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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 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class=md-tabs__link> Dict </a> </li> <li class=md-tabs__item> <a href=../../../howto/calculate-dc/ class=md-tabs__link> Howto </a> </li> <li class=md-tabs__item> <a href=../../../internals/shell_options/ class=md-tabs__link> Internals </a> </li> <li class=md-tabs__item> <a href=../../../misc/bashphorisms/ class=md-tabs__link> Misc </a> </li> <li class=md-tabs__item> <a href=../../../scripting/bashbehaviour/ class=md-tabs__link> Scripting </a> </li> <li class=md-tabs__item> <a href=../../../snipplets/ class=md-tabs__link> Snipplets </a> </li> <li class="md-tabs__item md-tabs__item--active"> <a href=../../arith_expr/ class=md-tabs__link> Syntax </a> </li> <li class=md-tabs__item> <a href=../../../tags/ class=md-tabs__link> Tags </a> </li> </ul> </div> </nav> <main class=md-main data-md-component=main> <div class="md-main__inner md-grid"> <div class="md-sidebar md-sidebar--primary" data-md-component=sidebar data-md-type=navigation> <div class=md-sidebar__scrollwrap> <div 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2.3 5.2 1 1.3-1 1-3.3-.7-5.2-1.6-1.6-3.9-2.3-5.2-1m-10.8-8.1c-.7 1.3.3 2.9 2.3 3.9 1.6 1 3.6.7 4.3-.7.7-1.3-.3-2.9-2.3-3.9-2-.6-3.6-.3-4.3.7m32.4 35.6c-1.6 1.3-1 4.3 1.3 6.2 2.3 2.3 5.2 2.6 6.5 1 1.3-1.3.7-4.3-1.3-6.2-2.2-2.3-5.2-2.6-6.5-1m-11.4-14.7c-1.6 1-1.6 3.6 0 5.9s4.3 3.3 5.6 2.3c1.6-1.3 1.6-3.9 0-6.2-1.4-2.3-4-3.3-5.6-2"/></svg> </div> <div class=md-source__repository> flokoe/bash-hackers-wiki </div> </a> </div> <ul class=md-nav__list data-md-scrollfix> <li class=md-nav__item> <a href=../../.. class=md-nav__link> <span class=md-ellipsis> Start </span> </a> </li> <li class="md-nav__item md-nav__item--nested"> <input class="md-nav__toggle md-toggle " type=checkbox id=__nav_2> <label class=md-nav__link for=__nav_2 id=__nav_2_label tabindex=0> <span class=md-ellipsis> Commands </span> <span class="md-nav__icon md-icon"></span> </label> <nav class=md-nav data-md-level=1 aria-labelledby=__nav_2_label aria-expanded=false> <label class=md-nav__title for=__nav_2> <span class="md-nav__icon md-icon"></span> Commands </label> <ul class=md-nav__list data-md-scrollfix> <li class=md-nav__item> <a href=../../../commands/classictest/ class=md-nav__link> <span class=md-ellipsis> The classic test command </span> </a> </li> <li class="md-nav__item md-nav__item--nested"> <input class="md-nav__toggle md-toggle " type=checkbox id=__nav_2_2> <label class=md-nav__link for=__nav_2_2 id=__nav_2_2_label tabindex=0> <span class=md-ellipsis> Builtin </span> <span class="md-nav__icon md-icon"></span> </label> <nav class=md-nav data-md-level=2 aria-labelledby=__nav_2_2_label aria-expanded=false> <label class=md-nav__title for=__nav_2_2> <span class="md-nav__icon md-icon"></span> Builtin </label> <ul class=md-nav__list data-md-scrollfix> <li class=md-nav__item> <a href=../../../commands/builtin/caller/ class=md-nav__link> <span class=md-ellipsis> The caller builtin command </span> </a> </li> <li class=md-nav__item> <a href=../../../commands/builtin/cd/ class=md-nav__link> <span class=md-ellipsis> The cd builtin command </span> </a> </li> <li class=md-nav__item> <a href=../../../commands/builtin/declare/ class=md-nav__link> <span class=md-ellipsis> The declare builtin command </span> </a> </li> <li class=md-nav__item> <a href=../../../commands/builtin/echo/ class=md-nav__link> <span class=md-ellipsis> The echo builtin command </span> </a> </li> <li class=md-nav__item> <a href=../../../commands/builtin/eval/ class=md-nav__link> <span class=md-ellipsis> The eval builtin command </span> </a> </li> <li class=md-nav__item> <a href=../../../commands/builtin/exec/ class=md-nav__link> <span class=md-ellipsis> The exec builtin command </span> </a> </li> <li class=md-nav__item> <a href=../../../commands/builtin/exit/ class=md-nav__link> <span class=md-ellipsis> The exit builtin command </span> </a> </li> <li class=md-nav__item> <a href=../../../commands/builtin/export/ class=md-nav__link> <span class=md-ellipsis> The export builtin command </span> </a> </li> <li class=md-nav__item> <a href=../../../commands/builtin/kill/ class=md-nav__link> <span class=md-ellipsis> The kill builtin command </span> </a> </li> <li class=md-nav__item> <a href=../../../commands/builtin/let/ class=md-nav__link> <span class=md-ellipsis> The let builtin command </span> </a> </li> <li class=md-nav__item> <a href=../../../commands/builtin/local/ class=md-nav__link> <span class=md-ellipsis> The local builtin command </span> </a> </li> <li class=md-nav__item> <a href=../../../commands/builtin/mapfile/ class=md-nav__link> <span class=md-ellipsis> The mapfile builtin command </span> </a> </li> <li class=md-nav__item> <a href=../../../commands/builtin/printf/ class=md-nav__link> <span class=md-ellipsis> The printf command </span> </a> </li> <li class=md-nav__item> <a href=../../../commands/builtin/read/ class=md-nav__link> <span class=md-ellipsis> The read builtin command </span> </a> </li> <li class=md-nav__item> <a href=../../../commands/builtin/readonly/ class=md-nav__link> <span class=md-ellipsis> The readonly builtin command </span> </a> </li> <li class=md-nav__item> <a href=../../../commands/builtin/return/ class=md-nav__link> <span class=md-ellipsis> The return builtin command </span> </a> </li> <li class=md-nav__item> <a href=../../../commands/builtin/set/ class=md-nav__link> <span class=md-ellipsis> The set builtin command </span> </a> </li> <li class=md-nav__item> <a href=../../../commands/builtin/shift/ class=md-nav__link> <span class=md-ellipsis> The shift builtin command </span> </a> </li> <li class=md-nav__item> <a href=../../../commands/builtin/shopt/ class=md-nav__link> <span class=md-ellipsis> The shopt builtin command </span> </a> </li> <li class=md-nav__item> <a href=../../../commands/builtin/trap/ class=md-nav__link> <span class=md-ellipsis> The trap builtin command </span> </a> </li> <li class=md-nav__item> <a href=../../../commands/builtin/unset/ class=md-nav__link> <span class=md-ellipsis> The unset builtin command </span> </a> </li> <li class=md-nav__item> <a href=../../../commands/builtin/wait/ class=md-nav__link> <span class=md-ellipsis> The wait builtin command </span> </a> </li> </ul> </nav> </li> </ul> </nav> </li> <li class="md-nav__item md-nav__item--nested"> <input class="md-nav__toggle md-toggle " type=checkbox id=__nav_3> <label class=md-nav__link for=__nav_3 id=__nav_3_label tabindex=0> <span class=md-ellipsis> Dict </span> <span class="md-nav__icon md-icon"></span> </label> <nav class=md-nav data-md-level=1 aria-labelledby=__nav_3_label aria-expanded=false> <label class=md-nav__title for=__nav_3> <span class="md-nav__icon md-icon"></span> Dict </label> <ul class=md-nav__list data-md-scrollfix> <li class=md-nav__item> <a href=../../../dict/directory/ class=md-nav__link> <span class=md-ellipsis> Directory </span> </a> </li> <li class=md-nav__item> <a href=../../../dict/end_of_options/ class=md-nav__link> <span class=md-ellipsis> End of Options </span> </a> </li> <li class=md-nav__item> <a href=../../../dict/exit_status/ class=md-nav__link> <span class=md-ellipsis> Exit Status </span> </a> </li> <li class=md-nav__item> <a href=../../../dict/file/ class=md-nav__link> <span class=md-ellipsis> File </span> </a> </li> <li class=md-nav__item> <a href=../../../dict/filetimes/ class=md-nav__link> <span class=md-ellipsis> File timestamp </span> </a> </li> <li class=md-nav__item> <a href=../../../dict/globbing/ class=md-nav__link> <span class=md-ellipsis> Globbing </span> </a> </li> <li class=md-nav__item> <a href=../../../dict/hardlink/ class=md-nav__link> <span class=md-ellipsis> Hardlink </span> </a> </li> <li class=md-nav__item> <a href=../../../dict/interpreter_directive/ class=md-nav__link> <span class=md-ellipsis> Interpreter Directive </span> </a> </li> <li class=md-nav__item> <a href=../../../dict/parameter/ class=md-nav__link> <span class=md-ellipsis> Parameter </span> </a> </li> <li class=md-nav__item> <a href=../../../dict/posix/ class=md-nav__link> <span class=md-ellipsis> POSIX </span> </a> </li> <li class=md-nav__item> <a href=../../../dict/shell/ class=md-nav__link> <span class=md-ellipsis> Shell </span> </a> </li> <li class=md-nav__item> <a href=../../../dict/special_file/ class=md-nav__link> <span class=md-ellipsis> Special file </span> </a> </li> <li class=md-nav__item> <a href=../../../dict/symlink/ class=md-nav__link> <span class=md-ellipsis> Symlink </span> </a> </li> </ul> </nav> </li> <li class="md-nav__item md-nav__item--nested"> <input class="md-nav__toggle md-toggle " type=checkbox id=__nav_4> <label class=md-nav__link for=__nav_4 id=__nav_4_label tabindex=0> <span class=md-ellipsis> Howto </span> <span class="md-nav__icon md-icon"></span> </label> <nav class=md-nav data-md-level=1 aria-labelledby=__nav_4_label aria-expanded=false> <label class=md-nav__title for=__nav_4> <span class="md-nav__icon md-icon"></span> Howto </label> <ul class=md-nav__list data-md-scrollfix> <li class=md-nav__item> <a href=../../../howto/calculate-dc/ class=md-nav__link> <span class=md-ellipsis> Calculating with dc </span> </a> </li> <li class=md-nav__item> <a href=../../../howto/collapsing_functions/ class=md-nav__link> <span class=md-ellipsis> Collapsing Functions </span> </a> </li> <li class=md-nav__item> <a href=../../../howto/conffile/ class=md-nav__link> <span class=md-ellipsis> Config files for your script </span> </a> </li> <li class=md-nav__item> <a href=../../../howto/dissectabadoneliner/ class=md-nav__link> <span class=md-ellipsis> Dissect a bad oneliner </span> </a> </li> <li class=md-nav__item> <a href=../../../howto/edit-ed/ class=md-nav__link> <span class=md-ellipsis> Editing files via scripts with ed </span> </a> </li> <li class=md-nav__item> <a href=../../../howto/getopts_tutorial/ class=md-nav__link> <span class=md-ellipsis> Small getopts tutorial </span> </a> </li> <li class=md-nav__item> <a href=../../../howto/mutex/ class=md-nav__link> <span class=md-ellipsis> Lock your script (against parallel execution) </span> </a> </li> <li class=md-nav__item> <a href=../../../howto/pax/ class=md-nav__link> <span class=md-ellipsis> pax - the POSIX archiver </span> </a> </li> <li class=md-nav__item> <a href=../../../howto/redirection_tutorial/ class=md-nav__link> <span class=md-ellipsis> Illustrated Redirection Tutorial </span> </a> </li> <li class=md-nav__item> <a href=../../../howto/testing-your-scripts/ class=md-nav__link> <span class=md-ellipsis> Testing your scripts </span> </a> </li> </ul> </nav> </li> <li class="md-nav__item md-nav__item--nested"> <input class="md-nav__toggle md-toggle " type=checkbox id=__nav_5> <label class=md-nav__link for=__nav_5 id=__nav_5_label tabindex=0> <span class=md-ellipsis> Internals </span> <span class="md-nav__icon md-icon"></span> </label> <nav class=md-nav data-md-level=1 aria-labelledby=__nav_5_label aria-expanded=false> <label class=md-nav__title for=__nav_5> <span class="md-nav__icon md-icon"></span> Internals </label> <ul class=md-nav__list data-md-scrollfix> <li class=md-nav__item> <a href=../../../internals/shell_options/ class=md-nav__link> <span class=md-ellipsis> List of shell options </span> </a> </li> </ul> </nav> </li> <li class="md-nav__item md-nav__item--nested"> <input class="md-nav__toggle md-toggle " type=checkbox id=__nav_6> <label class=md-nav__link for=__nav_6 id=__nav_6_label tabindex=0> <span class=md-ellipsis> Misc </span> <span class="md-nav__icon md-icon"></span> </label> <nav class=md-nav data-md-level=1 aria-labelledby=__nav_6_label aria-expanded=false> <label class=md-nav__title for=__nav_6> <span class="md-nav__icon md-icon"></span> Misc </label> <ul class=md-nav__list data-md-scrollfix> <li class=md-nav__item> <a href=../../../misc/bashphorisms/ class=md-nav__link> <span class=md-ellipsis> The Bashphorisms </span> </a> </li> <li class=md-nav__item> <a href=../../../misc/readthesourceluke/ class=md-nav__link> <span class=md-ellipsis> Readthesourceluke </span> </a> </li> <li class=md-nav__item> <a href=../../../misc/shell_humor/ class=md-nav__link> <span class=md-ellipsis> Shell Humor </span> </a> </li> </ul> </nav> </li> <li class="md-nav__item md-nav__item--nested"> <input class="md-nav__toggle md-toggle " type=checkbox id=__nav_7> <label class=md-nav__link for=__nav_7 id=__nav_7_label tabindex=0> <span class=md-ellipsis> Scripting </span> <span class="md-nav__icon md-icon"></span> </label> <nav class=md-nav data-md-level=1 aria-labelledby=__nav_7_label aria-expanded=false> <label class=md-nav__title for=__nav_7> <span class="md-nav__icon md-icon"></span> Scripting </label> <ul class=md-nav__list data-md-scrollfix> <li class=md-nav__item> <a href=../../../scripting/bashbehaviour/ class=md-nav__link> <span class=md-ellipsis> Bash's behaviour </span> </a> </li> <li class=md-nav__item> <a href=../../../scripting/bashchanges/ class=md-nav__link> <span class=md-ellipsis> Bash changes </span> </a> </li> <li class=md-nav__item> <a href=../../../scripting/basics/ class=md-nav__link> <span class=md-ellipsis> The basics of shell scripting </span> </a> </li> <li class=md-nav__item> <a href=../../../scripting/debuggingtips/ class=md-nav__link> <span class=md-ellipsis> Debugging a script </span> </a> </li> <li class=md-nav__item> <a href=../../../scripting/newbie_traps/ class=md-nav__link> <span class=md-ellipsis> Beginner Mistakes </span> </a> </li> <li class=md-nav__item> <a href=../../../scripting/nonportable/ class=md-nav__link> <span class=md-ellipsis> Portability talk </span> </a> </li> <li class=md-nav__item> <a href=../../../scripting/obsolete/ class=md-nav__link> <span class=md-ellipsis> Obsolete and deprecated syntax </span> </a> </li> <li class=md-nav__item> <a