Ubuntu Manpage: zsh - the Z shell

Provided by: zsh-common_5.9-6ubuntu2_all

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)