Provided by: csh_20110502-3_amd64 bug

NAME

     csh — a shell (command interpreter) with C-like syntax

SYNOPSIS

     csh [-bcefimnstVvXx] [argument ...]
     csh [-l]

DESCRIPTION

     csh is a command language interpreter incorporating a history mechanism (see History
     substitutions), job control facilities (see Jobs), interactive file name and user name
     completion (see File name completion), and a C-like syntax.  It is used both as an
     interactive login shell and a shell script command processor.

   Argument list processing
     If the first argument (argument 0) to the shell is a dash (‘-’), then this is a login shell.
     A login shell also can be specified by invoking the shell with the -l flag as the only
     argument.

     The rest of the flag arguments are interpreted as follows:

     -b     This flag forces a “break” from option processing, causing any further shell
            arguments to be treated as non-option arguments.  The remaining arguments will not be
            interpreted as shell options.  This may be used to pass options to a shell script
            without confusion or possible subterfuge.  The shell will not run a set-user-ID
            script without this option.

     -c     Commands are read from the (single) following argument which must be present.  Any
            remaining arguments are placed in argv.

     -e     The shell exits if any invoked command terminates abnormally or yields a non-zero
            exit status.

     -f     The shell will start faster, because it will neither search for nor execute commands
            from the file .cshrc in the invoker's home directory.  Note: if the environment
            variable HOME is not set, fast startup is the default.

     -i     The shell is interactive and prompts for its top-level input, even if it appears not
            to be a terminal.  Shells are interactive without this option if their inputs and
            outputs are terminals.

     -l     The shell is a login shell (only applicable if -l is the only flag specified).

     -m     Read .cshrc, regardless of its owner and group.  This option is dangerous and should
            only be used by su(1).

     -n     Commands are parsed, but not executed.  This aids in syntactic checking of shell
            scripts.  When used interactively, the shell can be terminated by pressing control-D
            (end-of-file character), since exit will not work.

     -s     Command input is taken from the standard input.

     -t     A single line of input is read and executed.  A backslash (‘\’) may be used to escape
            the newline at the end of this line and continue onto another line.

     -V     Causes the verbose variable to be set even before .cshrc is executed.

     -v     Causes the verbose variable to be set, with the effect that command input is echoed
            after history substitution.

     -X     Causes the echo variable to be set even before .cshrc is executed.

     -x     Causes the echo variable to be set, so that commands are echoed immediately before
            execution.

     After processing of flag arguments, if arguments remain but none of the -c, -i, -s, or -t
     options were given, the first argument is taken as the name of a file of commands to be
     executed.  The shell opens this file, and saves its name for possible resubstitution by
     ‘$0’.  Since many systems use either the standard version 6 or version 7 shells whose shell
     scripts are not compatible with this shell, the shell will execute such a “standard” shell
     if the first character of a script is not a hash mark (‘#’); i.e., if the script does not
     start with a comment.  Remaining arguments initialize the variable argv.

     An instance of csh begins by executing commands from the file /etc/csh.cshrc and, if this is
     a login shell, /etc/csh.login.  It then executes commands from .cshrc in the home directory
     of the invoker, and, if this is a login shell, the file .login in the same location.  It is
     typical for users on CRTs to put the command stty crt in their .login file, and to also
     invoke tset(1) there.

     In the normal case, the shell will begin reading commands from the terminal, prompting with
     ‘% .’ Processing of arguments and the use of the shell to process files containing command
     scripts will be described later.

     The shell repeatedly performs the following actions: a line of command input is read and
     broken into “words”.  This sequence of words is placed on the command history list and
     parsed.  Finally each command in the current line is executed.

     When a login shell terminates it executes commands from the files .logout in the user's home
     directory and /etc/csh.logout.

   Lexical structure
     The shell splits input lines into words at blanks and tabs with the following exceptions.
     The characters ‘&’, ‘|’, ‘;’, ‘<’, ‘>’, ‘(’, and ‘)’ form separate words.  If doubled in
     ‘&&’, ‘||’, ‘<<’, or ‘>>’, these pairs form single words.  These parser metacharacters may
     be made part of other words, or have their special meaning prevented, by preceding them with
     a backslash (‘\’).  A newline preceded by a ‘\’ is equivalent to a blank.

     Strings enclosed in matched pairs of quotations, ‘'’, ‘`’, or ‘"’, form parts of a word;
     metacharacters in these strings, including blanks and tabs, do not form separate words.
     These quotations have semantics to be described later.  Within pairs of ‘'’ or ‘"’
     characters, a newline preceded by a ‘\’ gives a true newline character.

     When the shell's input is not a terminal, the character ‘#’ introduces a comment that
     continues to the end of the input line.  This special meaning is prevented when preceded by
     ‘\’ and in quotations using ‘`’, ‘'’, and ‘"’.

   Commands
     A simple command is a sequence of words, the first of which specifies the command to be
     executed.  A simple command or a sequence of simple commands separated by ‘|’ characters
     forms a pipeline.  The output of each command in a pipeline is connected to the input of the
     next.  Sequences of pipelines may be separated by ‘;’, and are then executed sequentially.
     A sequence of pipelines may be executed without immediately waiting for it to terminate by
     following it with a ‘&’.

     Any of the above may be placed in ‘(’ ‘)’ to form a simple command (that may be a component
     of a pipeline, for example).  It is also possible to separate pipelines with ‘||’ or ‘&&’
     showing, as in the C language, that the second is to be executed only if the first fails or
     succeeds, respectively.  (See Expressions.)

   Jobs
     The shell associates a job with each pipeline.  It keeps a table of current jobs, printed by
     the jobs command, and assigns them small integer numbers.  When a job is started
     asynchronously with ‘&’, the shell prints a line that looks like:

           [1] 1234

     showing that the job which was started asynchronously was job number 1 and had one (top-
     level) process, whose process ID was 1234.

     If you are running a job and wish to do something else you may hit ^Z (control-Z), which
     sends a SIGSTOP signal to the current job.  The shell will then normally show that the job
     has been “Stopped”, and print another prompt.  You can then manipulate the state of this
     job, putting it in the background with the bg command, or run some other commands and
     eventually bring the job back into the foreground with the fg command.  A ^Z takes effect
     immediately and is like an interrupt in that pending output and unread input are discarded
     when it is typed.  There is another special key ^Y that does not generate a SIGSTOP signal
     until a program attempts to read(2) it.  This request can usefully be typed ahead when you
     have prepared some commands for a job that you wish to stop after it has read them.

     A job being run in the background will stop if it tries to read from the terminal.
     Background jobs are normally allowed to produce output, but this can be disabled by giving
     the command stty tostop.  If you set this tty option, then background jobs will stop when
     they try to produce output like they do when they try to read input.

