Ubuntu Manpage: zsh - the Z shell

NAME

       zsh - the Z shell

OVERVIEW

       Because  zsh  contains  many  features,  the  zsh  manual  has been split into a number of
       sections:

       zsh          Zsh overview (this section)
       zshroadmap   Informal introduction to the manual
       zshmisc      Anything not fitting into the other sections
       zshexpn      Zsh command and parameter expansion
       zshparam     Zsh parameters
       zshoptions   Zsh options
       zshbuiltins  Zsh built-in functions
       zshzle       Zsh command line editing
       zshcompwid   Zsh completion widgets
       zshcompsys   Zsh completion system
       zshcompctl   Zsh completion control
       zshmodules   Zsh loadable modules
       zshcalsys    Zsh built-in calendar functions
       zshtcpsys    Zsh built-in TCP functions
       zshzftpsys   Zsh built-in FTP client
       zshcontrib   Additional zsh functions and utilities
       zshall       Meta-man page containing all of the above

DESCRIPTION

       Zsh is a UNIX command interpreter (shell) usable as an interactive login shell  and  as  a
       shell  script  command  processor.  Of the standard shells, zsh most closely resembles ksh
       but includes many enhancements.  It does not provide compatibility  with  POSIX  or  other
       shells in its default operating mode:  see the section `Compatibility' below.

       Zsh   has   command  line  editing,  builtin  spelling  correction,  programmable  command
       completion, shell functions (with autoloading), a history mechanism, and a host  of  other
       features.

AUTHOR

       Zsh  was  originally written by Paul Falstad.  Zsh is now maintained by the members of the
       zsh-workers mailing list <zsh-workers@zsh.org>.  The development is currently  coordinated
       by    Peter   Stephenson   <pws@zsh.org>.    The   coordinator   can   be   contacted   at
       <coordinator@zsh.org>, but matters relating to the code should generally go to the mailing
       list.

AVAILABILITY

       Zsh is available from the following HTTP and anonymous FTP site.

       ftp://ftp.zsh.org/pub/
       https://www.zsh.org/pub/

       The   up-to-date   source   code   is   available   via   Git   from   Sourceforge.    See
       https://sourceforge.net/projects/zsh/ for details.  A  summary  of  instructions  for  the
       archive can be found at https://zsh.sourceforge.io/.

MAILING LISTS

       Zsh has several mailing lists:

       <zsh-announce@zsh.org>
              Announcements about releases, major changes in the shell and the monthly posting of
              the Zsh FAQ.  (moderated)

       <zsh-users@zsh.org>
              User discussions.

       <zsh-workers@zsh.org>
              Hacking, development, bug reports and patches.

       <zsh-security@zsh.org>
              Private mailing list (the general public cannot subscribe to it) for discussing bug
              reports with security implications, i.e., potential vulnerabilities.

              If you find a security problem in zsh itself, please mail this address.

       To  subscribe  or  unsubscribe, send mail to the associated administrative address for the
       mailing list.

       <zsh-announce-subscribe@zsh.org>
       <zsh-users-subscribe@zsh.org>
       <zsh-workers-subscribe@zsh.org>
       <zsh-announce-unsubscribe@zsh.org>
       <zsh-users-unsubscribe@zsh.org>
       <zsh-workers-unsubscribe@zsh.org>

       YOU ONLY NEED TO JOIN ONE OF THE MAILING LISTS AS THEY ARE  NESTED.   All  submissions  to
       zsh-announce  are  automatically forwarded to zsh-users.  All submissions to zsh-users are
       automatically forwarded to zsh-workers.

       If you have problems subscribing/unsubscribing to any of the mailing lists, send  mail  to
       <listmaster@zsh.org>.

       The  mailing  lists  are  archived;  the  archives  can be accessed via the administrative
       addresses  listed   above.    There   is   also   a   hypertext   archive   available   at
       https://www.zsh.org/mla/.