href=../../../scripting/posparams/ class=md-nav__link> <span class=md-ellipsis> Handling positional parameters </span> </a> </li> <li class=md-nav__item> <a href=../../../scripting/processtree/ class=md-nav__link> <span class=md-ellipsis> Bash and the process tree </span> </a> </li> <li class=md-nav__item> <a href=../../../scripting/style/ class=md-nav__link> <span class=md-ellipsis> Scripting with style </span> </a> </li> <li class=md-nav__item> <a href=../../../scripting/terminalcodes/ class=md-nav__link> <span class=md-ellipsis> Terminal codes (ANSI/VT100) introduction </span> </a> </li> </ul> </nav> </li> <li class="md-nav__item md-nav__item--nested"> <input class="md-nav__toggle md-toggle " type=checkbox id=__nav_8> <label class=md-nav__link for=__nav_8 id=__nav_8_label tabindex=0> <span class=md-ellipsis> Snipplets </span> <span class="md-nav__icon md-icon"></span> </label> <nav class=md-nav data-md-level=1 aria-labelledby=__nav_8_label aria-expanded=false> <label class=md-nav__title for=__nav_8> <span class="md-nav__icon md-icon"></span> Snipplets </label> <ul class=md-nav__list data-md-scrollfix> <li class=md-nav__item> <a href=../../../snipplets/ class=md-nav__link> <span class=md-ellipsis> Small code snipplets </span> </a> </li> <li class=md-nav__item> <a href=../../../snipplets/add_color_to_your_scripts/ class=md-nav__link> <span class=md-ellipsis> Add Color to your scripts </span> </a> </li> <li class=md-nav__item> <a href=../../../snipplets/awkcsv/ class=md-nav__link> <span class=md-ellipsis> Using awk to deal with CSV that uses quoted/unquoted delimiters </span> </a> </li> <li class=md-nav__item> <a href=../../../snipplets/filesize/ class=md-nav__link> <span class=md-ellipsis> Show size of a file </span> </a> </li> <li class=md-nav__item> <a href=../../../snipplets/kill_bg_job_without_message/ class=md-nav__link> <span class=md-ellipsis> Kill a background job without a message </span> </a> </li> <li class=md-nav__item> <a href=../../../snipplets/largestfile/ class=md-nav__link> <span class=md-ellipsis> Get largest file </span> </a> </li> <li class=md-nav__item> <a href=../../../snipplets/pause_command/ class=md-nav__link> <span class=md-ellipsis> Pausing a script (like MSDOS pause command) </span> </a> </li> <li class=md-nav__item> <a href=../../../snipplets/prargs/ class=md-nav__link> <span class=md-ellipsis> Print argument list for testing </span> </a> </li> <li class=md-nav__item> <a href=../../../snipplets/print_horizontal_line/ class=md-nav__link> <span class=md-ellipsis> Print a horizontal line </span> </a> </li> <li class=md-nav__item> <a href=../../../snipplets/rndstr/ class=md-nav__link> <span class=md-ellipsis> Print a random string or select random elements </span> </a> </li> <li class=md-nav__item> <a href=../../../snipplets/screen_saverestore/ class=md-nav__link> <span class=md-ellipsis> Save and restore terminal/screen content </span> </a> </li> <li class=md-nav__item> <a href=../../../snipplets/ssh_fetchkeys/ class=md-nav__link> <span class=md-ellipsis> Fetching SSH hostkeys without interaction </span> </a> </li> <li class=md-nav__item> <a href=../../../snipplets/ssh_local_var/ class=md-nav__link> <span class=md-ellipsis> Run some bash commands with SSH remotely using local variables </span> </a> </li> <li class=md-nav__item> <a href=../../../snipplets/wrapperargs/ class=md-nav__link> <span class=md-ellipsis> Generate code with own arguments properly quoted </span> </a> </li> <li