     There are several ways to refer to jobs in the shell.  The character ‘%’ introduces a job
     name.  If you wish to refer to job number 1, you can name it as ‘%1’.  Just naming a job
     brings it to the foreground; thus %1 is a synonym for fg %1, bringing job number 1 back into
     the foreground.  Similarly, saying %1 & resumes job number 1 in the background.  Jobs can
     also be named by prefixes of the string typed in to start them, if these prefixes are
     unambiguous; thus %ex would normally restart a suspended ex(1) job, if there were only one
     suspended job whose name began with the string "ex".  It is also possible to say %?string,
     which specifies a job whose text contains string, if there is only one such job.

     The shell maintains a notion of the current and previous jobs.  In output about jobs, the
     current job is marked with a ‘+’ and the previous job with a ‘-’.  The abbreviation ‘%+’
     refers to the current job and ‘%-’ refers to the previous job.  For close analogy with the
     syntax of the history mechanism (described below), ‘%%’ is also a synonym for the current
     job.

     The job control mechanism requires that the stty(1) option new be set.  It is an artifact
     from a new implementation of the tty driver that allows generation of interrupt characters
     from the keyboard to tell jobs to stop.  See stty(1) for details on setting options in the
     new tty driver.

   Status reporting
     The shell learns immediately whenever a process changes state.  It normally informs you
     whenever a job becomes blocked so that no further progress is possible, but only just before
     it prints a prompt.  This is done so that it does not otherwise disturb your work.  If,
     however, you set the shell variable notify, the shell will notify you immediately of changes
     of status in background jobs.  There is also a shell command notify that marks a single
     process so that its status changes will be immediately reported.  By default notify marks
     the current process; simply say notify after starting a background job to mark it.

     When you try to leave the shell while jobs are stopped, you will be warned that “You have
     stopped jobs”.  You may use the jobs command to see what they are.  If you try to exit again
     immediately, the shell will not warn you a second time, and the suspended jobs will be
     terminated.

   File name completion
     When the file name completion feature is enabled by setting the shell variable filec (see
     set), csh will interactively complete file names and user names from unique prefixes when
     they are input from the terminal followed by the escape character (the escape key, or
     control-[).  For example, if the current directory looks like

           DSC.OLD  bin      cmd      lib      xmpl.c
           DSC.NEW  chaosnet cmtest   mail     xmpl.o
           bench    class    dev      mbox     xmpl.out

     and the input is

           % vi ch<escape>

     csh will complete the prefix “ch” to the only matching file name “chaosnet”, changing the
     input line to

           % vi chaosnet

     However, given

           % vi D<escape>

     csh will only expand the input to

           % vi DSC.

     and will sound the terminal bell to indicate that the expansion is incomplete, since there
     are two file names matching the prefix ‘D’.

     If a partial file name is followed by the end-of-file character (usually control-D), then,
     instead of completing the name, csh will list all file names matching the prefix.  For
     example, the input

           % vi D<control-D>

     causes all files beginning with ‘D’ to be listed:

           DSC.NEW   DSC.OLD

     while the input line remains unchanged.

     The same system of escape and end-of-file can also be used to expand partial user names, if
     the word to be completed (or listed) begins with the tilde character (‘~’).  For example,
     typing

           cd ~ro<escape>

     may produce the expansion

           cd ~root

     The use of the terminal bell to signal errors or multiple matches can be inhibited by
     setting the variable nobeep.

     Normally, all files in the particular directory are candidates for name completion.  Files
     with certain suffixes can be excluded from consideration by setting the variable fignore to
     the list of suffixes to be ignored.  Thus, if fignore is set by the command

           % set fignore = (.o .out)

     then typing

           % vi x<escape>

     would result in the completion to

           % vi xmpl.c

     ignoring the files "xmpl.o" and "xmpl.out".  However, if the only completion possible
     requires not ignoring these suffixes, then they are not ignored.  In addition, fignore does
     not affect the listing of file names by control-D.  All files are listed regardless of their
     suffixes.

   Substitutions
     We now describe the various transformations the shell performs on the input in the order in
     which they occur.

   History substitutions
     History substitutions place words from previous command input as portions of new commands,
     making it easy to repeat commands, repeat arguments of a previous command in the current
     command, or fix spelling mistakes in the previous command with little typing and a high
     degree of confidence.  History substitutions begin with the character ‘!’ and may begin
     anywhere in the input stream (with the proviso that they do not nest).  This ‘!’ may be
     preceded by a ‘\’ to prevent its special meaning; for convenience, a ‘!’ character is passed
     unchanged when it is followed by a blank, tab, newline, ‘=’ or ‘(’.  (History substitutions
     also occur when an input line begins with ‘^’.  This special abbreviation will be described
     later.)  Any input line that contains history substitution is echoed on the terminal before
     it is executed as it would have been typed without history substitution.

     Commands input from the terminal that consist of one or more words are saved on the history
     list.  The history substitutions reintroduce sequences of words from these saved commands
     into the input stream.  The size of the history list is controlled by the history variable;
     the previous command is always retained, regardless of the value of the history variable.
     Commands are numbered sequentially from 1.

     For definiteness, consider the following output from the history command:

           09  write michael
           10  ex write.c
           11  cat oldwrite.c
           12  diff *write.c

     The commands are shown with their event numbers.  It is not usually necessary to use event
     numbers, but the current event number can be made part of the prompt by placing a ‘!’ in the
     prompt string.

     With the current event 13 we can refer to previous events by event number ‘!11’, relatively
     as in ‘!-2’ (referring to the same event), by a prefix of a command word as in ‘!d’ for
     event 12 or ‘!wri’ for event 9, or by a string contained in a word in the command as in
     ‘!?mic?’ also referring to event 9.  These forms, without further change, simply reintroduce
     the words of the specified events, each separated by a single blank.  As a special case,
     ‘!!’ refers to the previous command; thus ‘!!’ alone is a redo.

     To select words from an event we can follow the event specification by a ‘:’ and a
     designator for the desired words.  The words of an input line are numbered from 0, the first
     (usually command) word being 0, the second word (first argument) being 1, etc.  The basic
     word designators are:

           0       first (command) word
           n       n'th argument
           ^       first argument; i.e., ‘1’
           $       last argument
           %       word matched by (immediately preceding) ?s? search
           x-y     range of words
           -y      abbreviates ‘0-y’
           *       abbreviates ‘^-$’, or nothing if only 1 word in event
           x*      abbreviates ‘x-$’
           x-      like ‘x*’ but omitting word ‘$’

     The ‘:’ separating the event specification from the word designator can be omitted if the
     argument selector begins with a ‘^’, ‘$’, ‘*’, ‘-’, or ‘%’.  After the optional word
     designator, a sequence of modifiers can be placed, each preceded by a ‘:’.  The following
     modifiers are defined:

           h       Remove a trailing pathname component, leaving the head.
           r       Remove a trailing ‘.xxx’ component, leaving the root name.
           e       Remove all but the extension ‘.xxx’ part.
           s/l/r/  Substitute l for r.
           t       Remove all leading pathname components, leaving the tail.
           &       Repeat the previous substitution.
           g       Apply the change once on each word, prefixing the above; e.g., ‘g&’.
           a       Apply the change as many times as possible on a single word, prefixing the
                   above.  It can be used together with ‘g’ to apply a substitution globally.
           p       Print the new command line but do not execute it.
           q       Quote the substituted words, preventing further substitutions.
           x       Like ‘q’, but break into words at blanks, tabs, and newlines.

