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<!doctype html><html lang=en class=no-js> <head><meta charset=utf-8><meta name=viewport content="width=device-width,initial-scale=1"><link href=https://flokoe.github.io/bash-hackers-wiki/scripting/bashbehaviour/ rel=canonical><link href=../../misc/shell_humor/ rel=prev><link href=../bashchanges/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.5.44"><title>Bash's behaviour - The Bash Hackers Wiki</title><link rel=stylesheet href=../../assets/stylesheets/main.0253249f.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.06af60db.min.css><link rel=preconnect href=https://fonts.gstatic.com crossorigin><link rel=stylesheet href="https://fonts.googleapis.com/css?family=Roboto:300,300i,400,400i,700,700i%7CRoboto+Mono:400,400i,700,700i&display=fallback"><style>:root{--md-text-font:"Roboto";--md-code-font:"Roboto Mono"}</style><script>__md_scope=new URL("../..",location),__md_hash=e=>[...e].reduce(((e,_)=>(e<<5)-e+_.charCodeAt(0)),0),__md_get=(e,_=localStorage,t=__md_scope)=>JSON.parse(_.getItem(t.pathname+"."+e)),__md_set=(e,_,t=localStorage,a=__md_scope)=>{try{t.setItem(a.pathname+"."+e,JSON.stringify(_))}catch(e){}}</script></head> <body dir=ltr data-md-color-scheme=default data-md-color-primary=indigo data-md-color-accent=indigo> <input class=md-toggle data-md-toggle=drawer type=checkbox id=__drawer autocomplete=off> <input class=md-toggle data-md-toggle=search type=checkbox id=__search autocomplete=off> <label class=md-overlay for=__drawer></label> <div data-md-component=skip> <a href=#bashs-behaviour class=md-skip> Skip to content </a> </div> <div data-md-component=announce> </div> <header class=md-header data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../.. title="The Bash Hackers Wiki" class="md-header__button md-logo" aria-label="The Bash Hackers Wiki" data-md-component=logo> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a3 3 0 0 0 3-3 3 3 0 0 0-3-3 3 3 0 0 0-3 3 3 3 0 0 0 3 3m0 3.54C9.64 9.35 6.5 8 3 8v11c3.5 0 6.64 1.35 9 3.54 2.36-2.19 5.5-3.54 9-3.54V8c-3.5 0-6.64 1.35-9 3.54"/></svg> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> The Bash Hackers Wiki </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> Bash's behaviour </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media data-md-color-scheme=default data-md-color-primary=indigo data-md-color-accent=indigo aria-label="Switch to dark mode" type=radio name=__palette id=__palette_0> <label class="md-header__button md-icon" title="Switch to dark mode" for=__palette_1 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a4 4 0 0 0-4 4 4 4 0 0 0 4 4 4 4 0 0 0 4-4 4 4 0 0 0-4-4m0 10a6 6 0 0 1-6-6 6 6 0 0 1 6-6 6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> <input class=md-option data-md-color-media data-md-color-scheme=slate data-md-color-primary=indigo data-md-color-accent=indigo aria-label="Switch to light mode" type=radio name=__palette id=__palette_1> <label class="md-header__button md-icon" title="Switch to light mode" for=__palette_0 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 18c-.89 0-1.74-.2-2.5-.55C11.56 16.5 13 14.42 13 12s-1.44-4.5-3.5-5.45C10.26 6.2 11.11 6 12 6a6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(pref
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-o posix' while Bash is running will cause Bash to conform more closely
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to the POSIX standard by changing the behavior to match that specified
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by POSIX in areas where the Bash default differs.
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When invoked as `sh', Bash enters POSIX mode after reading the startup
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files.
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The following lists what's changed when Bash is in `POSIX mode':
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1. When a command in the hash table no longer exists, Bash will
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re-search `$PATH' to find the new location. This is also
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available with `shopt -s checkhash'.
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2. The message printed by the job control code and builtins when a job
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exits with a non-zero status is `Done(status)'.
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3. The message printed by the job control code and builtins when a job
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is stopped is `Stopped(SIGNAME)', where SIGNAME is, for example,
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`SIGTSTP'.
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4. The `bg' builtin uses the required format to describe each job
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placed in the background, which does not include an indication of
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whether the job is the current or previous job.
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5. Reserved words appearing in a context where reserved words are
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recognized do not undergo alias expansion.
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6. The POSIX `PS1' and `PS2' expansions of `!' to the history number
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and `!!' to `!' are enabled, and parameter expansion is performed
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on the values of `PS1' and `PS2' regardless of the setting of the
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`promptvars' option.
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7. The POSIX startup files are executed (`$ENV') rather than the
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normal Bash files.
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8. Tilde expansion is only performed on assignments preceding a
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command name, rather than on all assignment statements on the line.
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9. The default history file is `~/.sh_history' (this is the default
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value of `$HISTFILE').
