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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/basics/ rel=canonical><link href=../bashchanges/ rel=prev><link href=../debuggingtips/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.5.44"><title>The basics of shell scripting - 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=#the-basics-of-shell-scripting 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> The basics of shell scripting </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){
</code></pre></div> <p>There is <strong>no need to add a boring filename extension</strong> like <code>.bash</code> or <code>.sh</code>. That is a holdover from UNIX&reg;, where executables are not tagged by the extension, but by <strong>permissions</strong> (filemode). The file name can be any combination of legal filename characters. Adding a proper filename extension is a convention, nothing else.</p> <div class=highlight><pre><span></span><code>chmod +x ./myfile
</code></pre></div> <p>If the file is executable, and you want to use it by calling only the script name, the shebang must be included in the file.</p> <h2 id=the-shebang>The Shebang<a class=headerlink href=#the-shebang title="Permanent link">&para;</a></h2> <p>The in-file specification of the interpreter of that file, for example:</p> <div class=highlight><pre><span></span><code><a id=__codelineno-0-1 name=__codelineno-0-1 href=#__codelineno-0-1></a><span class=ch>#!/bin/bash</span>
<a id=__codelineno-0-2 name=__codelineno-0-2 href=#__codelineno-0-2></a><span class=nb>echo</span><span class=w> </span><span class=s2>&quot;Hello world...&quot;</span>
</code></pre></div> <p>This is interpreted by the kernel <sup id=fnref:1><a class=footnote-ref href=#fn:1>1</a></sup> of your system. In general, if a file is executable, but not an executable (binary) program, and such a line is present, the program specified after <code>#!</code> is started with the scriptname and all its arguments. These two characters <code>#</code> and <code>!</code> must be <strong>the first two bytes</strong> in the file!</p> <p>You can follow the process by using <code>echo</code> as a fake interpreter:</p> <div class=highlight><pre><span></span><code>#!/bin/echo
</code></pre></div> <p>We don't need a script body here, as the file will never be interpreted and executed by "<code>echo</code>". You can see what the Operating System does, it calls "<code>/bin/echo</code>" with the name of the executable file and following arguments.</p> <div class=highlight><pre><span></span><code>$ /home/bash/bin/test testword hello
/home/bash/bin/test testword hello
</code></pre></div> <p>The same way, with <code>#!/bin/bash</code> the shell "<code>/bin/bash</code>" is called with the script filename as an argument. It's the same as executing "<code>/bin/bash /home/bash/bin/test testword hello</code>"</p> <p>If the interpreter can be specified with arguments and how long it can be is system-specific (see <a href=http://www.in-ulm.de/~mascheck/various/shebang/ >#!-magic</a>). When Bash executes a file with a #!/bin/bash shebang, the shebang itself is ignored, since the first character is a hashmark "<code>#</code>", which indicates a comment. The shebang is for the operating system, not for the shell. Programs that don't ignore such lines, may not work as shebang driven interpreters.</p> <div class="admonition warning"> <p class=admonition-title>Warning</p> <p><u><strong>Attention:</strong></u>When the specified interpreter is unavailable or not executable (permissions), you usually get a "<code>bad interpreter</code>" error message., If you get nothing and it fails, check the shebang. Older Bash versions will respond with a "<code>no such file or directory</code>" error for a nonexistant interpreter specified by the shebang.</p> </div> <p><strong>Additional note:</strong> When you specify <code>#!/bin/sh</code> as shebang and that's a link to a Bash, then Bash will run in POSIX&reg; mode! See:</p> <ul> <li><a href=../bashbehaviour/ >Bash behaviour</a>.</li> </ul> <p>A common method is to specify a shebang like</p> <div class=highlight><pre><span></span><code>#!/usr/bin/env bash