     Unless preceded by a ‘g’ the change is applied only to the first modifiable word.  With
     substitutions, it is an error for no word to be applicable.

     The left-hand side of substitutions are not regular expressions in the sense of the editors,
     but instead strings.  Any character may be used as the delimiter in place of ‘/’; a ‘\’
     quotes the delimiter into the l   and r   strings.  The character ‘&’ in the right-hand side
     is replaced by the text from the left.  A ‘\’ also quotes ‘&’.  A NULL l (‘//’) uses the
     previous string either from an l or from a contextual scan string s in ‘!?s\?’.  The
     trailing delimiter in the substitution may be omitted if a newline follows immediately as
     may the trailing ‘?’ in a contextual scan.

     A history reference may be given without an event specification; e.g., ‘!$’.  Here, the
     reference is to the previous command unless a previous history reference occurred on the
     same line in which case this form repeats the previous reference.  Thus “!?foo?^ !$” gives
     the first and last arguments from the command matching “?foo?”.

     A special abbreviation of a history reference occurs when the first non-blank character of
     an input line is a ‘^’.  This is equivalent to “!:s^” providing a convenient shorthand for
     substitutions on the text of the previous line.  Thus ^lb^lib fixes the spelling of “lib” in
     the previous command.  Finally, a history substitution may be surrounded with ‘{’ and ‘}’ if
     necessary to insulate it from the characters that follow.  Thus, after ls -ld ~paul we might
     do !{l}a to do ls -ld ~paula, while !la would look for a command starting with “la”.

   Quotations with ´ and "
     The quotation of strings by ‘'’ and ‘"’ can be used to prevent all or some of the remaining
     substitutions.  Strings enclosed in ‘'’ are prevented from any further interpretation.
     Strings enclosed in ‘"’ may be expanded as described below.

     In both cases the resulting text becomes (all or part of) a single word; only in one special
     case (see Command Substitution below) does a ‘"’ quoted string yield parts of more than one
     word; ‘'’ quoted strings never do.

   Alias substitution
     The shell maintains a list of aliases that can be established, displayed and modified by the
     alias and unalias commands.  After a command line is scanned, it is parsed into distinct
     commands and the first word of each command, left-to-right, is checked to see if it has an
     alias.  If it does, then the text that is the alias for that command is reread with the
     history mechanism available as though that command were the previous input line.  The
     resulting words replace the command and argument list.  If no reference is made to the
     history list, then the argument list is left unchanged.

     Thus if the alias for “ls” is “ls -l”, the command ls /usr would map to ls -l /usr, the
     argument list here being undisturbed.  Similarly, if the alias for “lookup” was “grep !^
     /etc/passwd” then lookup bill would map to grep bill /etc/passwd.

     If an alias is found, the word transformation of the input text is performed and the
     aliasing process begins again on the reformed input line.  Looping is prevented if the first
     word of the new text is the same as the old by flagging it to prevent further aliasing.
     Other loops are detected and cause an error.

     Note that the mechanism allows aliases to introduce parser metasyntax.  Thus, we can alias
     print 'pr \!* | lpr' to make a command that pr's its arguments to the line printer.

   Variable substitution
     The shell maintains a set of variables, each of which has as value a list of zero or more
     words.  Some of these variables are set by the shell or referred to by it.  For instance,
     the argv variable is an image of the shell's argument list, and words of this variable's
     value are referred to in special ways.

     The values of variables may be displayed and changed by using the set and unset commands.
     Of the variables referred to by the shell a number are toggles; the shell does not care what
     their value is, only whether they are set or not.  For instance, the verbose variable is a
     toggle that causes command input to be echoed.  The setting of this variable results from
     the -v command-line option.

     Other operations treat variables numerically.  The @ command permits numeric calculations to
     be performed and the result assigned to a variable.  Variable values are, however, always
     represented as (zero or more) strings.  For the purposes of numeric operations, the null
     string is considered to be zero, and the second and additional words of multiword values are
     ignored.

     After the input line is aliased and parsed, and before each command is executed, variable
     substitution is performed, keyed by ‘$’ characters.  This expansion can be prevented by
     preceding the ‘$’ with a ‘\’ except within double quotes (`"'), where it always occurs, and
     within single quotes (`''), where it never occurs.  Strings quoted by backticks (` `) are
     interpreted later (see Command substitution below), so ‘$’ substitution does not occur there
     until later, if at all.  A ‘$’ is passed unchanged if followed by a blank, tab, or end-of-
     line.

     Input/output redirections are recognized before variable expansion, and are variable
     expanded separately.  Otherwise, the command name and entire argument list are expanded
     together.  It is thus possible for the first (command) word (to this point) to generate more
     than one word, the first of which becomes the command name, and the rest of which become
     arguments.

     Unless enclosed in ‘"’ or given the ‘:q’ modifier, the results of variable substitution may
     eventually be command and filename substituted.  Within ‘"’, a variable whose value consists
     of multiple words expands to (a portion of) a single word, with the words of the variable's
     value separated by blanks.  When the ‘:q’ modifier is applied to a substitution the variable
     will expand to multiple words with each word separated by a blank and quoted to prevent
     later command or filename substitution.

     The following metasequences are provided for introducing variable values into the shell
     input.  Except as noted, it is an error to reference a variable that is not set.

           $name
           ${name}
                   Are replaced by the words of the value of variable name, each separated by a
                   blank.  Braces insulate name from following characters that would otherwise be
                   part of it.  Shell variables have names consisting of up to 20 letters and
                   digits starting with a letter.  The underscore character is considered a
                   letter.  If name is not a shell variable, but is set in the environment, then
                   that value is returned (but ‘:’ modifiers and the other forms given below are
                   not available here).
           $name[selector]
           ${name[selector]}
                   May be used to select only some of the words from the value of name.  The
                   selector is subjected to ‘$’ substitution and may consist of a single number
                   or two numbers separated by a ‘-’.  The first word of a variable's value is
                   numbered ‘1’.  If the first number of a range is omitted it defaults to ‘1’.
                   If the last number of a range is omitted it defaults to ‘$#name’.  The
                   selector ‘*’ selects all words.  It is not an error for a range to be empty if
                   the second argument is omitted or in range.
           $#name
           ${#name}
                   Gives the number of words in the variable.  This is useful for later use in a
                   “$argv[selector]”.
           $0      Substitutes the name of the file from which command input is being read.  An
                   error occurs if the name is not known.
           $number
           ${number}
                   Equivalent to “$argv[number]”.
           $*      Equivalent to “$argv[*]”.