THE ZSH FAQ

       Zsh  has  a  list  of  Frequently  Asked  Questions  (FAQ), maintained by Peter Stephenson
       <pws@zsh.org>.   It  is  regularly  posted  to  the  newsgroup  comp.unix.shell  and   the
       zsh-announce  mailing  list.  The latest version can be found at any of the Zsh FTP sites,
       or  at  https://www.zsh.org/FAQ/.   The  contact  address  for  FAQ-related   matters   is
       <faqmaster@zsh.org>.

THE ZSH WEB PAGE

       Zsh  has  a  web  page  which is located at https://www.zsh.org/.  The contact address for
       web-related matters is <webmaster@zsh.org>.

THE ZSH USERGUIDE

       A userguide is currently in preparation.  It is intended to complement  the  manual,  with
       explanations  and  hints  on  issues where the manual can be cabbalistic, hierographic, or
       downright mystifying (for example, the word `hierographic' does not  exist).   It  can  be
       viewed in its current state at https://zsh.sourceforge.io/Guide/.  At the time of writing,
       chapters dealing with startup files and their contents and the new completion system  were
       essentially complete.

INVOCATION

The following flags are interpreted by the shell when invoked to determine where the shell
will read commands from:

-c Take the first argument as a command to execute, rather than reading commands from
a script or standard input. If any further arguments are given, the first one is
assigned to $0, rather than being used as a positional parameter.

-i Force shell to be interactive. It is still possible to specify a script to
execute.

-s Force shell to read commands from the standard input. If the -s flag is not
present and an argument is given, the first argument is taken to be the pathname of
a script to execute.

If there are any remaining arguments after option processing, and neither of the options
-c or -s was supplied, the first argument is taken as the file name of a script containing
shell commands to be executed. If the option PATH_SCRIPT is set, and the file name does
not contain a directory path (i.e. there is no `/' in the name), first the current
directory and then the command path given by the variable PATH are searched for the
script. If the option is not set or the file name contains a `/' it is used directly.

After the first one or two arguments have been appropriated as described above, the
remaining arguments are assigned to the positional parameters.

For further options, which are common to invocation and the set builtin, see
zshoptions(1).

The long option `--emulate' followed (in a separate word) by an emulation mode may be
passed to the shell. The emulation modes are those described for the emulate builtin, see
zshbuiltins(1). The `--emulate' option must precede any other options (which might
otherwise be overridden), but following options are honoured, so may be used to modify the
requested emulation mode. Note that certain extra steps are taken to ensure a smooth
emulation when this option is used compared with the emulate command within the shell: for
example, variables that conflict with POSIX usage such as path are not defined within the
shell.

Options may be specified by name using the -o option. -o acts like a single-letter
option, but takes a following string as the option name. For example,

zsh -x -o shwordsplit scr

runs the script scr, setting the XTRACE option by the corresponding letter `-x' and the
SH_WORD_SPLIT option by name. Options may be turned off by name by using +o instead of
-o. -o can be stacked up with preceding single-letter options, so for example `-xo
shwordsplit' or `-xoshwordsplit' is equivalent to `-x -o shwordsplit'.

Options may also be specified by name in GNU long option style, `--option-name'. When
this is done, `-' characters in the option name are permitted: they are translated into
`_', and thus ignored. So, for example, `zsh --sh-word-split' invokes zsh with the
SH_WORD_SPLIT option turned on. Like other option syntaxes, options can be turned off by
replacing the initial `-' with a `+'; thus `+-sh-word-split' is equivalent to
`--no-sh-word-split'. Unlike other option syntaxes, GNU-style long options cannot be
stacked with any other options, so for example `-x-shwordsplit' is an error, rather than
being treated like `-x --shwordsplit'.

The special GNU-style option `--version' is handled; it sends to standard output the
shell's version information, then exits successfully. `--help' is also handled; it sends
to standard output a list of options that can be used when invoking the shell, then exits
successfully.

Option processing may be finished, allowing following arguments that start with `-' or `+'
to be treated as normal arguments, in two ways. Firstly, a lone `-' (or `+') as an
argument by itself ends option processing. Secondly, a special option `--' (or `+-'),
which may be specified on its own (which is the standard POSIX usage) or may be stacked
with preceding options (so `-x-' is equivalent to `-x --'). Options are not permitted to
be stacked after `--' (so `-x-f' is an error), but note the GNU-style option form
discussed above, where `--shwordsplit' is permitted and does not end option processing.