</code></pre></div> <p>...which just moves the location of the potential problem to</p> <ul> <li>the <code>env</code> utility must be located in /usr/bin/</li> <li>the needed <code>bash</code> binary must be located in <code>PATH</code></li> </ul> <p>Which one you need, or whether you think which one is good, or bad, is up to you. There is no bulletproof portable way to specify an interpreter. <strong>It's a common misconception that it solves all problems. Period.</strong></p> <h2 id=the-standard-filedescriptors>The standard filedescriptors<a class=headerlink href=#the-standard-filedescriptors title="Permanent link">&para;</a></h2> <p>Once Initialized, every normal UNIX&reg;-program has <em>at least 3 open files</em>:</p> <ul> <li><strong>stdin</strong>: standard input</li> <li><strong>stdout</strong>: standard output</li> <li><strong>stderr</strong>: standard error output</li> </ul> <p>Usually, they're all connected to your terminal, stdin as input file (keyboard), stdout and stderr as output files (screen). When calling such a program, the invoking shell can change these filedescriptor connections away from the terminal to any other file (see redirection). Why two different output filedescriptors? It's convention to send error messages and warnings to stderr and only program output to stdout. This enables the user to decide if they want to see nothing, only the data, only the errors, or both - and where they want to see them.</p> <p>When you write a script:</p> <ul> <li>always read user-input from <code>stdin</code></li> <li>always write diagnostic/error/warning messages to <code>stderr</code></li> </ul> <p>To learn more about the standard filedescriptors, especially about redirection and piping, see:</p> <ul> <li><a href=../../howto/redirection_tutorial/ >An illustrated redirection tutorial</a></li> </ul> <h2 id=variable-names>Variable names<a class=headerlink href=#variable-names title="Permanent link">&para;</a></h2> <p>It's good practice to use lowercase names for your variables, as shell and system-variable names are usually all in UPPERCASE. However, you should avoid naming your variables any of the following (incomplete list!):</p> <table> <thead> <tr> <th></th> <th></th> <th></th> <th></th> <th></th> <th></th> </tr> </thead> <tbody> <tr> <td><code>BASH</code></td> <td><code>BASH_ARGC</code></td> <td><code>BASH_ARGV</code></td> <td><code>BASH_LINENO</code></td> <td><code>BASH_SOURCE</code></td> <td><code>BASH_VERSINFO</code></td> </tr> <tr> <td><code>BASH_VERSION</code></td> <td><code>COLUMNS</code></td> <td><code>DIRSTACK</code></td> <td><code>DISPLAY</code></td> <td><code>EDITOR</code></td> <td><code>EUID</code></td> </tr> <tr> <td><code>GROUPS</code></td> <td><code>HISTFILE</code></td> <td><code>HISTFILESIZE</code></td> <td><code>HISTSIZE</code></td> <td><code>HOME</code></td> <td><code>HOSTNAME</code></td> </tr> <tr> <td><code>IFS</code></td> <td><code>LANG</code></td> <td><code>LANGUAGE</code></td> <td><code>LC_ALL</code></td> <td><code>LINES</code></td> <td><code>LOGNAME</code></td> </tr> <tr> <td><code>LS_COLORS</code></td> <td><code>MACHTYPE</code></td> <td><code>MAILCHECK</code></td> <td><code>OLDPWD</code></td> <td><code>OPTERR</code></td> <td><code>OPTIND</code></td> </tr> <tr> <td><code>OSTYPE</code></td> <td><code>PATH</code></td> <td><code>PIPESTATUS</code></td> <td><code>PPID</code></td> <td><code>PROMPT_COMMAND</code></td> <td><code>PS1</code></td> </tr> <tr> <td><code>PS2</code></td> <td><code>PS4</code></td> <td><code>PS3</code></td> <td><code>PWD</code></td> <td><code>SHELL</code></td> <td><code>SHELLOPTS</code></td> </tr> <tr> <td><code>SHLVL</code></td> <td><code>TERM</code></td> <td><code>UID</code></td> <td><code>USER</code></td> <td><code>USERNAME</code></td> <td><code>XAUTHORITY</code></td> </tr> </tbody> </table> <p>This list is incomplete. <strong>The safest way is to use all-lowercase variable names.</strong></p> <h2 id=exit-codes>Exit codes<a class=headerlink href=#exit-codes title="Permanent link">&para;</a></h2> <p>Every program you start terminates with an exit code and reports