     The modifiers ‘:e’, ‘:h’, ‘:t’, ‘:r’, ‘:q’, and ‘:x’ may be applied to the substitutions
     above as may ‘:gh’, ‘:gt’, and ‘:gr’.  If braces ‘{’ ‘}’ appear in the command form then the
     modifiers must appear within the braces.  The current implementation allows only one ‘:’
     modifier on each ‘$’ expansion.

     The following substitutions may not be modified with ‘:’ modifiers.
           $?name
           ${?name}
                   Substitutes the string “1” if name is set, “0” if it is not.
           $?0     Substitutes ‘1’ if the current input filename is known, ‘0’ if it is not.
           $$      Substitute the (decimal) process number of the (parent) shell.  Do NOT use
                   this mechanism for generating temporary file names; see mktemp(1) instead.
           $!      Substitute the (decimal) process number of the last background process started
                   by this shell.
           $<      Substitutes a line from the standard input, with no further interpretation.
                   It can be used to read from the keyboard in a shell script.

   Command and filename substitution
     The remaining substitutions, command and filename substitution, are applied selectively to
     the arguments of built-in commands.  By selectively, we mean that portions of expressions
     which are not evaluated are not subjected to these expansions.  For commands that are not
     internal to the shell, the command name is substituted separately from the argument list.
     This occurs very late, after input-output redirection is performed, and in a child of the
     main shell.

   Command substitution
     Command substitution is shown by a command enclosed in ‘`’.  The output from such a command
     is normally broken into separate words at blanks, tabs, and newlines, with null words being
     discarded; this text then replaces the original string.  Within double quotes (`"'), only
     newlines force new words; blanks and tabs are preserved.

     In any case, the single final newline does not force a new word.  Note that it is thus
     possible for a command substitution to yield only part of a word, even if the command
     outputs a complete line.

   Filename substitution
     If a word contains any of the characters ‘*’, ‘?’, ‘[’, or ‘{’, or begins with the character
     ‘~’, then that word is a candidate for filename substitution, also known as “globbing”.
     This word is then regarded as a pattern, and replaced with an alphabetically sorted list of
     file names that match the pattern.  In a list of words specifying filename substitution it
     is an error for no pattern to match an existing file name, but it is not required for each
     pattern to match.  Only the metacharacters ‘*’, ‘?’, and ‘[’ imply pattern matching, the
     characters ‘~’ and ‘{’ being more akin to abbreviations.

     In matching filenames, the character ‘.’ at the beginning of a filename or immediately
     following a ‘/’, as well as the character ‘/’ must be matched explicitly.  The character ‘*’
     matches any string of characters, including the null string.  The character ‘?’ matches any
     single character.

     The sequence “[...]” matches any one of the characters enclosed.  Within “[...]”, a pair of
     characters separated by ‘-’ matches any character lexically between the two (inclusive).
     Within “[...]”, the name of a character class enclosed in ‘[:’ and ‘:]’ stands for the list
     of all characters belonging to that class.  Supported character classes:

           alnum     cntrl     lower     space
           alpha     digit     print     upper
           blank     graph     punct     xdigit

     These match characters using the macros specified in ctype(3).  A character class may not be
     used as an endpoint of a range.

     The character ‘~’ at the beginning of a filename refers to home directories.  Standing
     alone, i.e., ‘~’, it expands to the invoker's home directory as reflected in the value of
     the variable home.  When followed by a name consisting of letters, digits, and ‘-’
     characters, the shell searches for a user with that name and substitutes their home
     directory; thus “~ken” might expand to “/usr/ken” and “~ken/chmach” to “/usr/ken/chmach”.
     If the character ‘~’ is followed by a character other than a letter or ‘/’, or does not
     appear at the beginning of a word, it is left undisturbed.

     The metanotation “a{b,c,d}e” is a shorthand for “abe ace ade”.  Left to right order is
     preserved, with results of matches being sorted separately at a low level to preserve this
     order.  This construct may be nested.  Thus, “~source/s1/{oldls,ls}.c” expands to
     “/usr/source/s1/oldls.c /usr/source/s1/ls.c” without chance of error if the home directory
     for “source” is “/usr/source”.  Similarly “../{memo,*box}” might expand to “../memo ../box
     ../mbox”.  (Note that “memo” was not sorted with the results of the match to “*box”.)  As a
     special case ‘{’, ‘}’, and ‘{}’ are passed undisturbed.

   Input/output
     The standard input and the standard output of a command may be redirected with the following
     syntax:

           < name  Open file name (which is first variable, command, and filename expanded) as
                   the standard input.
           << word
                   Read the shell input up to a line that is identical to word.  word is not
                   subjected to variable, command, or filename substitution, and each input line
                   is compared to word before any substitutions are done on the input line.
                   Unless a quoting ‘\’, ‘"’, ‘'’ or ‘`’ appears in word, variable and command
                   substitution is performed on the intervening lines, allowing ‘\’ to quote ‘$’,
                   ‘\’ and ‘`’.  Commands that are substituted have all blanks, tabs, and
                   newlines preserved, except for the final newline which is dropped.  The
                   resultant text is placed in an anonymous temporary file that is given to the
                   command as its standard input.
           > name
           >! name
           >& name
           >&! name
                   The file name is used as the standard output.  If the file does not exist then
                   it is created; if the file exists, it is truncated; its previous contents are
                   lost.

                   If the variable noclobber is set, then the file must not exist or be a
                   character special file (e.g., a terminal or /dev/null) or an error results.
                   This helps prevent accidental destruction of files.  Here, the ‘!’ forms can
                   be used to suppress this check.

                   The forms involving ‘&’ route the standard error output into the specified
                   file as well as the standard output.  name is expanded in the same way as ‘<’
                   input filenames are.
           >> name
           >>& name
           >>! name
           >>&! name
                   Uses file name as the standard output; like ‘>’ but places output at the end
                   of the file.  If the variable noclobber is set, then it is an error for the
                   file not to exist unless one of the ‘!’ forms is given.  Otherwise similar to
                   ‘>’.

     A command receives the environment in which the shell was invoked as modified by the input-
     output parameters and the presence of the command in a pipeline.  Thus, unlike some previous
     shells, commands run from a file of shell commands have no access to the text of the
     commands by default; instead they receive the original standard input of the shell.  The
     ‘<<’ mechanism should be used to present inline data.  This permits shell command scripts to
     function as components of pipelines and allows the shell to block read its input.  Note that
     the default standard input for a command run detached is not modified to be the empty file
     /dev/null; instead the standard input remains as the original standard input of the shell.
     If this is a terminal and if the process attempts to read from the terminal, then the
     process will block and the user will be notified (see Jobs above).

     The standard error output may be directed through a pipe with the standard output.  Simply
     use the form ‘|&’ instead of just ‘|’.

