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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/keywords/coproc/ rel=canonical><link href=../../grammar/parser_exec/ rel=prev><link href=../../../tags/ 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 coproc keyword - 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-coproc-keyword 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 coproc keyword </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
</code></pre></div> <h2 id=description>Description<a class=headerlink href=#description title="Permanent link">&para;</a></h2> <p>Bash 4.0 introduced <em>coprocesses</em>, a feature certainly familiar to ksh users. The <code>coproc</code> keyword starts a command as a background job, setting up pipes connected to both its stdin and stdout so that you can interact with it bidirectionally. Optionally, the co-process can have a name <code>NAME</code>. If <code>NAME</code> is given, the command that follows <strong>must be a compound command</strong>. If no <code>NAME</code> is given, then the command can be either simple or compound.</p> <p>The process ID of the shell spawned to execute the coprocess is available through the value of the variable named by <code>NAME</code> followed by a <code>_PID</code> suffix. For example, the variable name used to store the PID of a coproc started with no <code>NAME</code> given would be <code>COPROC_PID</code> (because <code>COPROC</code> is the default <code>NAME</code>). <a href=../../../commands/builtin/wait/ >wait</a> may be used to wait for the coprocess to terminate. Additionally, coprocesses may be manipulated through their <code>jobspec</code>.</p> <h3 id=return-status>Return status<a class=headerlink href=#return-status title="Permanent link">&para;</a></h3> <p>The return status of a coprocess is the exit status of its command.</p> <h3 id=redirections>Redirections<a class=headerlink href=#redirections title="Permanent link">&para;</a></h3> <p>The optional redirections are applied after the pipes have been set up. Some examples:</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># redirecting stderr in the pipe</span>
<a id=__codelineno-0-2 name=__codelineno-0-2 href=#__codelineno-0-2></a>$<span class=w> </span>coproc<span class=w> </span><span class=o>{</span><span class=w> </span>ls<span class=w> </span>thisfiledoesntexist<span class=p>;</span><span class=w> </span>read<span class=p>;</span><span class=w> </span><span class=o>}</span><span class=w> </span><span class=m>2</span>&gt;<span class=p>&amp;</span><span class=m>1</span>
<a id=__codelineno-0-3 name=__codelineno-0-3 href=#__codelineno-0-3></a><span class=o>[</span><span class=m>2</span><span class=o>]</span><span class=w> </span><span class=m>23084</span>
<a id=__codelineno-0-4 name=__codelineno-0-4 href=#__codelineno-0-4></a>$<span class=w> </span><span class=nv>IFS</span><span class=o>=</span><span class=w> </span><span class=nb>read</span><span class=w> </span>-ru<span class=w> </span><span class=si>${</span><span class=nv>COPROC</span><span class=p>[0]</span><span class=si>}</span><span class=w> </span>x<span class=p>;</span><span class=w> </span><span class=nb>printf</span><span class=w> </span><span class=s1>&#39;%s\n&#39;</span><span class=w> </span><span class=s2>&quot;</span><span class=nv>$x</span><span class=s2>&quot;</span>
<a id=__codelineno-0-5 name=__codelineno-0-5 href=#__codelineno-0-5></a>ls:<span class=w> </span>cannot<span class=w> </span>access<span class=w> </span>thisfiledoesntexist:<span class=w> </span>No<span class=w> </span>such<span class=w> </span>file<span class=w> </span>or<span class=w> </span>directory
</code></pre></div> <div class=highlight><pre><span></span><code><a id=__codelineno-1-1 name=__codelineno-1-1 href=#__codelineno-1-1></a><span class=c1>#let the output of the coprocess go to stdout</span>
