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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/redirection/ rel=canonical><link href=../quoting/ rel=prev><link href=../shellvars/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.5.44"><title>Redirection - 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=#redirection 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> Redirection </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("(prefers-color-scheme: light)"),input=
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cat some_file.txt 2>/dev/null
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</code></pre></div> <p>Whenever you <strong>reference</strong> a descriptor, to point to its current target file, then you use a "<code>&</code>" followed by a the descriptor number:</p> <div class=highlight><pre><span></span><code># this executes the echo-command and redirects its normal output (stdout) to the standard error target
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echo "There was an error" 1>&2
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</code></pre></div> <p>The redirection operation can be <strong>anywhere</strong> in a simple command, so these examples are equivalent:</p> <div class=highlight><pre><span></span><code>cat foo.txt bar.txt >new.txt
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cat >new.txt foo.txt bar.txt
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>new.txt cat foo.txt bar.txt
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</code></pre></div> <div class="admonition info"> <p class=admonition-title>important</p> <p>Every redirection operator takes one or two words as operands. If you have to use operands (e.g. filenames to redirect to) that contain spaces you <strong>must</strong> quote them!</p> </div> <h2 id=valid-redirection-targets-and-sources>Valid redirection targets and sources<a class=headerlink href=#valid-redirection-targets-and-sources title="Permanent link">¶</a></h2> <p>This syntax is recognized whenever a <code>TARGET</code> or a <code>SOURCE</code> specification (like below in the details descriptions) is used.</p> <table> <thead> <tr> <th>Syntax</th> <th>Description</th> </tr> </thead> <tbody> <tr> <td><code>FILENAME</code></td> <td>references a normal, ordinary filename from the filesystem (which can of course be a FIFO, too. Simply everything you can reference in the filesystem)</td> </tr> <tr> <td><code>&N</code></td> <td>references the current target/source of the filedescriptor <code>N</code> ("duplicates" the filedescriptor)</td> </tr> <tr> <td><code>&-</code></td> <td>closes the redirected filedescriptor, useful instead of <code>> /dev/null</code> constructs (<code>> &-</code>)</td> </tr> <tr> <td><code>/dev/fd/N</code></td> <td>duplicates the filedescriptor <code>N</code>, if <code>N</code> is a valid integer</td> </tr> <tr> <td><code>/dev/stdin</code></td> <td>duplicates filedescriptor 0 (<code>stdin</code>)</td> </tr> <tr> <td><code>/dev/stdout</code></td> <td>duplicates filedescriptor 1 (<code>stdout</code>)</td> </tr> <tr> <td><code>/dev/stderr</code></td> <td>duplicates filedescriptor 2 (<code>stderr</code>)</td> </tr> <tr> <td><code>/dev/tcp/HOST/PORT</code></td> <td>assuming <code>HOST</code> is a valid hostname or IP address, and <code>PORT</code> is a valid port number or service name: redirect from/to the corresponding TCP socket</td> </tr> <tr> <td><code>/dev/udp/HOST/PORT</code></td> <td>assuming <code>HOST</code> is a valid hostname or IP address, and <code>PORT</code> is a valid port number or service name: redirect from/to the corresponding UDP socket</td> </tr> </tbody> </table> <p>If a target/source specification fails to open, the whole redirection operation fails. Avoid referencing file descriptors above 9, since you may collide with file descriptors Bash uses internally.</p> <h2 id=redirecting-output>Redirecting output<a class=headerlink href=#redirecting-output title="Permanent link">¶</a></h2> <div class=highlight><pre><span></span><code>N > TARGET
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</code></pre></div> <p>This redirects the file descriptor number <code>N</code> to the target <code>TARGET</code>. If <code>N</code> is omitted, <code>stdout</code> is assumed (FD 1). The <code>TARGET</code> is <strong>truncated</strong> before writing starts.</p> <p>If the option <code>noclobber</code> is set with <a href=../../commands/builtin/set/ >the set builtin</a>, with cause the redirection to fail, when <code>TARGET</code> names a regular file that already exists. You can manually override that behaviour by forcing overwrite with the redirection operator <code>>|</code> instead of <code>></code>.</p> <h2 id=appending-redirected-output>Appending redirected output<a class=headerlink href=#appending-redirected-output title="Permanent link">¶</a></h2> <div class=highlight><pre><span></span><code>N >> TARGET
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</code></pre></div> <p>This redirects the file descriptor number <code>N</code> to the target <code>TARGET</code>. If <code>N</code> is omitted, <code>stdout</code> is assumed (FD 1). The <code>TARGET</code> is <strong>not truncated</strong> before writing starts.</p> <h2 id=redirecting-output-and-error-output>Redirecting output and error output<a class=headerlink href=#redirecting-output-and-error-output title="Permanent link">¶</a></h2> <div class=highlight><pre><span></span><code>&> TARGET