   Expressions
     Several of the built-in commands (to be described later) take expressions, in which the
     operators are similar to those of C, with the same precedence, but with the opposite
     grouping: right to left.  These expressions appear in the @, exit, if, and while commands.
     The following operators are available:

           ||  &&  | ↑  &  ==  !=  =~  !~  <=  >= <  > <<  >>  +  -  *  /  %  !  ~  (  )

     Here the precedence increases to the right, ‘==’ ‘!=’ ‘=~’ and ‘!~’, ‘<=’ ‘>=’ ‘<’ and ‘>’,
     ‘<<’ and ‘>>’, ‘+’ and ‘-’, ‘*’ ‘/’ and ‘%’ being, in groups, at the same level.  The ‘==’
     ‘!=’ ‘=~’ and ‘!~’ operators compare their arguments as strings; all others operate on
     numbers.  The operators ‘=~’ and ‘!~’ are like ‘!=’ and ‘==’ except that the right hand side
     is a pattern (containing, e.g., *'s, ?'s, and instances of “[...]”) against which the left-
     hand operand is matched.  This reduces the need for use of the switch statement in shell
     scripts when all that is really needed is pattern matching.

     Strings that begin with ‘0’ are considered octal numbers.  Null or missing arguments are
     considered ‘0’.  The results of all expressions are strings, which represent decimal
     numbers.  It is important to note that no two components of an expression can appear in the
     same word; except when adjacent to components of expressions that are syntactically
     significant to the parser (‘&’, ‘|’, ‘<’, ‘>’, ‘(’, and ‘)’), they should be surrounded by
     spaces.

     Also available in expressions as primitive operands are command executions enclosed in ‘{’
     and ‘}’ and file enquiries of the form -l name where l is one of:

           r       read access
           w       write access
           x       execute access
           e       existence
           o       ownership
           z       zero size
           f       plain file
           d       directory

     The specified name is command and filename expanded and then tested to see if it has the
     specified relationship to the real user.  If the file does not exist or is inaccessible then
     all enquiries return false, i.e., ‘0’.  Command executions succeed, returning true, i.e.,
     ‘1’, if the command exits with status 0, otherwise they fail, returning false, i.e., ‘0’.
     If more detailed status information is required then the command should be executed outside
     an expression and the variable status examined.

   Control flow
     The shell contains several commands that can be used to regulate the flow of control in
     command files (shell scripts) and (in limited but useful ways) from terminal input.  These
     commands all operate by forcing the shell to reread or skip in its input and, because of the
     implementation, restrict the placement of some of the commands.

     The foreach, switch, and while statements, as well as the if-then-else form of the if
     statement require that the major keywords appear in a single simple command on an input line
     as shown below.

     If the shell's input is not seekable, the shell buffers up input whenever a loop is being
     read and performs seeks in this internal buffer to accomplish the rereading implied by the
     loop.  (To the extent that this allows, backward goto's will succeed on non-seekable
     inputs.)

   Built-in commands
     Built-in commands are executed within the shell.  If a built-in command occurs as any
     component of a pipeline except the last then it is executed in a sub-shell.

           alias
           alias name
           alias name wordlist
                   The first form prints all aliases.  The second form prints the alias for name.
                   The final form assigns the specified wordlist as the alias of name; wordlist
                   is command and filename substituted.  name is not allowed to be “alias” or
                   “unalias”.

           alloc   Shows the amount of dynamic memory acquired, broken down into used and free
                   memory.  With an argument shows the number of free and used blocks in each
                   size category.  The categories start at size 8 and double at each step.  This
                   command's output may vary across system types, since systems other than the
                   VAX may use a different memory allocator.

           bg
           bg %job ...
                   Puts the current or specified jobs into the background, continuing them if
                   they were stopped.

           break   Causes execution to resume after the end of the nearest enclosing foreach or
                   while.  The remaining commands on the current line are executed.  Multi-level
                   breaks are thus possible by writing them all on one line.

           breaksw
                   Causes a break from a switch, resuming after the endsw.

           case label:
                   A label in a switch statement as discussed below.

           cd
           cd name
           chdir
           chdir name
                   Change the shell's working directory to directory name.  If no argument is
                   given then change to the home directory of the user.  If name is not found as
                   a subdirectory of the current directory (and does not begin with ‘/’, ‘./’ or
                   ‘../’), then each component of the variable cdpath is checked to see if it has
                   a subdirectory name.  Finally, if all else fails but name is a shell variable
                   whose value begins with ‘/’, then this is tried to see if it is a directory.

           continue
                   Continue execution of the nearest enclosing while or foreach.  The rest of the
                   commands on the current line are executed.

           default:
                   Labels the default case in a switch statement.  The default should come after
                   all case labels.

           dirs    Prints the directory stack; the top of the stack is at the left, the first
                   directory in the stack being the current directory.

           echo wordlist
           echo -n wordlist
                   The specified words are written to the shell's standard output, separated by
                   spaces, and terminated with a newline unless the -n option is specified.

           else
           end
           endif
           endsw   See the description of the foreach, if, switch, and while statements below.

           eval arg ...
                   (As in sh(1).)  The arguments are read as input to the shell and the resulting
                   command(s) executed in the context of the current shell.  This is usually used
                   to execute commands generated as the result of command or variable
                   substitution, since parsing occurs before these substitutions.  See tset(1)
                   for an example of using eval.

           exec command
                   The specified command is executed in place of the current shell.

           exit
           exit (expr)
                   The shell exits either with the value of the status variable (first form) or
                   with the value of the specified expr (second form).

           fg
           fg %job ...
                   Brings the current or specified jobs into the foreground, continuing them if
                   they were stopped.

           foreach name (wordlist)
           ...
           end     The variable name is successively set to each member of wordlist and the
                   sequence of commands between this command and the matching end are executed.
                   (Both foreach and end must appear alone on separate lines.)  The built-in
                   command continue may be used to continue the loop prematurely and the built-in
                   command break to terminate it prematurely.  When this command is read from the
                   terminal, the loop is read once prompting with ‘?’ before any statements in
                   the loop are executed.  If you make a mistake typing in a loop at the terminal
                   you can rub it out.

           glob wordlist
                   Like echo but no ‘\’ escapes are recognized and words are delimited by NUL
                   characters in the output.  Useful for programs that wish to use the shell to
                   filename expand a list of words.

           goto word
                   The specified word is filename and command expanded to yield a string of the
                   form ‘label’.  The shell rewinds its input as much as possible and searches
                   for a line of the form “label:”, possibly preceded by blanks or tabs.
                   Execution continues after the specified line.

           hashstat
                   Print a statistics line showing how effective the internal hash table has been
                   at locating commands (and avoiding exec´s).  An exec is attempted for each
                   component of the path where the hash function indicates a possible hit, and in
                   each component that does not begin with a ‘/’.

           history
           history n
           history -h n
           history -r n
                   Displays the history event list; if n is given, only the n most recent events
                   are printed.  The -h option causes the history list to be printed without
                   leading numbers.  This format produces files suitable for sourcing using the
                   -h option to source.  The -r option reverses the order of printout to be most
                   recent first instead of oldest first.

           if (expr) command
                   If the specified expression evaluates to true, then the single command with
                   arguments is executed.  Variable substitution on command happens early, at the
                   same time it does for the rest of the if command.  command must be a simple
                   command, not a pipeline, a command list, or a parenthesized command list.
                   Input/output redirection occurs even if expr is false, i.e., when command is
                   not executed (this is a bug).