class=md-nav__item> <a href=../../../snipplets/xclip/ class=md-nav__link> <span class=md-ellipsis> X-Clipboard on Commandline </span> </a> </li> </ul> </nav> </li> <li class="md-nav__item md-nav__item--active md-nav__item--section md-nav__item--nested"> <input class="md-nav__toggle md-toggle " type=checkbox id=__nav_9 checked> <label class=md-nav__link for=__nav_9 id=__nav_9_label tabindex> <span class=md-ellipsis> Syntax </span> <span class="md-nav__icon md-icon"></span> </label> <nav class=md-nav data-md-level=1 aria-labelledby=__nav_9_label aria-expanded=true> <label class=md-nav__title for=__nav_9> <span class="md-nav__icon md-icon"></span> Syntax </label> <ul class=md-nav__list data-md-scrollfix> <li class=md-nav__item> <a href=../../arith_expr/ class=md-nav__link> <span class=md-ellipsis> Arithmetic expressions </span> </a> </li> <li class=md-nav__item> <a href=../../arrays/ class=md-nav__link> <span class=md-ellipsis> Arrays </span> </a> </li> <li class=md-nav__item> <a href=../../basicgrammar/ class=md-nav__link> <span class=md-ellipsis> Basic grammar rules of Bash </span> </a> </li> <li class=md-nav__item> <a href=../../pattern/ class=md-nav__link> <span class=md-ellipsis> Patterns and pattern matching </span> </a> </li> <li class=md-nav__item> <a href=../../pe/ class=md-nav__link> <span class=md-ellipsis> Parameter expansion </span> </a> </li> <li class=md-nav__item> <a href=../../quoting/ class=md-nav__link> <span class=md-ellipsis> Quotes and escaping </span> </a> </li> <li class=md-nav__item> <a href=../../redirection/ class=md-nav__link> <span class=md-ellipsis> Redirection </span> </a> </li> <li class=md-nav__item> <a href=../../shellvars/ class=md-nav__link> <span class=md-ellipsis> Special parameters and shell variables </span> </a> </li> <li class=md-nav__item> <a href=../../words/ class=md-nav__link> <span class=md-ellipsis> Words... </span> </a> </li> <li class="md-nav__item md-nav__item--section md-nav__item--nested"> <input class="md-nav__toggle md-toggle " type=checkbox id=__nav_9_10> <label class=md-nav__link for=__nav_9_10 id=__nav_9_10_label tabindex> <span class=md-ellipsis> Ccmd </span> <span class="md-nav__icon md-icon"></span> </label> <nav class=md-nav data-md-level=2 aria-labelledby=__nav_9_10_label aria-expanded=false> <label class=md-nav__title for=__nav_9_10> <span class="md-nav__icon md-icon"></span> Ccmd </label> <ul class=md-nav__list data-md-scrollfix> <li class=md-nav__item> <a href=../../ccmd/arithmetic_eval/ class=md-nav__link> <span class=md-ellipsis> Arithmetic evaluation (command) </span> </a> </li> <li class=md-nav__item> <a href=../../ccmd/c_for/ class=md-nav__link> <span class=md-ellipsis> The C-style for-loop </span> </a> </li> <li class=md-nav__item> <a href=../../ccmd/case/ class=md-nav__link> <span class=md-ellipsis> The case statement </span> </a> </li> <li class=md-nav__item> <a href=../../ccmd/classic_for/ class=md-nav__link> <span class=md-ellipsis> The classic for-loop </span> </a> </li> <li class=md-nav__item> <a href=../../ccmd/conditional_expression/ class=md-nav__link> <span class=md-ellipsis> The conditional expression </span> </a> </li> <li class=md-nav__item> <a href=../../ccmd/grouping_plain/ class=md-nav__link> <span class=md-ellipsis> Grouping commands </span> </a> </li> <li class=md-nav__item> <a href=../../ccmd/grouping_subshell/ class=md-nav__link> <span class=md-ellipsis> Grouping commands in a subshell </span> </a> </li> <li