4 Invocation

4.1 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 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. 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.

4.2 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/.

4.3 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.