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10. The output of `kill -l' prints all the signal names on a single
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line, separated by spaces, without the `SIG' prefix.
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11. The `kill' builtin does not accept signal names with a `SIG'
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prefix.
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12. Non-interactive shells exit if FILENAME in `.' FILENAME is not
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found.
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13. Non-interactive shells exit if a syntax error in an arithmetic
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expansion results in an invalid expression.
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14. Redirection operators do not perform filename expansion on the word
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in the redirection unless the shell is interactive.
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15. Redirection operators do not perform word splitting on the word in
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the redirection.
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16. Function names must be valid shell names. That is, they may not
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contain characters other than letters, digits, and underscores, and
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may not start with a digit. Declaring a function with an invalid
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name causes a fatal syntax error in non-interactive shells.
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17. POSIX special builtins are found before shell functions during
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command lookup.
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18. If a POSIX special builtin returns an error status, a
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non-interactive shell exits. The fatal errors are those listed in
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the POSIX standard, and include things like passing incorrect
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options, redirection errors, variable assignment errors for
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assignments preceding the command name, etc.
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19. If `CDPATH' is set, the `cd' builtin will not implicitly append
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the current directory to it. This means that `cd' will fail if no
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valid directory name can be constructed from any of the entries in
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`$CDPATH', even if the a directory with the same name as the name
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given as an argument to `cd' exists in the current directory.
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20. A non-interactive shell exits with an error status if a variable
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assignment error occurs when no command name follows the assignment
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statements. A variable assignment error occurs, for example, when
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trying to assign a value to a readonly variable.
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21. A non-interactive shell exits with an error status if the iteration
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variable in a `for' statement or the selection variable in a
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`select' statement is a readonly variable.
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22. Process substitution is not available.
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23. Assignment statements preceding POSIX special builtins persist in
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the shell environment after the builtin completes.
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24. Assignment statements preceding shell function calls persist in the
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shell environment after the function returns, as if a POSIX
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special builtin command had been executed.
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25. The `export' and `readonly' builtin commands display their output
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in the format required by POSIX.
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26. The `trap' builtin displays signal names without the leading `SIG'.
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27. The `trap' builtin doesn't check the first argument for a possible
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signal specification and revert the signal handling to the original
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disposition if it is, unless that argument consists solely of
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digits and is a valid signal number. If users want to reset the
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handler for a given signal to the original disposition, they
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should use `-' as the first argument.
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28. The `.' and `source' builtins do not search the current directory
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for the filename argument if it is not found by searching `PATH'.
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29. Subshells spawned to execute command substitutions inherit the
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value of the `-e' option from the parent shell. When not in POSIX
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mode, Bash clears the `-e' option in such subshells.
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30. Alias expansion is always enabled, even in non-interactive shells.
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31. When the `alias' builtin displays alias definitions, it does not
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display them with a leading `alias ' unless the `-p' option is
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supplied.
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32. When the `set' builtin is invoked without options, it does not
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display shell function names and definitions.
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33. When the `set' builtin is invoked without options, it displays
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variable values without quotes, unless they contain shell
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metacharacters, even if the result contains nonprinting characters.
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34. When the `cd' builtin is invoked in LOGICAL mode, and the pathname
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constructed from `$PWD' and the directory name supplied as an
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argument does not refer to an existing directory, `cd' will fail
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instead of falling back to PHYSICAL mode.
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35. When the `pwd' builtin is supplied the `-P' option, it resets
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`$PWD' to a pathname containing no symlinks.
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36. The `pwd' builtin verifies that the value it prints is the same as
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the current directory, even if it is not asked to check the file
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system with the `-P' option.
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37. When listing the history, the `fc' builtin does not include an
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indication of whether or not a history entry has been modified.
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38. The default editor used by `fc' is `ed'.
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39. The `type' and `command' builtins will not report a non-executable
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file as having been found, though the shell will attempt to
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execute such a file if it is the only so-named file found in
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`$PATH'.
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40. The `vi' editing mode will invoke the `vi' editor directly when
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the `v' command is run, instead of checking `$FCEDIT' and
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`$EDITOR'.
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41. When the `xpg_echo' option is enabled, Bash does not attempt to
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interpret any arguments to `echo' as options. Each argument is
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displayed, after escape characters are converted.
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There is other POSIX behavior that Bash does not implement by default
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even when in POSIX mode. Specifically:
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1. The `fc' builtin checks `$EDITOR' as a program to edit history
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entries if `FCEDIT' is unset, rather than defaulting directly to
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`ed'. `fc' uses `ed' if `EDITOR' is unset.
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2. As noted above, Bash requires the `xpg_echo' option to be enabled
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for the `echo' builtin to be fully conformant.
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Bash can be configured to be POSIX-conformant by default, by specifying
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the `--enable-strict-posix-default' to `configure' when building.