<a id=__codelineno-1-2 name=__codelineno-1-2 href=#__codelineno-1-2></a>$<span class=w> </span><span class=o>{</span><span class=w> </span>coproc<span class=w> </span>mycoproc<span class=w> </span><span class=o>{</span><span class=w> </span>awk<span class=w> </span><span class=s1>&#39;{print &quot;foo&quot; $0;fflush()}&#39;</span><span class=p>;</span><span class=w> </span><span class=o>}</span><span class=w> </span>&gt;<span class=p>&amp;</span><span class=m>3</span><span class=p>;</span><span class=w> </span><span class=o>}</span><span class=w> </span><span class=m>3</span>&gt;<span class=p>&amp;</span><span class=m>1</span>
<a id=__codelineno-1-3 name=__codelineno-1-3 href=#__codelineno-1-3></a><span class=o>[</span><span class=m>2</span><span class=o>]</span><span class=w> </span><span class=m>23092</span>
<a id=__codelineno-1-4 name=__codelineno-1-4 href=#__codelineno-1-4></a>$<span class=w> </span><span class=nb>echo</span><span class=w> </span>bar<span class=w> </span>&gt;<span class=p>&amp;</span><span class=si>${</span><span class=nv>mycoproc</span><span class=p>[1]</span><span class=si>}</span>
<a id=__codelineno-1-5 name=__codelineno-1-5 href=#__codelineno-1-5></a>$<span class=w> </span>foobar
</code></pre></div> <p>Here we need to save the previous file descriptor of stdout, because by the time we redirect the fds of the coprocess, stdout has already been redirected to the pipe.</p> <h3 id=pitfalls>Pitfalls<a class=headerlink href=#pitfalls title="Permanent link">&para;</a></h3> <h4 id=avoid-the-final-pipeline-subshell>Avoid the final pipeline subshell<a class=headerlink href=#avoid-the-final-pipeline-subshell title="Permanent link">&para;</a></h4> <p>The traditional Ksh workaround to avoid the subshell when doing <code>command | while read</code> is to use a coprocess. Unfortunately, Bash's behavior differs.</p> <p>In Ksh you would do:</p> <div class=highlight><pre><span></span><code><a id=__codelineno-2-1 name=__codelineno-2-1 href=#__codelineno-2-1></a><span class=c1># ksh93 or mksh/pdksh derivatives</span>
<a id=__codelineno-2-2 name=__codelineno-2-2 href=#__codelineno-2-2></a>ls<span class=w> </span><span class=p>|&amp;</span><span class=w> </span><span class=c1># start a coprocess</span>
<a id=__codelineno-2-3 name=__codelineno-2-3 href=#__codelineno-2-3></a><span class=k>while</span><span class=w> </span><span class=nv>IFS</span><span class=o>=</span><span class=w> </span><span class=nb>read</span><span class=w> </span>-rp<span class=w> </span>file<span class=p>;</span><span class=w> </span><span class=k>do</span><span class=w> </span>print<span class=w> </span>-r<span class=w> </span>--<span class=w> </span><span class=s2>&quot;</span><span class=nv>$file</span><span class=s2>&quot;</span><span class=p>;</span><span class=w> </span><span class=k>done</span><span class=w> </span><span class=c1># read its output</span>
</code></pre></div> <p>In bash:</p> <div class=highlight><pre><span></span><code><a id=__codelineno-3-1 name=__codelineno-3-1 href=#__codelineno-3-1></a><span class=c1>#DOESN&#39;T WORK</span>
<a id=__codelineno-3-2 name=__codelineno-3-2 href=#__codelineno-3-2></a>$<span class=w> </span>coproc<span class=w> </span>ls
<a id=__codelineno-3-3 name=__codelineno-3-3 href=#__codelineno-3-3></a><span class=o>[</span><span class=m>1</span><span class=o>]</span><span class=w> </span><span class=m>23232</span>
<a id=__codelineno-3-4 name=__codelineno-3-4 href=#__codelineno-3-4></a>$<span class=w> </span><span class=k>while</span><span class=w> </span><span class=nv>IFS</span><span class=o>=</span><span class=w> </span><span class=nb>read</span><span class=w> </span>-ru<span class=w> </span><span class=si>${</span><span class=nv>COPROC</span><span class=p>[0]</span><span class=si>}</span><span class=w> </span>line<span class=p>;</span><span class=w> </span><span class=k>do</span><span class=w> </span><span class=nb>printf</span><span class=w> </span><span class=s1>&#39;%s\n&#39;</span><span class=w> </span><span class=s2>&quot;</span><span class=nv>$line</span><span class=s2>&quot;</span><span class=p>;</span><span class=w> </span><span class=k>done</span>