<a id=__codelineno-1-2 name=__codelineno-1-2 href=#__codelineno-1-2></a><span class=w> </span><span class=nb>echo</span><span class=w> </span><span class=s2>&quot;The user root was found&quot;</span>
<a id=__codelineno-1-3 name=__codelineno-1-3 href=#__codelineno-1-3></a><span class=k>else</span>
<a id=__codelineno-1-4 name=__codelineno-1-4 href=#__codelineno-1-4></a><span class=w> </span><span class=nb>echo</span><span class=w> </span><span class=s2>&quot;The user root was not found&quot;</span>
<a id=__codelineno-1-5 name=__codelineno-1-5 href=#__codelineno-1-5></a><span class=k>fi</span>
</code></pre></div> <p>A common decision making command is "<code>test</code>" or its equivalent "<code>[</code>". But note that, when calling test with the name "<code>[</code>", the square brackets are not part of the shell syntax, the left bracket <strong>is</strong> the test command!</p> <div class=highlight><pre><span></span><code><a id=__codelineno-2-1 name=__codelineno-2-1 href=#__codelineno-2-1></a><span class=k>if</span><span class=w> </span><span class=o>[</span><span class=w> </span><span class=s2>&quot;</span><span class=nv>$mystring</span><span class=s2>&quot;</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=s2>&quot;Hello world&quot;</span><span class=w> </span><span class=o>]</span><span class=p>;</span><span class=w> </span><span class=k>then</span>
<a id=__codelineno-2-2 name=__codelineno-2-2 href=#__codelineno-2-2></a><span class=w> </span><span class=nb>echo</span><span class=w> </span><span class=s2>&quot;Yeah dude, you entered the right words...&quot;</span>
<a id=__codelineno-2-3 name=__codelineno-2-3 href=#__codelineno-2-3></a><span class=k>else</span>
<a id=__codelineno-2-4 name=__codelineno-2-4 href=#__codelineno-2-4></a><span class=w> </span><span class=nb>echo</span><span class=w> </span><span class=s2>&quot;Eeeek - go away...&quot;</span>
<a id=__codelineno-2-5 name=__codelineno-2-5 href=#__codelineno-2-5></a><span class=k>fi</span>
</code></pre></div> <p>Read more about <a href=../../commands/classictest/ >the test command</a></p> <p>A common exit code check method uses the "<code>||</code>" or "<code>&amp;&amp;</code>" operators. This lets you execute a command based on whether or not the previous command completed successfully:</p> <div class=highlight><pre><span></span><code><a id=__codelineno-3-1 name=__codelineno-3-1 href=#__codelineno-3-1></a>grep<span class=w> </span>^root:<span class=w> </span>/etc/passwd<span class=w> </span>&gt;/dev/null<span class=w> </span><span class=o>||</span><span class=w> </span><span class=nb>echo</span><span class=w> </span><span class=s2>&quot;root was not found - check the pub at the corner.&quot;</span>
<a id=__codelineno-3-2 name=__codelineno-3-2 href=#__codelineno-3-2></a>which<span class=w> </span>vi<span class=w> </span><span class=o>&amp;&amp;</span><span class=w> </span><span class=nb>echo</span><span class=w> </span><span class=s2>&quot;Your favourite editor is installed.&quot;</span>
</code></pre></div> <p>Please, when your script exits on errors, provide a "<code>FALSE</code>" exit code, so others can check the script execution.</p> <h2 id=comments>Comments<a class=headerlink href=#comments title="Permanent link">&para;</a></h2> <p>In a larger, or complex script, it's wise to comment the code. Comments can help with debugging or tests. Comments start with the <code>#</code> character (hashmark) and continue to the end of the line:</p> <div class=highlight><pre><span></span><code><a id=__codelineno-4-1 name=__codelineno-4-1 href=#__codelineno-4-1></a><span class=ch>#!/bin/bash</span>
<a id=__codelineno-4-2 name=__codelineno-4-2 href=#__codelineno-4-2></a><span class=c1># This is a small script to say something.</span>
<a id=__codelineno-4-3 name=__codelineno-4-3 href=#__codelineno-4-3></a><span class=nb>echo</span><span class=w> </span><span class=s2>&quot;Be liberal in what you accept, and conservative in what you send&quot;</span><span class=w> </span><span class=c1># say something</span>
