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**Table of Contents**  *generated with [DocToc](https://github.com/thlorenz/doctoc)*

- [4 Invocation](#4-invocation)
  - [4.1 Invocation](#41-invocation)
  - [4.2 Compatibility](#42-compatibility)
  - [4.3 Restricted Shell](#43-restricted-shell)

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<span id="Invocation"></span> <span id="Invocation-2"></span>

# 4 Invocation

<span id="index-invocation"></span>

------------------------------------------------------------------------

<span id="Invocation-1"></span>

## 4.1 Invocation

<span id="index-shell-options"></span> <span
id="index-options_002c-shell"></span> <span
id="index-shell-flags"></span> <span id="index-flags_002c-shell"></span>

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 [Options](Options.html#Options).

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 [Shell Builtin
Commands](Shell-Builtin-Commands.html#Shell-Builtin-Commands). 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,

<div class="example">

```zsh
zsh -x -o shwordsplit scr
```

</div>

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’.

<span id="index-long-option"></span>

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’.

<span id="index-_002d_002dversion"></span> <span
id="index-_002d_002dhelp"></span>

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.

------------------------------------------------------------------------

<span id="Compatibility"></span> <span id="Compatibility-1"></span>

## 4.2 Compatibility

<span id="index-compatibility"></span> <span
id="index-sh-compatibility"></span> <span
id="index-ksh-compatibility"></span>

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.

<span id="index-ENV_002c-use-of"></span>

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/>.

------------------------------------------------------------------------

<span id="Restricted-Shell"></span> <span
id="Restricted-Shell-1"></span>

## 4.3 Restricted Shell

<span id="index-restricted-shell"></span> <span
id="index-RESTRICTED"></span>

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

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.

------------------------------------------------------------------------

This document was generated on *May 14, 2022* using [*texi2html
5.0*](http://www.nongnu.org/texi2html/).  
Zsh version 5.9, released on May 14, 2022.