<a id=__codelineno-3-5 name=__codelineno-3-5 href=#__codelineno-3-5></a>bash:<span class=w> </span>read:<span class=w> </span>line:<span class=w> </span>invalid<span class=w> </span>file<span class=w> </span>descriptor<span class=w> </span>specification
<a id=__codelineno-3-6 name=__codelineno-3-6 href=#__codelineno-3-6></a><span class=o>[</span><span class=m>1</span><span class=o>]</span>+<span class=w> </span>Done<span class=w> </span>coproc<span class=w> </span>COPROC<span class=w> </span>ls
</code></pre></div> <p>By the time we start reading from the output of the coprocess, the file descriptor has been closed.</p> <p>See <a href=http://mywiki.wooledge.org/BashFAQ/024>this FAQ entry on Greg's wiki</a> for other pipeline subshell workarounds.</p> <h4 id=buffering>Buffering<a class=headerlink href=#buffering title="Permanent link">&para;</a></h4> <p>In the first example, we GNU awk's <code>fflush()</code> command. As always, when you use pipes the I/O operations are buffered. Let's see what happens with <code>sed</code>:</p> <div class=highlight><pre><span></span><code><a id=__codelineno-4-1 name=__codelineno-4-1 href=#__codelineno-4-1></a>$<span class=w> </span>coproc<span class=w> </span>sed<span class=w> </span>s/^/foo/
<a id=__codelineno-4-2 name=__codelineno-4-2 href=#__codelineno-4-2></a><span class=o>[</span><span class=m>1</span><span class=o>]</span><span class=w> </span><span class=m>22981</span>
<a id=__codelineno-4-3 name=__codelineno-4-3 href=#__codelineno-4-3></a>$<span class=w> </span><span class=nb>echo</span><span class=w> </span>bar<span class=w> </span>&gt;<span class=p>&amp;</span><span class=si>${</span><span class=nv>COPROC</span><span class=p>[1]</span><span class=si>}</span>
<a id=__codelineno-4-4 name=__codelineno-4-4 href=#__codelineno-4-4></a>$<span class=w> </span><span class=nb>read</span><span class=w> </span>-t<span class=w> </span><span class=m>3</span><span class=w> </span>-ru<span class=w> </span><span class=si>${</span><span class=nv>COPROC</span><span class=p>[0]</span><span class=si>}</span><span class=w> </span>_<span class=p>;</span><span class=w> </span><span class=o>((</span><span class=w> </span><span class=nv>$?</span><span class=w> </span>&gt;<span class=w> </span><span class=m>127</span><span class=w> </span><span class=o>))</span><span class=w> </span><span class=o>&amp;&amp;</span><span class=w> </span><span class=nb>echo</span><span class=w> </span><span class=s2>&quot;nothing read&quot;</span>
<a id=__codelineno-4-5 name=__codelineno-4-5 href=#__codelineno-4-5></a>nothing<span class=w> </span><span class=nb>read</span>
</code></pre></div> <p>Even though this example is the same as the first <code>awk</code> example, the <code>read</code> doesn't return because the output is waiting in a buffer.</p> <p>See <a href=http://mywiki.wooledge.org/BashFAQ/009>this faq entry on Greg's wiki</a> for some workarounds and more information on buffering issues.</p> <h4 id=background-processes>background processes<a class=headerlink href=#background-processes title="Permanent link">&para;</a></h4> <p>A coprocess' file descriptors are accessible only to the process from which the <code>coproc</code> was started. They are not inherited by subshells.</p> <p>Here is a not-so-meaningful illustration. Suppose we want to continuously read the output of a coprocess and <code>echo</code> the result:</p> <div class=highlight><pre><span></span><code><a id=__codelineno-5-1 name=__codelineno-5-1 href=#__codelineno-5-1></a><span class=c1>#NOT WORKING</span>
