -p baz        # will, however, do so

     mksh provides a consistent set of 32-bit integer arithmetics, both signed and unsigned, with
     defined wraparound and sign of the result of a modulo operation, even (defying POSIX) on
     64-bit systems.  If you require 64-bit integer arithmetics, use lksh (legacy mksh) instead,
     but be aware that, in POSIX, it's legal for the OS to make print $((2147483647 + 1)) delete
     all files on your system, as it's Undefined Behaviour.

BUGS

     Suspending (using ^Z) pipelines like the one below will only suspend the currently running
     part of the pipeline; in this example, “fubar” is immediately printed on suspension (but not
     later after an fg).

           $ /bin/sleep 666 && echo fubar

     This document attempts to describe mksh R46 and up, compiled without any options impacting
     functionality, such as MKSH_SMALL, when not called as /bin/sh which, on some systems only,
     enables set -o sh automatically (whose behaviour differs across targets), for an operating
     environment supporting all of its advanced needs.  Please report bugs in mksh to the MirOS
     mailing list at <miros-mksh@mirbsd.org> or in the #!/bin/mksh (or #ksh) IRC channel at
     irc.freenode.net (Port 6697 SSL, 6667 unencrypted), or at: https://launchpad.net/mksh