</code></pre></div> <p>The first thing was already explained, it's the so-called shebang, for the shell, <strong>only a comment</strong>. The second one is a comment from the beginning of the line, the third comment starts after a valid command. All three syntactically correct.</p> <h3 id=block-commenting>Block commenting<a class=headerlink href=#block-commenting title="Permanent link">&para;</a></h3> <p>To temporarily disable complete blocks of code you would normally have to prefix every line of that block with a <code>#</code> (hashmark) to make it a comment. There's a little trick, using the pseudo command <code>:</code> (colon) and input redirection. The <code>:</code> does nothing, it's a pseudo command, so it does not care about standard input. In the following code example, you want to test mail and logging, but not dump the database, or execute a shutdown:</p> <div class=highlight><pre><span></span><code><a id=__codelineno-5-1 name=__codelineno-5-1 href=#__codelineno-5-1></a><span class=ch>#!/bin/bash</span>
<a id=__codelineno-5-2 name=__codelineno-5-2 href=#__codelineno-5-2></a><span class=c1># Write info mails, do some tasks and bring down the system in a safe way</span>
<a id=__codelineno-5-3 name=__codelineno-5-3 href=#__codelineno-5-3></a><span class=nb>echo</span><span class=w> </span><span class=s2>&quot;System halt requested&quot;</span><span class=w> </span><span class=p>|</span><span class=w> </span>mail<span class=w> </span>-s<span class=w> </span><span class=s2>&quot;System halt&quot;</span><span class=w> </span>netadmin@example.com
<a id=__codelineno-5-4 name=__codelineno-5-4 href=#__codelineno-5-4></a>logger<span class=w> </span>-t<span class=w> </span>SYSHALT<span class=w> </span><span class=s2>&quot;System halt requested&quot;</span>
<a id=__codelineno-5-5 name=__codelineno-5-5 href=#__codelineno-5-5></a>
<a id=__codelineno-5-6 name=__codelineno-5-6 href=#__codelineno-5-6></a><span class=c1>##### The following &quot;code block&quot; is effectively ignored</span>
<a id=__codelineno-5-7 name=__codelineno-5-7 href=#__codelineno-5-7></a>:<span class=w> </span>&lt;&lt;<span class=s2>&quot;SOMEWORD&quot;</span>
<a id=__codelineno-5-8 name=__codelineno-5-8 href=#__codelineno-5-8></a>/etc/init.d/mydatabase<span class=w> </span>clean_stop
<a id=__codelineno-5-9 name=__codelineno-5-9 href=#__codelineno-5-9></a>mydatabase_dump<span class=w> </span>/var/db/db1<span class=w> </span>/mnt/fsrv0/backups/db1
<a id=__codelineno-5-10 name=__codelineno-5-10 href=#__codelineno-5-10></a>logger<span class=w> </span>-t<span class=w> </span>SYSHALT<span class=w> </span><span class=s2>&quot;System halt: pre-shutdown actions done, now shutting down the system&quot;</span>
<a id=__codelineno-5-11 name=__codelineno-5-11 href=#__codelineno-5-11></a>shutdown<span class=w> </span>-h<span class=w> </span>NOW
<a id=__codelineno-5-12 name=__codelineno-5-12 href=#__codelineno-5-12></a>SOMEWORD
<a id=__codelineno-5-13 name=__codelineno-5-13 href=#__codelineno-5-13></a><span class=c1>##### The ignored codeblock ends here</span>
</code></pre></div> <p>What happened? The <code>:</code> pseudo command was given some input by redirection (a here-document) - the pseudo command didn't care about it, effectively, the entire block was ignored.</p> <p>The here-document-tag was quoted here <strong>to avoid substitutions</strong> in the "commented" text! Check <a href=../../syntax/redirection/#tag_heredoc>redirection with here-documents</a> for more</p> <h2 id=variable-scope>Variable scope<a class=headerlink href=#variable-scope title="Permanent link">&para;</a></h2> <p>In Bash, the scope of user variables is generally <em>global</em>. That means, it does <strong>not</strong> matter whether a variable is set in the "main program" or in a "function", the variable is defined everywhere.</p> <p>Compare the following <em>equivalent</em> code snippets:</p> <div class=highlight><pre><span></span><code><a id=__codelineno-6-1 name=__codelineno-6-1 href=#__codelineno-6-1></a><span class=nv>myvariable</span><span class=o>=</span><span class=nb>test</span>
<a id=__codelineno-6-2 name=__codelineno-6-2 href=#__codelineno-6-2></a><span class=nb>echo</span><span class=w> </span><span class=nv>$myvariable</span>
</code></pre></div> <div class=highlight><pre><span></span><code><a id=__codelineno-7-1 name=__codelineno-7-1 href=#__codelineno-7-1></a>myfunction<span class=o>()</span><span class=w> </span><span class=o>{</span>