<a id=__codelineno-5-2 name=__codelineno-5-2 href=#__codelineno-5-2></a>$<span class=w> </span>coproc<span class=w> </span>awk<span class=w> </span><span class=s1>&#39;{print &quot;foo&quot; $0;fflush()}&#39;</span>
<a id=__codelineno-5-3 name=__codelineno-5-3 href=#__codelineno-5-3></a><span class=o>[</span><span class=m>2</span><span class=o>]</span><span class=w> </span><span class=m>23100</span>
<a id=__codelineno-5-4 name=__codelineno-5-4 href=#__codelineno-5-4></a>$<span class=w> </span><span class=k>while</span><span class=w> </span><span class=nv>IFS</span><span class=o>=</span><span class=w> </span><span class=nb>read</span><span class=w> </span>-ru<span class=w> </span><span class=si>${</span><span class=nv>COPROC</span><span class=p>[0]</span><span class=si>}</span><span class=w> </span>x<span class=p>;</span><span class=w> </span><span class=k>do</span><span class=w> </span><span class=nb>printf</span><span class=w> </span><span class=s1>&#39;%s\n&#39;</span><span class=w> </span><span class=s2>&quot;</span><span class=nv>$x</span><span class=s2>&quot;</span><span class=p>;</span><span class=w> </span><span class=k>done</span><span class=w> </span><span class=p>&amp;</span>
<a id=__codelineno-5-5 name=__codelineno-5-5 href=#__codelineno-5-5></a><span class=o>[</span><span class=m>3</span><span class=o>]</span><span class=w> </span><span class=m>23104</span>
<a id=__codelineno-5-6 name=__codelineno-5-6 href=#__codelineno-5-6></a>bash:<span class=w> </span>line<span class=w> </span><span class=m>243</span>:<span class=w> </span>read:<span class=w> </span><span class=m>61</span>:<span class=w> </span>invalid<span class=w> </span>file<span class=w> </span>descriptor:<span class=w> </span>Bad<span class=w> </span>file<span class=w> </span>descriptor
</code></pre></div> <p>This fails because the file descriptors created by the parent are not available to the subshell created by &amp;.</p> <p>A possible workaround:</p> <div class=highlight><pre><span></span><code><a id=__codelineno-6-1 name=__codelineno-6-1 href=#__codelineno-6-1></a><span class=c1>#WARNING: for illustration purpose ONLY</span>
<a id=__codelineno-6-2 name=__codelineno-6-2 href=#__codelineno-6-2></a><span class=c1># this is not the way to make the coprocess print its output</span>
<a id=__codelineno-6-3 name=__codelineno-6-3 href=#__codelineno-6-3></a><span class=c1># to stdout, see the redirections above.</span>
<a id=__codelineno-6-4 name=__codelineno-6-4 href=#__codelineno-6-4></a>$<span class=w> </span>coproc<span class=w> </span>awk<span class=w> </span><span class=s1>&#39;{print &quot;foo&quot; $0;fflush()}&#39;</span>
<a id=__codelineno-6-5 name=__codelineno-6-5 href=#__codelineno-6-5></a><span class=o>[</span><span class=m>2</span><span class=o>]</span><span class=w> </span><span class=m>23109</span>
<a id=__codelineno-6-6 name=__codelineno-6-6 href=#__codelineno-6-6></a>$<span class=w> </span><span class=nb>exec</span><span class=w> </span><span class=m>3</span>&lt;<span class=p>&amp;</span><span class=si>${</span><span class=nv>COPROC</span><span class=p>[0]</span><span class=si>}</span>
<a id=__codelineno-6-7 name=__codelineno-6-7 href=#__codelineno-6-7></a>$<span class=w> </span><span class=k>while</span><span class=w> </span><span class=nv>IFS</span><span class=o>=</span><span class=w> </span><span class=nb>read</span><span class=w> </span>-ru<span class=w> </span><span class=m>3</span><span class=w> </span>x<span class=p>;</span><span class=w> </span><span class=k>do</span><span class=w> </span><span class=nb>printf</span><span class=w> </span><span class=s1>&#39;%s\n&#39;</span><span class=w> </span><span class=s2>&quot;</span><span class=nv>$x</span><span class=s2>&quot;</span><span class=p>;</span><span class=w> </span><span class=k>done</span><span class=w> </span><span class=p>&amp;</span>