class=md-nav__item> <a href=../../ccmd/if_clause/ class=md-nav__link> <span class=md-ellipsis> The if-clause </span> </a> </li> <li class=md-nav__item> <a href=../../ccmd/intro/ class=md-nav__link> <span class=md-ellipsis> Bash compound commands </span> </a> </li> <li class=md-nav__item> <a href=../../ccmd/until_loop/ class=md-nav__link> <span class=md-ellipsis> The until loop </span> </a> </li> <li class=md-nav__item> <a href=../../ccmd/user_select/ class=md-nav__link> <span class=md-ellipsis> User selections </span> </a> </li> <li class=md-nav__item> <a href=../../ccmd/while_loop/ class=md-nav__link> <span class=md-ellipsis> The while-loop </span> </a> </li> </ul> </nav> </li> <li class="md-nav__item md-nav__item--section md-nav__item--nested"> <input class="md-nav__toggle md-toggle " type=checkbox id=__nav_9_11> <label class=md-nav__link for=__nav_9_11 id=__nav_9_11_label tabindex> <span class=md-ellipsis> Expansion </span> <span class="md-nav__icon md-icon"></span> </label> <nav class=md-nav data-md-level=2 aria-labelledby=__nav_9_11_label aria-expanded=false> <label class=md-nav__title for=__nav_9_11> <span class="md-nav__icon md-icon"></span> Expansion </label> <ul class=md-nav__list data-md-scrollfix> <li class=md-nav__item> <a href=../../expansion/arith/ class=md-nav__link> <span class=md-ellipsis> Arithmetic expansion </span> </a> </li> <li class=md-nav__item> <a href=../../expansion/brace/ class=md-nav__link> <span class=md-ellipsis> Brace expansion </span> </a> </li> <li class=md-nav__item> <a href=../../expansion/cmdsubst/ class=md-nav__link> <span class=md-ellipsis> Command substitution </span> </a> </li> <li class=md-nav__item> <a href=../../expansion/globs/ class=md-nav__link> <span class=md-ellipsis> Pathname expansion (globbing) </span> </a> </li> <li class=md-nav__item> <a href=../../expansion/intro/ class=md-nav__link> <span class=md-ellipsis> Expansions and substitutions </span> </a> </li> <li class=md-nav__item> <a href=../../expansion/proc_subst/ class=md-nav__link> <span class=md-ellipsis> Process substitution </span> </a> </li> <li class=md-nav__item> <a href=../../expansion/tilde/ class=md-nav__link> <span class=md-ellipsis> Tilde expansion </span> </a> </li> <li class=md-nav__item> <a href=../../expansion/wordsplit/ class=md-nav__link> <span class=md-ellipsis> Word splitting </span> </a> </li> </ul> </nav> </li> <li class="md-nav__item md-nav__item--section md-nav__item--nested"> <input class="md-nav__toggle md-toggle " type=checkbox id=__nav_9_12> <label class=md-nav__link for=__nav_9_12 id=__nav_9_12_label tabindex> <span class=md-ellipsis> Grammar </span> <span class="md-nav__icon md-icon"></span> </label> <nav class=md-nav data-md-level=2 aria-labelledby=__nav_9_12_label aria-expanded=false> <label class=md-nav__title for=__nav_9_12> <span class="md-nav__icon md-icon"></span> Grammar </label> <ul class=md-nav__list data-md-scrollfix> <li class=md-nav__item> <a href=../../grammar/parser_exec/ class=md-nav__link> <span class=md-ellipsis> Parser exec </span> </a> </li> </ul> </nav> </li> <li class="md-nav__item md-nav__item--active md-nav__item--section md-nav__item--nested"> <input class="md-nav__toggle md-toggle " type=checkbox id=__nav_9_13 checked> <label class=md-nav__link for=__nav_9_13 id=__nav_9_13_label tabindex> <span class=md-ellipsis> Keywords </span> <span class="md-nav__icon md-icon"></span> </label> <nav class=md-nav data-md-level=2 aria-labelledby=__nav_9_13_label