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>& TARGET
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</code></pre></div> <p>This special syntax redirects both, <code>stdout</code> and <code>stderr</code> to the specified target. It's <strong>equivalent</strong> to</p> <div class=highlight><pre><span></span><code>> TARGET 2>&1
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</code></pre></div> <p>Since Bash4, there's <code>&>>TARGET</code>, which is equivalent to <code>>> TARGET 2>&1</code>.</p> <div class="admonition info"> <p class=admonition-title>important</p> <p>This syntax is deprecated and should not be used. See the page about <a href=../../scripting/obsolete/ >obsolete and deprecated syntax</a>.</p> </div> <h2 id=appending-redirected-output-and-error-output>Appending redirected output and error output<a class=headerlink href=#appending-redirected-output-and-error-output title="Permanent link">¶</a></h2> <p>To append the cumulative redirection of <code>stdout</code> and <code>stderr</code> to a file you simply do</p> <div class=highlight><pre><span></span><code>>> FILE 2>&1
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&>> FILE
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</code></pre></div> <h2 id=transporting-stdout-and-stderr-through-a-pipe>Transporting stdout and stderr through a pipe<a class=headerlink href=#transporting-stdout-and-stderr-through-a-pipe title="Permanent link">¶</a></h2> <div class=highlight><pre><span></span><code>COMMAND1 2>&1 | COMMAND2
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COMMAND1 |& COMMAND2
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</code></pre></div> <h2 id=redirecting-input>Redirecting input<a class=headerlink href=#redirecting-input title="Permanent link">¶</a></h2> <div class=highlight><pre><span></span><code>N < SOURCE
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</code></pre></div> <p>The input descriptor <code>N</code> uses <code>SOURCE</code> as its data source. If <code>N</code> is omitted, filedescriptor 0 (<code>stdin</code>) is assumed.</p> <h2 id=here-documents>Here documents<a class=headerlink href=#here-documents title="Permanent link">¶</a></h2> <p><bookmark:tag_heredoc></p> <div class=highlight><pre><span></span><code><<TAG
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...
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TAG
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<<-TAG
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...
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TAG
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</code></pre></div> <p>A here-document is an input redirection using source data specified directly at the command line (or in the script), no "external" source. The redirection-operator <code><<</code> is used together with a tag <code>TAG</code> that's used to mark the end of input later:</p> <div class=highlight><pre><span></span><code># display help
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cat <<EOF
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Sorry...
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No help available yet for $PROGRAM.
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Hehe...
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EOF
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</code></pre></div> <p>As you see, substitutions are possible. To be precise, the following substitutions and expansions are performed in the here-document data:</p> <ul> <li><a href=../pe/ >Parameter expansion</a></li> <li><a href=../expansion/cmdsubst/ >Command substitution</a></li> <li><a href=../expansion/arith/ >Arithmetic expansion</a></li> </ul> <p>You can avoid that by quoting the tag:</p> <div class=highlight><pre><span></span><code>cat <<"EOF"
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This won't be expanded: $PATH
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EOF
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</code></pre></div> <p>Last but not least, if the redirection operator <code><<</code> is followed by a <code>-</code> (dash), all <strong>leading TAB</strong> from the document data will be ignored. This might be useful to have optical nice code also when using here-documents.</p> <p>The tag you use <strong>must</strong> be the only word in the line, to be recognized as end-of-here-document marker.</p> <div class="admonition info"> <p class=admonition-title>info</p> <p>It seems that here-documents (tested on versions <code>1.14.7</code>, <code>2.05b</code> and <code>3.1.17</code>) are correctly terminated when there is an EOF before the end-of-here-document tag. The reason is unknown, but it seems to be done on purpose. Bash 4 introduced a warning message when end-of-file is seen before the tag is reached.</p> </div> <h2 id=here-strings>Here strings<a class=headerlink href=#here-strings title="Permanent link">¶</a></h2> <div class=highlight><pre><span></span><code><<< WORD
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</code></pre></div> <p>The here-strings are a variation of the here-documents. The word <code>WORD</code> is taken for the input redirection:</p> <div class=highlight><pre><span></span><code>cat <<< "Hello world... $NAME is here..."