<a id=__codelineno-6-8 name=__codelineno-6-8 href=#__codelineno-6-8></a><span class=o>[</span><span class=m>3</span><span class=o>]</span><span class=w> </span><span class=m>23110</span>
<a id=__codelineno-6-9 name=__codelineno-6-9 href=#__codelineno-6-9></a>$<span class=w> </span><span class=nb>echo</span><span class=w> </span>bar<span class=w> </span>&gt;<span class=p>&amp;</span><span class=si>${</span><span class=nv>COPROC</span><span class=p>[1]</span><span class=si>}</span>
<a id=__codelineno-6-10 name=__codelineno-6-10 href=#__codelineno-6-10></a>$<span class=w> </span>foobar
</code></pre></div> <p>Here, fd 3 is inherited.</p> <h2 id=examples>Examples<a class=headerlink href=#examples title="Permanent link">&para;</a></h2> <h3 id=anonymous-coprocess>Anonymous Coprocess<a class=headerlink href=#anonymous-coprocess title="Permanent link">&para;</a></h3> <p>Unlike ksh, Bash doesn't have true anonymous coprocesses. Instead, Bash assigns FDs to a default array named <code>COPROC</code> if no <code>NAME</code> is supplied. Here's an example:</p> <div class=highlight><pre><span></span><code><a id=__codelineno-7-1 name=__codelineno-7-1 href=#__codelineno-7-1></a>$<span class=w> </span>coproc<span class=w> </span>awk<span class=w> </span><span class=s1>&#39;{print &quot;foo&quot; $0;fflush()}&#39;</span>
<a id=__codelineno-7-2 name=__codelineno-7-2 href=#__codelineno-7-2></a><span class=o>[</span><span class=m>1</span><span class=o>]</span><span class=w> </span><span class=m>22978</span>
</code></pre></div> <p>This command starts in the background, and <code>coproc</code> returns immediately. Two new file descriptors are now available via the <code>COPROC</code> array. We can send data to our command:</p> <div class=highlight><pre><span></span><code><a id=__codelineno-8-1 name=__codelineno-8-1 href=#__codelineno-8-1></a>$<span class=w> </span><span class=nb>echo</span><span class=w> </span>bar<span class=w> </span>&gt;<span class=p>&amp;</span><span class=si>${</span><span class=nv>COPROC</span><span class=p>[1]</span><span class=si>}</span>
</code></pre></div> <p>And then read its output:</p> <div class=highlight><pre><span></span><code><a id=__codelineno-9-1 name=__codelineno-9-1 href=#__codelineno-9-1></a>$<span class=w> </span><span class=nv>IFS</span><span class=o>=</span><span class=w> </span><span class=nb>read</span><span class=w> </span>-ru<span class=w> </span><span class=si>${</span><span class=nv>COPROC</span><span class=p>[0]</span><span class=si>}</span><span class=w> </span>x<span class=p>;</span><span class=w> </span><span class=nb>printf</span><span class=w> </span><span class=s1>&#39;%s\n&#39;</span><span class=w> </span><span class=s2>&quot;</span><span class=nv>$x</span><span class=s2>&quot;</span>
<a id=__codelineno-9-2 name=__codelineno-9-2 href=#__codelineno-9-2></a>foobar
</code></pre></div> <p>When we don't need our command anymore, we can kill it via its pid:</p> <div class=highlight><pre><span></span><code>$ kill $COPROC_PID
$
[1]+ Terminated coproc COPROC awk &#39;{print &quot;foo&quot; $0;fflush()}&#39;
</code></pre></div> <h3 id=named-coprocess>Named Coprocess<a class=headerlink href=#named-coprocess title="Permanent link">&para;</a></h3> <p>Using a named coprocess is simple. We just need a compound command (like when defining a function), and the resulting FDs will be assigned to the indexed array <code>NAME</code> we supply instead.</p> <div class=highlight><pre><span></span><code><a id=__codelineno-10-1 name=__codelineno-10-1 href=#__codelineno-10-1></a>$<span class=w> </span>coproc<span class=w> </span>mycoproc<span class=w> </span><span class=o>{</span><span class=w> </span>awk<span class=w> </span><span class=s1>&#39;{print &quot;foo&quot; $0;fflush()}&#39;</span><span class=w> </span><span class=p>;</span><span class=o>}</span>