Except when the sh/ksh emulation single-letter options are in effect, the option `-b' (or
`+b') ends option processing. `-b' is like `--', except that further single-letter
options can be stacked after the `-b' and will take effect as normal.

COMPATIBILITY

Zsh tries to emulate sh or ksh when it is invoked as sh or ksh respectively; more
precisely, it looks at the first letter of the name by which it was invoked, excluding any
initial `r' (assumed to stand for `restricted'), and if that is `b', `s' or `k' it will
emulate sh or ksh. Furthermore, if invoked as su (which happens on certain systems when
the shell is executed by the su command), the shell will try to find an alternative name
from the SHELL environment variable and perform emulation based on that.

In sh and ksh compatibility modes the following parameters are not special and not
initialized by the shell: ARGC, argv, cdpath, fignore, fpath, HISTCHARS, mailpath,
MANPATH, manpath, path, prompt, PROMPT, PROMPT2, PROMPT3, PROMPT4, psvar, status.

The usual zsh startup/shutdown scripts are not executed. Login shells source /etc/profile
followed by $HOME/.profile. If the ENV environment variable is set on invocation, $ENV is
sourced after the profile scripts. The value of ENV is subjected to parameter expansion,
command substitution, and arithmetic expansion before being interpreted as a pathname.
Note that the PRIVILEGED option also affects the execution of startup files.

The following options are set if the shell is invoked as sh or ksh: NO_BAD_PATTERN,
NO_BANG_HIST, NO_BG_NICE, NO_EQUALS, NO_FUNCTION_ARGZERO, GLOB_SUBST, NO_GLOBAL_EXPORT,
NO_HUP, INTERACTIVE_COMMENTS, KSH_ARRAYS, NO_MULTIOS, NO_NOMATCH, NO_NOTIFY,
POSIX_BUILTINS, NO_PROMPT_PERCENT, RM_STAR_SILENT, SH_FILE_EXPANSION, SH_GLOB,
SH_OPTION_LETTERS, SH_WORD_SPLIT. Additionally the BSD_ECHO and IGNORE_BRACES options are
set if zsh is invoked as sh. Also, the KSH_OPTION_PRINT, LOCAL_OPTIONS, PROMPT_BANG,
PROMPT_SUBST and SINGLE_LINE_ZLE options are set if zsh is invoked as ksh.

Please note that, whilst reasonable efforts are taken to address incompatibilities when
they arise, zsh does not guarantee complete emulation of other shells, nor POSIX
compliance. For more information on the differences between zsh and other shells, please
refer to chapter 2 of the shell FAQ, https://www.zsh.org/FAQ/.

RESTRICTED SHELL

When the basename of the command used to invoke zsh starts with the letter `r' or the `-r'
command line option is supplied at invocation, the shell becomes restricted. Emulation
mode is determined after stripping the letter `r' from the invocation name. The following
are disabled in restricted mode:

• changing directories with the cd builtin

• changing or unsetting the EGID, EUID, GID, HISTFILE, HISTSIZE, IFS,
LD_AOUT_LIBRARY_PATH, LD_AOUT_PRELOAD, LD_LIBRARY_PATH, LD_PRELOAD, MODULE_PATH,
module_path, PATH, path, SHELL, UID and USERNAME parameters

• specifying command names containing /

• specifying command pathnames using hash

• redirecting output to files

• using the exec builtin command to replace the shell with another command

• using jobs -Z to overwrite the shell process' argument and environment space

• using the ARGV0 parameter to override argv[0] for external commands

• turning off restricted mode with set +r or unsetopt RESTRICTED

These restrictions are enforced after processing the startup files. The startup files
should set up PATH to point to a directory of commands which can be safely invoked in the
restricted environment. They may also add further restrictions by disabling selected
builtins.

Restricted mode can also be activated any time by setting the RESTRICTED option. This
immediately enables all the restrictions described above even if the shell still has not
processed all startup files.

A shell Restricted Mode is an outdated way to restrict what users may do: modern systems
have better, safer and more reliable ways to confine user actions, such as chroot jails,
containers and zones.