<a id=__codelineno-7-2 name=__codelineno-7-2 href=#__codelineno-7-2></a><span class=w> </span><span class=nv>myvariable</span><span class=o>=</span><span class=nb>test</span>
<a id=__codelineno-7-3 name=__codelineno-7-3 href=#__codelineno-7-3></a><span class=o>}</span>
<a id=__codelineno-7-4 name=__codelineno-7-4 href=#__codelineno-7-4></a>
<a id=__codelineno-7-5 name=__codelineno-7-5 href=#__codelineno-7-5></a>myfunction
<a id=__codelineno-7-6 name=__codelineno-7-6 href=#__codelineno-7-6></a><span class=nb>echo</span><span class=w> </span><span class=nv>$myvariable</span>
</code></pre></div> <p>In both cases, the variable <code>myvariable</code> is set and accessible from everywhere in that script, both in functions and in the "main program".</p> <p><strong><u>Attention:</u></strong> When you set variables in a child process, for example a <em>subshell</em>, they will be set there, but you will <strong>never</strong> have access to them outside of that subshell. One way to create a subshell is the pipe. It's all mentioned in a small article about <a href=../processtree/ >Bash in the processtree</a>!</p> <h3 id=local-variables>Local variables<a class=headerlink href=#local-variables title="Permanent link">&para;</a></h3> <p>Bash provides ways to make a variable's scope <em>local</em> to a function:</p> <ul> <li>Using the <code>local</code> keyword, or</li> <li>Using <code>declare</code> (which will <em>detect</em> when it was called from within a function and make the variable(s) local).</li> </ul> <div class=highlight><pre><span></span><code><a id=__codelineno-8-1 name=__codelineno-8-1 href=#__codelineno-8-1></a>myfunc<span class=o>()</span><span class=w> </span><span class=o>{</span>
<a id=__codelineno-8-2 name=__codelineno-8-2 href=#__codelineno-8-2></a><span class=nb>local</span><span class=w> </span><span class=nv>var</span><span class=o>=</span>VALUE
<a id=__codelineno-8-3 name=__codelineno-8-3 href=#__codelineno-8-3></a>
<a id=__codelineno-8-4 name=__codelineno-8-4 href=#__codelineno-8-4></a><span class=c1># alternative, only when used INSIDE a function</span>
<a id=__codelineno-8-5 name=__codelineno-8-5 href=#__codelineno-8-5></a><span class=nb>declare</span><span class=w> </span><span class=nv>var</span><span class=o>=</span>VALUE
<a id=__codelineno-8-6 name=__codelineno-8-6 href=#__codelineno-8-6></a>
<a id=__codelineno-8-7 name=__codelineno-8-7 href=#__codelineno-8-7></a>...
<a id=__codelineno-8-8 name=__codelineno-8-8 href=#__codelineno-8-8></a><span class=o>}</span>
</code></pre></div> <p>The <em>local</em> keyword (or declaring a variable using the <code>declare</code> command) tags a variable to be treated <em>completely local and separate</em> inside the function where it was declared:</p> <div class=highlight><pre><span></span><code><a id=__codelineno-9-1 name=__codelineno-9-1 href=#__codelineno-9-1></a><span class=nv>foo</span><span class=o>=</span>external
<a id=__codelineno-9-2 name=__codelineno-9-2 href=#__codelineno-9-2></a>
<a id=__codelineno-9-3 name=__codelineno-9-3 href=#__codelineno-9-3></a>printvalue<span class=o>()</span><span class=w> </span><span class=o>{</span>
<a id=__codelineno-9-4 name=__codelineno-9-4 href=#__codelineno-9-4></a><span class=nb>local</span><span class=w> </span><span class=nv>foo</span><span class=o>=</span>internal
<a id=__codelineno-9-5 name=__codelineno-9-5 href=#__codelineno-9-5></a>
<a id=__codelineno-9-6 name=__codelineno-9-6 href=#__codelineno-9-6></a><span class=nb>echo</span><span class=w> </span><span class=nv>$foo</span>
<a id=__codelineno-9-7 name=__codelineno-9-7 href=#__codelineno-9-7></a><span class=o>}</span>
<a id=__codelineno-9-8 name=__codelineno-9-8 href=#__codelineno-9-8></a>
<a id=__codelineno-9-9 name=__codelineno-9-9 href=#__codelineno-9-9></a>
<a id=__codelineno-9-10 name=__codelineno-9-10 href=#__codelineno-9-10></a><span class=c1># this will print &quot;external&quot;</span>
<a id=__codelineno-9-11 name=__codelineno-9-11 href=#__codelineno-9-11></a><span class=nb>echo</span><span class=w> </span><span class=nv>$foo</span>
<a id=__codelineno-9-12 name=__codelineno-9-12 href=#__codelineno-9-12></a>