<a id=__codelineno-10-2 name=__codelineno-10-2 href=#__codelineno-10-2></a><span class=o>[</span><span class=m>1</span><span class=o>]</span><span class=w> </span><span class=m>23058</span>
<a id=__codelineno-10-3 name=__codelineno-10-3 href=#__codelineno-10-3></a>$<span class=w> </span><span class=nb>echo</span><span class=w> </span>bar<span class=w> </span>&gt;<span class=p>&amp;</span><span class=si>${</span><span class=nv>mycoproc</span><span class=p>[1]</span><span class=si>}</span>
<a id=__codelineno-10-4 name=__codelineno-10-4 href=#__codelineno-10-4></a>$<span class=w> </span><span class=nv>IFS</span><span class=o>=</span><span class=w> </span><span class=nb>read</span><span class=w> </span>-ru<span class=w> </span><span class=si>${</span><span class=nv>mycoproc</span><span class=p>[0]</span><span class=si>}</span><span class=w> </span>x<span class=p>;</span><span class=w> </span><span class=nb>printf</span><span class=w> </span><span class=s1>&#39;%s\n&#39;</span><span class=w> </span><span class=s2>&quot;</span><span class=nv>$x</span><span class=s2>&quot;</span>
<a id=__codelineno-10-5 name=__codelineno-10-5 href=#__codelineno-10-5></a>foobar
<a id=__codelineno-10-6 name=__codelineno-10-6 href=#__codelineno-10-6></a>$<span class=w> </span><span class=nb>kill</span><span class=w> </span><span class=nv>$mycoproc_PID</span>
<a id=__codelineno-10-7 name=__codelineno-10-7 href=#__codelineno-10-7></a>$
<a id=__codelineno-10-8 name=__codelineno-10-8 href=#__codelineno-10-8></a><span class=o>[</span><span class=m>1</span><span class=o>]</span>+<span class=w> </span>Terminated<span class=w> </span>coproc<span class=w> </span>mycoproc<span class=w> </span><span class=o>{</span><span class=w> </span>awk<span class=w> </span><span class=s1>&#39;{print &quot;foo&quot; $0;fflush()}&#39;</span><span class=p>;</span><span class=w> </span><span class=o>}</span>
</code></pre></div> <h3 id=redirecting-the-output-of-a-script-to-a-file-and-to-the-screen>Redirecting the output of a script to a file and to the screen<a class=headerlink href=#redirecting-the-output-of-a-script-to-a-file-and-to-the-screen title="Permanent link">&para;</a></h3> <div class=highlight><pre><span></span><code><a id=__codelineno-11-1 name=__codelineno-11-1 href=#__codelineno-11-1></a><span class=ch>#!/bin/bash</span>
<a id=__codelineno-11-2 name=__codelineno-11-2 href=#__codelineno-11-2></a><span class=c1># we start tee in the background</span>
<a id=__codelineno-11-3 name=__codelineno-11-3 href=#__codelineno-11-3></a><span class=c1># redirecting its output to the stdout of the script</span>
<a id=__codelineno-11-4 name=__codelineno-11-4 href=#__codelineno-11-4></a><span class=o>{</span><span class=w> </span>coproc<span class=w> </span>tee<span class=w> </span><span class=o>{</span><span class=w> </span>tee<span class=w> </span>logfile<span class=w> </span><span class=p>;</span><span class=o>}</span><span class=w> </span>&gt;<span class=p>&amp;</span><span class=m>3</span><span class=w> </span><span class=p>;</span><span class=o>}</span><span class=w> </span><span class=m>3</span>&gt;<span class=p>&amp;</span><span class=m>1</span>
<a id=__codelineno-11-5 name=__codelineno-11-5 href=#__codelineno-11-5></a><span class=c1># we redirect stding and stdout of the script to our coprocess</span>
<a id=__codelineno-11-6 name=__codelineno-11-6 href=#__codelineno-11-6></a><span class=nb>exec</span><span class=w> </span>&gt;<span class=p>&amp;</span><span class=si>${</span><span class=nv>tee</span><span class=p>[1]</span><span class=si>}</span><span class=w> </span><span class=m>2</span>&gt;<span class=p>&amp;</span><span class=m>1</span>