A restricted shell is very difficult to implement safely. The feature may be removed in a
future version of zsh.

It is important to realise that the restrictions only apply to the shell, not to the
commands it runs (except for some shell builtins). While a restricted shell can only run
the restricted list of commands accessible via the predefined `PATH' variable, it does not
prevent those commands from running any other command.

As an example, if `env' is among the list of allowed commands, then it allows the user to
run any command as `env' is not a shell builtin command and can run arbitrary executables.

So when implementing a restricted shell framework it is important to be fully aware of
what actions each of the allowed commands or features (which may be regarded as modules)
can perform.

Many commands can have their behaviour affected by environment variables. Except for the
few listed above, zsh does not restrict the setting of environment variables.

If a `perl', `python', `bash', or other general purpose interpreted script it treated as a
restricted command, the user can work around the restriction by setting specially crafted
`PERL5LIB', `PYTHONPATH', `BASHENV' (etc.) environment variables. On GNU systems, any
command can be made to run arbitrary code when performing character set conversion
(including zsh itself) by setting a `GCONV_PATH' environment variable. Those are only a
few examples.

Bear in mind that, contrary to some other shells, `readonly' is not a security feature in
zsh as it can be undone and so cannot be used to mitigate the above.

A restricted shell only works if the allowed commands are few and carefully written so as
not to grant more access to users than intended. It is also important to restrict what
zsh module the user may load as some of them, such as `zsh/system', `zsh/mapfile' and
`zsh/files', allow bypassing most of the restrictions.

STARTUP/SHUTDOWN FILES

Commands are first read from /etc/zsh/zshenv; this cannot be overridden. Subsequent
behaviour is modified by the RCS and GLOBAL_RCS options; the former affects all startup
files, while the second only affects global startup files (those shown here with an path
starting with a /). If one of the options is unset at any point, any subsequent startup
file(s) of the corresponding type will not be read. It is also possible for a file in
$ZDOTDIR to re-enable GLOBAL_RCS. Both RCS and GLOBAL_RCS are set by default.

Commands are then read from $ZDOTDIR/.zshenv. If the shell is a login shell, commands are
read from /etc/zsh/zprofile and then $ZDOTDIR/.zprofile. Then, if the shell is
interactive, commands are read from /etc/zsh/zshrc and then $ZDOTDIR/.zshrc. Finally, if
the shell is a login shell, /etc/zsh/zlogin and $ZDOTDIR/.zlogin are read.

When a login shell exits, the files $ZDOTDIR/.zlogout and then /etc/zsh/zlogout are read.
This happens with either an explicit exit via the exit or logout commands, or an implicit
exit by reading end-of-file from the terminal. However, if the shell terminates due to
exec'ing another process, the logout files are not read. These are also affected by the
RCS and GLOBAL_RCS options. Note also that the RCS option affects the saving of history
files, i.e. if RCS is unset when the shell exits, no history file will be saved.

If ZDOTDIR is unset, HOME is used instead. Files listed above as being in /etc may be in
another directory, depending on the installation.

As /etc/zsh/zshenv is run for all instances of zsh, it is important that it be kept as
small as possible. In particular, it is a good idea to put code that does not need to be
run for every single shell behind a test of the form `if [[ -o rcs ]]; then ...' so that
it will not be executed when zsh is invoked with the `-f' option.

Any of these files may be pre-compiled with the zcompile builtin command (see
zshbuiltins(1)). If a compiled file exists (named for the original file plus the .zwc
extension) and it is newer than the original file, the compiled file will be used instead.

FILES

       $ZDOTDIR/.zshenv
       $ZDOTDIR/.zprofile
       $ZDOTDIR/.zshrc
       $ZDOTDIR/.zlogin
       $ZDOTDIR/.zlogout
       ${TMPPREFIX}*   (default is /tmp/zsh*)
       /etc/zsh/zshenv
       /etc/zsh/zprofile
       /etc/zsh/zshrc
       /etc/zsh/zlogin
       /etc/zsh/zlogout    (installation-specific - /etc is the default)