           if (expr) then
           ...
           else if (expr2) then
           ...
           else
           ...
           endif   If the specified expr is true then the commands up to the first else are
                   executed; otherwise if expr2 is true then the commands up to the second else
                   are executed, etc.  Any number of else-if pairs are possible; only one endif
                   is needed.  The else part is likewise optional.  (The words else and endif
                   must appear at the beginning of input lines; the if must appear alone on its
                   input line or after an else.)

           jobs
           jobs -l
                   Lists the active jobs; the -l option lists process IDs in addition to the
                   normal information.

           kill %job
           kill    [-s signal_name] pid
           kill -sig pid ...
           kill -l [exit_status]
                   Sends either the SIGTERM (terminate) signal or the specified signal to the
                   specified jobs or processes.  Signals are either given by number or by names
                   (as given in ⟨signal.h⟩, stripped of the prefix “SIG”).  The signal names are
                   listed by “kill -l”; if an exit_status is specified, only the corresponding
                   signal name will be written.  There is no default; just saying “kill” does not
                   send a signal to the current job.  If the signal being sent is SIGTERM
                   (terminate) or SIGHUP (hangup), then the job or process will be sent a SIGCONT
                   (continue) signal as well.

           limit
           limit resource
           limit resource maximum-use
           limit -h
           limit -h resource
           limit -h resource maximum-use
                   Limits the consumption by the current process and each process it creates to
                   not individually exceed maximum-use on the specified resource.  If no
                   maximum-use is given, then the current limit is printed; if no resource is
                   given, then all limitations are given.  If the -h flag is given, the hard
                   limits are used instead of the current limits.  The hard limits impose a
                   ceiling on the values of the current limits.  Only the superuser may raise the
                   hard limits, but a user may lower or raise the current limits within the legal
                   range.

                   Resources controllable currently include:

                   cputime       the maximum number of CPU-seconds to be used by each process.

                   filesize      the largest single file (in bytes) that can be created.

                   datasize      the maximum growth of the data+stack region via sbrk(2) beyond
                                 the end of the program text.

                   stacksize     the maximum size of the automatically-extended stack region.

                   coredumpsize  the size of the largest core dump (in bytes) that will be
                                 created.

                   memoryuse     the maximum size (in bytes) to which a process's resident set
                                 size (RSS) may grow.

                   memorylocked  The maximum size (in bytes) which a process may lock into memory
                                 using the mlock(2) function.

                   maxproc       The maximum number of simultaneous processes for this user ID.

                   openfiles     The maximum number of simultaneous open files for this user ID.

                   vmemoryuse    the maximum size (in bytes) to which a process's total size may
                                 grow.

                   The maximum-use may be given as a (floating point or integer) number followed
                   by a scale factor.  For all limits other than cputime the default scale is ‘k’
                   or “kilobytes” (1024 bytes); a scale factor of ‘m’ or “megabytes” may also be
                   used.  For cputime the default scale is “seconds”; a scale factor of ‘m’ for
                   minutes or ‘h’ for hours, or a time of the form “mm:ss” giving minutes and
                   seconds also may be used.

                   For both resource names and scale factors, unambiguous prefixes of the names
                   suffice.

           login   Terminate a login shell, replacing it with an instance of /usr/bin/login.
                   This is one way to log off, included for compatibility with sh(1).

           logout  Terminate a login shell.  Especially useful if ignoreeof is set.

           nice
           nice +number
           nice command
           nice +number command
                   The first form sets the scheduling priority for this shell to 4.  The second
                   form sets the priority to the given number.  The final two forms run command
                   at priority 4 and number respectively.  The greater the number, the less CPU
                   the process will get.  The superuser may specify negative priority by using
                   “nice -number ...”.  command is always executed in a sub-shell, and the
                   restrictions placed on commands in simple if statements apply.

           nohup
           nohup command
                   The first form can be used in shell scripts to cause hangups to be ignored for
                   the remainder of the script.  The second form causes the specified command to
                   be run with hangups ignored.  All processes detached with ‘&’ are effectively
                   nohup´ed.

           notify
           notify %job ...
                   Causes the shell to notify the user asynchronously when the status of the
                   current or specified jobs change; normally notification is presented before a
                   prompt.  This is automatic if the shell variable notify is set.

           onintr
           onintr -
           onintr label
                   Control the action of the shell on interrupts.  The first form restores the
                   default action of the shell on interrupts, which is to terminate shell scripts
                   or to return to the terminal command input level.  The second form onintr -
                   causes all interrupts to be ignored.  The final form causes the shell to
                   execute a goto label when an interrupt is received or a child process
                   terminates because it was interrupted.

                   In any case, if the shell is running detached and interrupts are being
                   ignored, all forms of onintr have no meaning and interrupts continue to be
                   ignored by the shell and all invoked commands.  Finally, onintr statements are
                   ignored in the system startup files where interrupts are disabled
                   (/etc/csh.cshrc, /etc/csh.login).

           popd
           popd +n
                   Pops the directory stack, returning to the new top directory.  With an
                   argument “+n” discards the n´th entry in the stack.  The members of the
                   directory stack are numbered from the top starting at 0.

           pushd
           pushd name
           pushd +n
                   With no arguments, pushd exchanges the top two elements of the directory
                   stack.  Given a name argument, pushd changes to the new directory (ala cd) and
                   pushes the old current working directory (as in cwd) onto the directory stack.
                   With a numeric argument, pushd rotates the n´th argument of the directory
                   stack around to be the top element and changes to it.  The members of the
                   directory stack are numbered from the top starting at 0.

           rehash  Causes the internal hash table of the contents of the directories in the path
                   variable to be recomputed.  This is needed if new commands are added to
                   directories in the path while you are logged in.  This should only be
                   necessary if you add commands to one of your own directories, or if a systems
                   programmer changes the contents of a system directory.

           repeat count command
                   The specified command, which is subject to the same restrictions as the
                   command in the one line if statement above, is executed count times.  I/O
                   redirections occur exactly once, even if count is 0.

           set
           set name
           set name=word
           set name[index]=word
           set name=(wordlist)
                   The first form of the command shows the value of all shell variables.
                   Variables that have other than a single word as their value print as a
                   parenthesized word list.  The second form sets name to the null string.  The
                   third form sets name to the single word.  The fourth form sets the index'th
                   component of name to word; this component must already exist.  The final form
                   sets name to the list of words in wordlist.  The value is always command and
                   filename expanded.