</code></pre></div> <h2 id=portability-considerations>Portability considerations<a class=headerlink href=#portability-considerations title="Permanent link">&para;</a></h2> <ul> <li>The <code>coproc</code> keyword is not specified by POSIX(R)</li> <li>The <code>coproc</code> keyword appeared in Bash version 4.0-alpha</li> <li>The <code>-p</code> option to Bash's <code>print</code> loadable is a NOOP and not connected to Bash coprocesses in any way. It is only recognized as an option for ksh compatibility, and has no effect.</li> <li>The <code>-p</code> option to Bash's <code>read</code> builtin conflicts with that of all kshes and zsh. The equivalent in those shells is to add a <code>\?prompt</code> suffix to the first variable name argument to <code>read</code>. i.e., if the first variable name given contains a <code>?</code> character, the remainder of the argument is used as the prompt string. Since this feature is pointless and redundant, I suggest not using it in either shell. Simply precede the <code>read</code> command with a <code>printf %s prompt &gt;&amp;2</code>.</li> </ul> <h3 id=other-shells>Other shells<a class=headerlink href=#other-shells title="Permanent link">&para;</a></h3> <p>ksh93, mksh, zsh, and Bash all support something called "coprocesses" which all do approximately the same thing. ksh93 and mksh have virtually identical syntax and semantics for coprocs. A <em>list</em> operator: <code>|&amp;</code> is added to the language which runs the preceding <em>pipeline</em> as a coprocess (This is another reason not to use the special <code>|&amp;</code> pipe operator in Bash -- its syntax is conflicting). The <code>-p</code> option to the <code>read</code> and <code>print</code> builtins can then be used to read and write to the pipe of the coprocess (whose FD isn't yet known). Special redirects are added to move the last spawned coprocess to a different FD: <code>&lt;&amp;p</code> and <code>&gt;&amp;p</code>, at which point it can be accessed at the new FD using ordinary redirection, and another coprocess may then be started, again using <code>|&amp;</code>.</p> <p>zsh coprocesses are very similar to ksh except in the way they are started. zsh adds the shell reserved word <code>coproc</code> to the pipeline syntax (similar to the way Bash's <code>time</code> keyword works), so that the pipeline that follows is started as a coproc. The coproc's input and output FDs can then be accessed and moved using the same <code>read</code>/<code>print</code> <code>-p</code> and redirects used by the ksh shells.</p> <p>It is unfortunate that Bash chose to go against existing practice in their coproc implementation, especially considering it was the last of the major shells to incorporate this feature. However, Bash's method accomplishes the same without requiring nearly as much additional syntax. The <code>coproc</code> keyword is easy enough to wrap in a function such that it takes Bash code as an ordinary argument and/or stdin like <code>eval</code>. Coprocess functionality in other shells can be similarly wrapped to create a <code>COPROC</code> array automatically.</p> <h3 id=only-one-coprocess-at-a-time>Only one coprocess at a time<a class=headerlink href=#only-one-coprocess-at-a-time title="Permanent link">&para;</a></h3> <p>The title says it all, complain to the bug-bash mailing list if you want more. See <a href=http://lists.gnu.org/archive/html/bug-bash/2011-04/msg00056.html>http://lists.gnu.org/archive/html/bug-bash/2011-04/msg00056.html</a> for more details</p> <p>The ability to use multiple coprocesses in Bash is considered "experimental". Bash will throw an error if you attempt to start more than one. This may be overridden at compile-time with the <code>MULTIPLE_COPROCS</code> option. However, at this time there are still issues -- see the above mailing list discussion.</p> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">&para;</a></h2> <ul> <li><a href=http://www.ict.griffith.edu.au/anthony/info/shell/co-processes.hints>Anthony Thyssen's Coprocess Hints</a> - excellent
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