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</code></pre></div> <p>Just beware to quote the <code>WORD</code> if it contains spaces. Otherwise the rest will be given as normal parameters.</p> <p>The here-string will append a newline (<code>\n</code>) to the data.</p> <h2 id=multiple-redirections>Multiple redirections<a class=headerlink href=#multiple-redirections title="Permanent link">¶</a></h2> <p>More redirection operations can occur in a line of course. The order is <strong>important</strong>! They're evaluated from <strong>left to right</strong>. If you want to redirect both, <code>stderr</code> and <code>stdout</code> to the same file (like <code>/dev/null</code>, to hide it), this is <strong>the wrong way</strong>:</p> <div class=highlight><pre><span></span><code><a id=__codelineno-0-1 name=__codelineno-0-1 href=#__codelineno-0-1></a><span class=c1># { echo OUTPUT; echo ERRORS >&2; } is to simulate something that outputs to STDOUT and STDERR</span>
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<a id=__codelineno-0-2 name=__codelineno-0-2 href=#__codelineno-0-2></a><span class=c1># you can test with it</span>
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<a id=__codelineno-0-3 name=__codelineno-0-3 href=#__codelineno-0-3></a><span class=o>{</span><span class=w> </span><span class=nb>echo</span><span class=w> </span>OUTPUT<span class=p>;</span><span class=w> </span><span class=nb>echo</span><span class=w> </span>ERRORS<span class=w> </span>><span class=p>&</span><span class=m>2</span><span class=p>;</span><span class=w> </span><span class=o>}</span><span class=w> </span><span class=m>2</span>><span class=p>&</span><span class=m>1</span><span class=w> </span><span class=m>1</span>>/dev/null
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</code></pre></div> <p>Why? Relatively easy:</p> <ul> <li>initially, <code>stdout</code> points to your terminal (you read it)</li> <li>same applies to <code>stderr</code>, it's connected to your terminal</li> <li><code>2>&1</code> redirects <code>stderr</code> away from the terminal to the target for <code>stdout</code>: <strong>the terminal</strong> (again...)</li> <li><code>1>/dev/null</code> redirects <code>stdout</code> away from your terminal to the file <code>/dev/null</code></li> </ul> <p>What remains? <code>stdout</code> goes to <code>/dev/null</code>, <code>stderr</code> still (or better: "again") goes to the terminal. You have to swap the order to make it do what you want:</p> <div class=highlight><pre><span></span><code><a id=__codelineno-1-1 name=__codelineno-1-1 href=#__codelineno-1-1></a><span class=o>{</span><span class=w> </span><span class=nb>echo</span><span class=w> </span>OUTPUT<span class=p>;</span><span class=w> </span><span class=nb>echo</span><span class=w> </span>ERRORS<span class=w> </span>><span class=p>&</span><span class=m>2</span><span class=p>;</span><span class=w> </span><span class=o>}</span><span class=w> </span><span class=m>1</span>>/dev/null<span class=w> </span><span class=m>2</span>><span class=p>&</span><span class=m>1</span>
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</code></pre></div> <h2 id=examples>Examples<a class=headerlink href=#examples title="Permanent link">¶</a></h2> <p>How to make a program quiet (assuming all output goes to <code>STDOUT</code> and <code>STDERR</code>?</p> <div class=highlight><pre><span></span><code>command >/dev/null 2>&1
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</code></pre></div> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li>Internal: <a href=../../howto/redirection_tutorial/ >Illustrated Redirection Tutorial</a></li> <li>Internal: <a href=../../commands/builtin/set/#tag_noclobber>The noclobber option</a></li> <li>Internal: <a href=../../commands/builtin/exec/ >The exec builtin command</a></li> <li>Internal: <a href=../grammar/parser_exec/ >Simple commands parsing and execution</a></li> <li>Internal: <a href=../expansion/proc_subst/ >Process substitution syntax</a></li> <li>Internal: <a href=../../scripting/obsolete/ >Obsolete and deprecated syntax</a></li> <li>Internal: <a href=../../scripting/nonportable/ >Nonportable syntax and command uses</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">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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