                   These arguments may be repeated to set multiple values in a single set
                   command.  Note however, that variable expansion happens for all arguments
                   before any setting occurs.

           setenv
           setenv name
           setenv name value
                   The first form lists all current environment variables.  It is equivalent to
                   printenv(1).  The last form sets the value of environment variable name to be
                   value, a single string.  The second form sets name to an empty string.  The
                   most commonly used environment variables USER, TERM, and PATH are
                   automatically imported to and exported from the csh variables user, term, and
                   path; there is no need to use setenv for these.

           shift
           shift variable
                   The members of argv are shifted to the left, discarding argv[1].  It is an
                   error for argv not to be set or to have less than one word as value.  The
                   second form performs the same function on the specified variable.

           source name
           source -h name
                   The shell reads commands from name.  source commands may be nested; if they
                   are nested too deeply the shell may run out of file descriptors.  An error in
                   a source at any level terminates all nested source commands.  Normally input
                   during source commands is not placed on the history list; the -h option causes
                   the commands to be placed on the history list without being executed.

           stop
           stop %job ...
                   Stops the current or specified jobs that are executing in the background.

           suspend
                   Causes the shell to stop in its tracks, much as if it had been sent a stop
                   signal with ^Z.  This is most often used to stop shells started by su(1).

           switch (string)
           case str1:
               ...
               breaksw
               ...
           default:
               ...
               breaksw
           endsw   Each case label is successively matched against the specified string, which is
                   first command and filename expanded.  The file metacharacters ‘*’, ‘?’ and
                   “[...]” may be used in the case labels, which are variable expanded.  If none
                   of the labels match before the “default” label is found, then the execution
                   begins after the default label.  Each case label and the default label must
                   appear at the beginning of a line.  The command breaksw causes execution to
                   continue after the endsw.  Otherwise control may fall through case labels and
                   the default label as in C.  If no label matches and there is no default,
                   execution continues after the endsw.

           time
           time command
                   With no argument, a summary of time used by this shell and its children is
                   printed.  If arguments are given the specified simple command is timed and a
                   time summary as described under the time variable is printed.  If necessary,
                   an extra shell is created to print the time statistic when the command
                   completes.

           umask
           umask value
                   The file creation mask is displayed (first form) or set to the specified value
                   (second form).  The mask is given in octal.  Common values for the mask are
                   002 giving all access to the group and read and execute access to others or
                   022 giving all access except write access for users in the group or others.

           unalias pattern
                   All aliases whose names match the specified pattern are discarded.  Thus all
                   aliases are removed by unalias *.  It is not an error for nothing to be
                   unaliased.

           unhash  Use of the internal hash table to speed location of executed programs is
                   disabled.

           unlimit
           unlimit resource
           unlimit -h
           unlimit -h resource
                   Removes the limitation on resource.  If no resource is specified, then all
                   resource limitations are removed.  If -h is given, the corresponding hard
                   limits are removed.  Only the superuser may do this.

           unset pattern
                   All variables whose names match the specified pattern are removed.  Thus all
                   variables are removed by unset *; this has noticeably distasteful side-
                   effects.  It is not an error for nothing to be unset.

           unsetenv pattern
                   Removes all variables whose names match the specified pattern from the
                   environment.  See also the setenv command above and printenv(1).

           wait    Wait for all background jobs.  If the shell is interactive, then an interrupt
                   can disrupt the wait.  After the interrupt, the shell prints names and job
                   numbers of all jobs known to be outstanding.

           which command
                   Displays the resolved command that will be executed by the shell.

           while (expr)
           ...
           end     While the specified expression evaluates to non-zero, the commands between the
                   while and the matching end are evaluated.  break and continue may be used to
                   terminate or continue the loop prematurely.  (The while and end must appear
                   alone on their input lines.)  Prompting occurs here the first time through the
                   loop as for the foreach statement if the input is a terminal.

           %job    Brings the specified job into the foreground.

           %job &  Continues the specified job in the background.

           @
           @ name= expr
           @ name[index]= expr
                   The first form prints the values of all the shell variables.  The second form
                   sets the specified name to the value of expr.  If the expression contains ‘<’,
                   ‘>’, ‘&’ or ‘|’ then at least this part of the expression must be placed
                   within ‘(’ ‘)’.  The third form assigns the value of expr to the index'th
                   argument of name.  Both name and its index'th component must already exist.

                   The operators ‘*=’, ‘+=’, etc. are available as in C.  The space separating
                   the name from the assignment operator is optional.  Spaces are, however,
                   mandatory in separating components of expr, which would otherwise be single
                   words.

                   Special postfix ‘++’ and ‘--’ operators increment and decrement name
                   respectively; i.e., “@ i++”.

   Pre-defined and environment variables
     The following variables have special meaning to the shell.  Of these, argv, cwd, home, path,
     prompt, shell and status are always set by the shell.  Except for cwd and status, this
     setting occurs only at initialization; these variables will not then be modified unless done
     explicitly by the user.

     The shell copies the environment variable USER into the variable user, TERM into term, and
     HOME into home, and copies these back into the environment whenever the normal shell
     variables are reset.  The environment variable PATH is likewise handled; it is not necessary
     to worry about its setting other than in the file .cshrc as inferior csh processes will
     import the definition of path from the environment, and re-export it if you then change it.

     argv       Set to the arguments to the shell, it is from this variable that positional
                parameters are substituted; i.e., “$1” is replaced by “$argv[1]”, etc.

     cdpath     Gives a list of alternate directories searched to find subdirectories in chdir
                commands.

     cwd        The full pathname of the current directory.

     echo       Set when the -x command-line option is given.  Causes each command and its
                arguments to be echoed just before it is executed.  For non-built-in commands all
                expansions occur before echoing.  Built-in commands are echoed before command and
                filename substitution, since these substitutions are then done selectively.

     filec      Enable file name completion.

     histchars  Can be given a string value to change the characters used in history
                substitution.  The first character of its value is used as the history
                substitution character, replacing the default character ‘!’.  The second
                character of its value replaces the character ‘^’ in quick substitutions.

     histfile   Can be set to the pathname where history is going to be saved/restored.

     history    Can be given a numeric value to control the size of the history list.  Any
                command that has been referenced in this many events will not be discarded.  Too
                large values of history may run the shell out of memory.  The last executed
                command is always saved on the history list.

     home       The home directory of the invoker, initialized from the environment.  The
                filename expansion of “~” refers to this variable.

     ignoreeof  If set the shell ignores end-of-file from input devices which are terminals.
                This prevents shells from accidentally being killed by control-Ds.

     mail       The files where the shell checks for mail.  This checking is done after each
                command completion that will result in a prompt, if a specified interval has
                elapsed.  The shell says “You have new mail.” if the file exists with an access
                time not greater than its modify time.

                If the first word of the value of mail is numeric it specifies a different mail
                checking interval, in seconds, than the default, which is 10 minutes.

