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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/syntax/expansion/cmdsubst/ rel=canonical><link href=../brace/ rel=prev><link href=../globs/ rel=next><link rel=icon href=../../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.5.40"><title>Command substitution - The Bash Hackers Wiki</title><link rel=stylesheet href=../../../assets/stylesheets/main.8c3ca2c6.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=#command-substitution 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> Command substitution </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=matchMe
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` <COMMANDS> `
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</code></pre></div> <p>The command substitution expands to the output of commands. These commands are executed in a subshell, and their <code>stdout</code> data is what the substitution syntax expands to.</p> <p>All <strong>trailing</strong> newlines are removed (below is an example for a workaround).</p> <p>In later steps, <strong>if not quoted</strong>, the results undergo <a href=../wordsplit/ >word splitting</a> and <a href=../globs/ >pathname expansion</a>. You have to remember that, because the word splitting will also remove embedded newlines and other <code>IFS</code> characters and break the results up into several words. Also you'll probably get unexpected pathname matches. <strong>If you need the literal results, quote the command substitution!</strong></p> <p>The second form <code>`COMMAND`</code> is more or less obsolete for Bash, since it has some trouble with nesting ("inner" backticks need to be escaped) and escaping characters. Use <code>$(COMMAND)</code>, it's also POSIX!</p> <p>When you <a href=../../ccmd/grouping_subshell/ >call an explicit subshell</a> <code>(COMMAND)</code> inside the command substitution <code>$()</code>, then take care, this way is <strong>wrong</strong>:</p> <div class=highlight><pre><span></span><code>$((COMMAND))
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</code></pre></div> <p>Why? because it collides with the syntax for <a href=../arith/ >arithmetic expansion</a>. You need to separate the command substitution from the inner <code>(COMMAND)</code>:</p> <div class=highlight><pre><span></span><code>$( (COMMAND) )
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</code></pre></div> <h2 id=specialities>Specialities<a class=headerlink href=#specialities title="Permanent link">¶</a></h2> <p>When the inner command is only an input redirection, and nothing else, for example</p> <div class=highlight><pre><span></span><code>$( <FILE )
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# or
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` <FILE `
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</code></pre></div> <p>then Bash attempts to read the given file and act just if the given command was <code>cat FILE</code>.</p> <h2 id=a-closer-look-at-the-two-forms>A closer look at the two forms<a class=headerlink href=#a-closer-look-at-the-two-forms title="Permanent link">¶</a></h2> <p>In general you really should only use the form <code>$()</code>, it's escaping-neutral, it's nestable, it's also POSIX. But take a look at the following code snips to decide yourself which form you need under specific circumstances:</p> <p><strong><u>Nesting</u></strong></p> <p>Backtick form <code>`...`</code> is not directly nestable. You will have to escape the "inner" backticks. Also, the deeper you go, the more escape characters you need. Ugly.</p> <div class=highlight><pre><span></span><code>echo `echo `ls`` # INCORRECT
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echo `echo \`ls\`` # CORRECT
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echo $(echo $(ls)) # CORRECT
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</code></pre></div> <p><strong><u>Parsing</u></strong></p> <p>All is based on the fact that the backquote-form is simple character substitution, while every <code>$()</code>-construct opens an own, subsequent parsing step. Everything inside <code>$()</code> is interpreted as if written normal on a commandline. No special escaping of <strong>nothing</strong> is needed:</p> <div class=highlight><pre><span></span><code>echo "$(echo "$(ls)")" # nested double-quotes - no problem
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</code></pre></div> <p><strong><u>Constructs you should avoid</u></strong></p> <p>It's not all shiny with <code>$()</code>, at least for my current Bash (<code>3.1.17(1)-release</code>. :!: <u><strong>Update:</strong> Fixed since <code>3.2-beta</code> together with a misinterpretion of '))' being recognized as arithmetic expansion by redduck666</u>). This command seems to incorrectly close the substitution step and echo prints "ls" and ")":</p> <div class=highlight><pre><span></span><code>echo $(
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# some comment ending with a )
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ls
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)
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</code></pre></div> <p>It seems that every closing ")" confuses this construct. Also a (very uncommon ;-)) construct like:</p> <div class=highlight><pre><span></span><code>echo $(read VAR; case "$var" in foo) blah ;; esac) # spits out some error, when it sees the ";;"
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# fixes it:
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echo $(read VAR; case "$var" in (foo) blah ;; esac) # will work, but just let it be, please ;-)
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</code></pre></div> <p><strong><u>Conclusion:</u></strong></p> <p>In general, the <code>$()</code> should be the preferred method:</p> <ul> <li>it's clean syntax</li> <li>it's intuitive syntax</li> <li>it's more readable</li> <li>it's nestable</li> <li>its inner parsing is separate</li> </ul> <h2 id=examples>Examples<a class=headerlink href=#examples title="Permanent link">¶</a></h2> <p><strong>To get the date:</strong></p> <div class=highlight><pre><span></span><code>DATE="$(date)"
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</code></pre></div> <p><strong>To copy a file and get <code>cp</code> error output:</strong></p> <div class=highlight><pre><span></span><code>COPY_OUTPUT="$(cp file.txt /some/where 2>&1)"
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</code></pre></div> <p>Attention: Here, you need to redirect <code>cp</code> <code>STDERR</code> to its <code>STDOUT</code> target, because command substitution only catches <code>STDOUT</code>!</p> <p><strong>Catch stdout and preserve trailing newlines:</strong></p> <div class=highlight><pre><span></span><code>var=$(echo -n $'\n'); echo -n "$var"; # $var == ""
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var=$(echo -n $'\n'; echo -n x); var="${var%x}"; echo -n "$var" # $var == "\n"
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</code></pre></div> <p>This adds "x" to the output, which prevents the trailing newlines of the previous commands' output from being deleted by <code>$()</code>.</p> <p>By removing this "x" later on, we are left with the previous commands' output with its trailing newlines.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li>Internal: <a href=../intro/ >Introduction to expansion and substitution</a></li> <li>Internal: <a href=../../../scripting/obsolete/ >Obsolete and deprecated syntax</a></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 0-8 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">October 12, 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">October 12, 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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