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</code></pre></div> <div class="admonition warning"> <p class=admonition-title>FIXME</p> <p>help me to find out what breaks in POSIX® mode!</p> </div> <p><u><strong>The POSIX® mode can be switched on by:</strong></u></p> <ul> <li>Bash starting as <code>sh</code> (the basename of <code>argv[0]</code> is <code>sh</code>)</li> <li>starting Bash with the commandline option <code>--posix</code></li> <li>on startup, the environment variable <a href=../../syntax/shellvars/#POSIXLY_CORRECT>POSIXLY_CORRECT</a> is set</li> <li>the command <code>set -o posix</code></li> </ul> <p><u><strong>Tests for the POSIX® mode:</strong></u></p> <ul> <li>the variable <a href=../../syntax/shellvars/#SHELLOPTS>SHELLOPTS</a> contains <code>posix</code> in its list</li> </ul> <h3 id=restricted-shell>Restricted shell<a class=headerlink href=#restricted-shell title="Permanent link">¶</a></h3> <p>In restricted mode, Bash sets up (and runs) a shell environment that's far more controlled and limited than the standard shell mode. It acts like normal Bash with the following restrictions:</p> <ul> <li>the <code>cd</code> command can't be used to change directories</li> <li>the variables <a href=../../syntax/shellvars/#SHELL>SHELL</a>, <a href=../../syntax/shellvars/#PATH>PATH</a>, <a href=../../syntax/shellvars/#ENV>ENV</a> and <a href=../../syntax/shellvars/#BASH_ENV>BASH_ENV</a> can't be set or unset</li> <li>command names that contain a <code>/</code> (slash) can't be called (hence you're limited to <code>PATH</code>)</li> <li>filenames containing a <code>/</code> (slash) can't be specified as argument to the <code>source</code> or <code>.</code> builtin command</li> <li>filenames containing a <code>/</code> (slash) can't be specified as argument to the <code>-p</code> option of the <code>hash</code> builtin command</li> <li>function definitions are not inherited from the environment at shell startup</li> <li>the environment variable <a href=../../syntax/shellvars/#SHELLOPTS>SHELLOPTS</a> is ignored at startup</li> <li>redirecting output using the <code>></code>, <code>>|</code>, <code><></code>, <code>>&</code>, <code>&></code>, and <code>>></code> redirection operators isn't allowed</li> <li>the <code>exec</code> builtin command can't replace the shell with another process</li> <li>adding or deleting builtin commands with the <code>-f</code> and <code>-d</code> options to the enable builtin command is forbidden</li> <li>using the <code>enable</code> builtin command to enable disabled shell builtins doesn't work</li> <li>the <code>-p</code> option to the <code>command</code> builtin command doesn't work</li> <li>turning off restricted mode with <code>set +r</code> or <code>set +o restricted</code> is (of course) forbidden</li> </ul> <p>The "-r" restrictions are turned on <strong>after</strong> Bash has read its startup files.</p> <p>When the command that is run is a shell script, then the restrictions are <strong>turned off</strong> for the (sub-)shell that runs that shell script.</p> <p><u><strong>The restricted shell can be switched on by:</strong></u></p> <ul> <li>calling Bash as <code>rbash</code> (the basename of <code>argv[0]</code> is <code>rbash</code>)</li> <li>calling Bash with the <code>-r</code> option</li> <li>calling Bash with the <code>--restricted</code> option</li> </ul> <p><u><strong>Tests for restricted mode:</strong></u></p> <ul> <li>the special parameter <code>$-</code> contains the letter <code>r</code> (lowercase R)</li> <li>the shell option <code>restricted_shell</code> is set and can be checked by the <code>shopt</code> builtin command</li> </ul> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-
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</script> <script>
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var giscus = document.querySelector("script[src*=giscus]")
|
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|
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|
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|
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/* Set palette on initial load */
|
||
|
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var palette = __md_get("__palette")
|
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|
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if (palette && typeof palette.color === "object") {
|
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|
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var theme = palette.color.scheme === "slate" ? "dark" : "light"
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||
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giscus.setAttribute("data-theme", theme)
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|
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}
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|
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/* Register event handlers after documented loaded */
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||
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document.addEventListener("DOMContentLoaded", function() {
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var ref = document.querySelector("[data-md-component=palette]")
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ref.addEventListener("change", function() {
|
||
|
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var palette = __md_get("__palette")
|
||
|
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if (palette && typeof palette.color === "object") {
|
||
|
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var theme = palette.color.scheme === "slate" ? "dark" : "light"
|
||
|
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|
||
|
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/* Instruct Giscus to change theme */
|
||
|
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var frame = document.querySelector(".giscus-frame")
|
||
|
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frame.contentWindow.postMessage(
|
||
|
|
{ giscus: { setConfig: { theme } } },
|
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|
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"https://giscus.app"
|
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|
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)
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|
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}
|
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|
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})
|
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|
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})
|
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