                If multiple mail files are specified, then the shell says “New mail in name” when
                there is mail in the file name.

     noclobber  As described in the section on Input/output, restrictions are placed on output
                redirection to ensure that files are not accidentally destroyed, and that ‘>>’
                redirections refer to existing files.

     noglob     If set, filename expansion is inhibited.  This inhibition is most useful in shell
                scripts that are not dealing with filenames, or after a list of filenames has
                been obtained and further expansions are not desirable.

     nonomatch  If set, it is not an error for a filename expansion to not match any existing
                files; instead the primitive pattern is returned.  It is still an error for the
                primitive pattern to be malformed; i.e., “echo [” still gives an error.

     notify     If set, the shell notifies asynchronously of job completions; the default is to
                present job completions just before printing a prompt.

     path       Each word of the path variable specifies a directory in which commands are to be
                sought for execution.  A null word specifies the current directory.  If there is
                no path variable then only full path names will execute.  The usual search path
                is “.”, “/bin”, “/usr/bin”, “/sbin” and “/usr/sbin”, but this may vary from
                system to system.  For the superuser the default search path is “/bin”,
                “/usr/bin”, “/sbin”, and “/usr/sbin”.  A shell that is given neither the -c nor
                the -t option will normally hash the contents of the directories in the path
                variable after reading .cshrc, and each time the path variable is reset.  If new
                commands are added to these directories while the shell is active, it may be
                necessary to do a rehash or the commands may not be found.

     prompt     The string that is printed before each command is read from an interactive
                terminal input.  If a ‘!’ appears in the string it will be replaced by the
                current event number unless a preceding ‘\’ is given.  Default is “%”, or “#” for
                the superuser.

     savehist   Is given a numeric value to control the number of entries of the history list
                that are saved in ~/.history when the user logs out.  Any command that has been
                referenced in this many events will be saved.  During start up the shell sources
                ~/.history into the history list enabling history to be saved across logins.  Too
                large values of savehist will slow down the shell during start up.  If savehist
                is just set, the shell will use the value of history.

     shell      The file in which the shell resides.  This variable is used in forking shells to
                interpret files that have execute bits set, but which are not executable by the
                system.  (See the description of Non-built-in command execution below.)
                Initialized to the (system-dependent) home of the shell.

     status     The status returned by the last command.  If it terminated abnormally, then 0200
                is added to the status.  Built-in commands that fail return exit status 1, all
                other built-in commands set status to 0.

     time       Controls automatic timing of commands.  If set, then any command that takes more
                than this many CPU seconds will cause a line giving user, system, and real times,
                and a utilization percentage which is the ratio of user plus system times to real
                time to be printed when it terminates.

     verbose    Set by the -v command-line option, causes the words of each command to be printed
                after history substitution.

   Non-built-in command execution
     When a command to be executed is found to not be a built-in command the shell attempts to
     execute the command via execve(2).  Each word in the variable path names a directory from
     which the shell will attempt to execute the command.  If it is given neither a -c nor a -t
     option, the shell will hash the names in these directories into an internal table so that it
     will only try an exec in a directory if there is a possibility that the command resides
     there.  This shortcut greatly speeds command location when many directories are present in
     the search path.  If this mechanism has been turned off (via unhash), or if the shell was
     given a -c or -t argument, and in any case for each directory component of path that does
     not begin with a ‘/’, the shell concatenates with the given command name to form a path name
     of a file which it then attempts to execute.

     Parenthesized commands are always executed in a sub-shell.  Thus

           (cd; pwd); pwd

     prints the home directory; leaving you where you were (printing this after the home
     directory), while

           cd; pwd

     leaves you in the home directory.  Parenthesized commands are most often used to prevent
     chdir from affecting the current shell.

     If the file has execute permissions but is not an executable binary to the system, then it
     is assumed to be a file containing shell commands and a new shell is spawned to read it.

     If there is an alias for shell then the words of the alias will be prepended to the argument
     list to form the shell command.  The first word of the alias should be the full path name of
     the shell (e.g., “$shell”).  Note that this is a special, late occurring, case of alias
     substitution, and only allows words to be prepended to the argument list without change.

   Signal handling
     The shell normally ignores SIGQUIT signals.  Jobs running detached (either by & or the bg or
     %... & commands) are immune to signals generated from the keyboard, including hangups.
     Other signals have the values which the shell inherited from its parent.  The shell's
     handling of interrupts and terminate signals in shell scripts can be controlled by onintr.
     Login shells catch the SIGTERM (terminate) signal; otherwise this signal is passed on to
     children from the state in the shell's parent.  Interrupts are not allowed when a login
     shell is reading the file .logout.

LIMITATIONS

     Word lengths - Words can be no longer than 1024 characters.  The number of arguments to a
     command that involves filename expansion is limited to 1/6th the number of characters
     allowed in an argument list.  Command substitutions may substitute no more characters than
     are allowed in an argument list.  To detect looping, the shell restricts the number of alias
     substitutions on a single line to 20.

FILES

     ~/.cshrc     read at beginning of execution by each shell
     ~/.login     read by login shell, after .cshrc at login
     ~/.logout    read by login shell, at logout
     /bin/sh      standard shell, for shell scripts not starting with a ‘#’
     /tmp/sh.*    temporary file for ‘<<’
     /etc/passwd  source of home directories for “~name”

SEE ALSO

     sh(1), access(2), execve(2), fork(2), pipe(2), setrlimit(2), umask(2), wait(2), killpg(3),
     sigvec(3), tty(4), a.out(5), environ(7), script(7)

HISTORY

     csh appeared in 3BSD.  It was a first implementation of a command language interpreter
     incorporating a history mechanism (see History substitutions), job control facilities (see
     Jobs), interactive file name and user name completion (see File name completion), and a C-
     like syntax.  There are now many shells that also have these mechanisms, plus a few more
     (and maybe some bugs too), which are available through the usenet.

AUTHORS

     William Joy.  Job control and directory stack features first implemented by J.E. Kulp of
     IIASA, Laxenburg, Austria, with different syntax than that used now.  File name completion
     code written by Ken Greer, HP Labs.  Eight-bit implementation Christos S. Zoulas, Cornell
     University.

BUGS

     When a command is restarted from a stop, the shell prints the directory it started in if
     this is different from the current directory; this can be misleading (i.e., wrong) as the
     job may have changed directories internally.

     Shell built-in functions are not stoppable/restartable.  Command sequences of the form “a ;
     b ; c” are also not handled gracefully when stopping is attempted.  If you suspend ‘b’, the
     shell will immediately execute ‘c’.  This is especially noticeable if this expansion results
     from an alias.  It suffices to place the sequence of commands in ()'s to force it to a sub-
     shell; i.e., “(a ; b ; c)”.

     Control over tty output after processes are started is primitive; perhaps this will inspire
     someone to work on a good virtual terminal interface.  In a virtual terminal interface much
     more interesting things could be done with output control.

     Alias substitution is most often used to clumsily simulate shell procedures; shell
     procedures should be provided instead of aliases.

     Commands within loops, prompted for by ‘?’, are not placed on the history list.  Control
     structure should be parsed instead of being recognized as built-in commands.  This would
     allow control commands to be placed anywhere, to be combined with ‘|’, and to be used with
     ‘&’ and ‘;’ metasyntax.

     It should be possible to use the ‘:’ modifiers on the output of command substitutions.

     The way the filec facility is implemented is ugly and expensive.