<a id=__codelineno-9-13 name=__codelineno-9-13 href=#__codelineno-9-13></a><span class=c1># this will print &quot;internal&quot;</span>
<a id=__codelineno-9-14 name=__codelineno-9-14 href=#__codelineno-9-14></a>printvalue
<a id=__codelineno-9-15 name=__codelineno-9-15 href=#__codelineno-9-15></a>
<a id=__codelineno-9-16 name=__codelineno-9-16 href=#__codelineno-9-16></a><span class=c1># this will print - again - &quot;external&quot;</span>
<a id=__codelineno-9-17 name=__codelineno-9-17 href=#__codelineno-9-17></a><span class=nb>echo</span><span class=w> </span><span class=nv>$foo</span>
</code></pre></div> <h3 id=environment-variables>Environment variables<a class=headerlink href=#environment-variables title="Permanent link">&para;</a></h3> <p>The environment space is not directly related to the topic about scope, but it's worth mentioning.</p> <p>Every UNIX&reg; process has a so-called <em>environment</em>. Other items, in addition to variables, are saved there, the so-called <em>environment variables</em>. When a child process is created (in Bash e.g. by simply executing another program, say <code>ls</code> to list files), the whole environment <em>including the environment variables</em> is copied to the new process. Reading that from the other side means: <strong>Only variables that are part of the environment are available in the child process.</strong></p> <p>A variable can be tagged to be part of the environment using the <code>export</code> command:</p> <div class=highlight><pre><span></span><code><a id=__codelineno-10-1 name=__codelineno-10-1 href=#__codelineno-10-1></a><span class=c1># create a new variable and set it:</span>
<a id=__codelineno-10-2 name=__codelineno-10-2 href=#__codelineno-10-2></a><span class=c1># -&gt; This is a normal shell variable, not an environment variable!</span>
<a id=__codelineno-10-3 name=__codelineno-10-3 href=#__codelineno-10-3></a><span class=nv>myvariable</span><span class=o>=</span><span class=s2>&quot;Hello world.&quot;</span>
<a id=__codelineno-10-4 name=__codelineno-10-4 href=#__codelineno-10-4></a>
<a id=__codelineno-10-5 name=__codelineno-10-5 href=#__codelineno-10-5></a><span class=c1># make the variable visible to all child processes:</span>
<a id=__codelineno-10-6 name=__codelineno-10-6 href=#__codelineno-10-6></a><span class=c1># -&gt; Make it an environment variable: &quot;export&quot; it</span>
<a id=__codelineno-10-7 name=__codelineno-10-7 href=#__codelineno-10-7></a><span class=nb>export</span><span class=w> </span>myvariable
</code></pre></div> <p>Remember that the <em>exported</em> variable is a <strong>copy</strong>. There is no provision to "copy it back to the parent." See the article about <a href=../processtree/ >Bash in the process tree</a>!</p> <div class=footnote> <hr> <ol> <li id=fn:1> <p>under specific circumstances, also by the shell itself&#160;<a class=footnote-backref href=#fnref:1 title="Jump back to footnote 1 in the text">&#8617;</a></p> </li> </ol> </div> <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 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date">November 13, 2024</span> </span> <span class=md-source-file__fact> <span class=md-icon title=Created> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M14.47 15.08 11 13V7h1.5v5.25l3.08 1.83c-.41.28-.79.62-1.11 1m-1.39 4.84c-.36.05-.71.08-1.08.08-4.42 0-8-3.58-8-8s3.58-8 8-8 8 3.58 8 8c0 .37-.03.72-.08 1.08.69.1 1.33.32 1.92.64.1-.56.16-1.13.16-1.72 0-5.5-4.5-10-10-10S2 6.5 2 12s4.47 10 10 10c.59 0 1.16-.06 1.72-.16-.32-.59-.54-1.23-.64-1.92M18 15v3h-3v2h3v3h2v-3h3v-2h-3v-3z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date">November 13, 2024</span> </span> </aside> <h2 id=__comments>Comments</h2> <script src=https://giscus.app/client.js data-repo=flokoe/bash-hackers-wiki data-repo-id=R_kgDOJ3Nr6Q data-category="Giscus Page Comments" data-category-id=DIC_kwDOJ3Nr6c4CXq9t data-mapping=pathname data-strict=1 data-reactions-enabled=1 data-emit-metadata=0 data-input-position=top data-theme=preferred_color_scheme data-lang=en data-loading=lazy crossorigin=anonymous async>
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