aria-expanded=true> <label class=md-nav__title for=__nav_9_13> <span class="md-nav__icon md-icon"></span> Keywords </label> <ul class=md-nav__list data-md-scrollfix> <li class="md-nav__item md-nav__item--active"> <input class="md-nav__toggle md-toggle" type=checkbox id=__toc> <label class="md-nav__link md-nav__link--active" for=__toc> <span class=md-ellipsis> The coproc keyword </span> <span class="md-nav__icon md-icon"></span> </label> <a href=./ class="md-nav__link md-nav__link--active"> <span class=md-ellipsis> The coproc keyword </span> </a> <nav class="md-nav md-nav--secondary" aria-label="Table of contents"> <label class=md-nav__title for=__toc> <span class="md-nav__icon md-icon"></span> Table of contents </label> <ul class=md-nav__list data-md-component=toc data-md-scrollfix> <li class=md-nav__item> <a href=#synopsis class=md-nav__link> <span class=md-ellipsis> Synopsis </span> </a> </li> <li class=md-nav__item> <a href=#description class=md-nav__link> <span class=md-ellipsis> Description </span> </a> <nav class=md-nav aria-label=Description> <ul class=md-nav__list> <li class=md-nav__item> <a href=#return-status class=md-nav__link> <span class=md-ellipsis> Return status </span> </a> </li> <li class=md-nav__item> <a href=#redirections class=md-nav__link> <span class=md-ellipsis> Redirections </span> </a> </li> <li class=md-nav__item> <a href=#pitfalls class=md-nav__link> <span class=md-ellipsis> Pitfalls </span> </a> <nav class=md-nav aria-label=Pitfalls> <ul class=md-nav__list> <li class=md-nav__item> <a href=#avoid-the-final-pipeline-subshell class=md-nav__link> <span class=md-ellipsis> Avoid the final pipeline subshell </span> </a> </li> <li class=md-nav__item> <a href=#buffering class=md-nav__link> <span class=md-ellipsis> Buffering </span> </a> </li> <li class=md-nav__item> <a href=#background-processes class=md-nav__link> <span class=md-ellipsis> background processes </span> </a> </li> </ul> </nav> </li> </ul> </nav> </li> <li class=md-nav__item> <a href=#examples class=md-nav__link> <span class=md-ellipsis> Examples </span> </a> <nav class=md-nav aria-label=Examples> <ul class=md-nav__list> <li class=md-nav__item> <a href=#anonymous-coprocess class=md-nav__link> <span class=md-ellipsis> Anonymous Coprocess </span> </a> </li> <li class=md-nav__item> <a href=#named-coprocess class=md-nav__link> <span class=md-ellipsis> Named Coprocess </span> </a> </li> <li class=md-nav__item> <a href=#redirecting-the-output-of-a-script-to-a-file-and-to-the-screen class=md-nav__link> <span class=md-ellipsis> Redirecting the output of a script to a file and to the screen </span> </a> </li> </ul> </nav> </li> <li class=md-nav__item> <a href=#portability-considerations class=md-nav__link> <span class=md-ellipsis> Portability considerations </span> </a> <nav class=md-nav aria-label="Portability considerations"> <ul class=md-nav__list> <li class=md-nav__item> <a href=#other-shells class=md-nav__link> <span class=md-ellipsis> Other shells </span> </a> </li> <li class=md-nav__item> <a href=#only-one-coprocess-at-a-time class=md-nav__link> <span class=md-ellipsis> Only one coprocess at a time </span> </a> </li> </ul> </nav> </li> <li class=md-nav__item> <a href=#see-also class=md-nav__link> <span class=md-ellipsis> See also </span> </a> </li> </ul> </nav> </li> </ul> </nav> </li> </ul> </nav> </li> <li class=md-nav__item> <a href=../../../tags/ class=md-nav__link> <span class=md-ellipsis> Tags </span> </a> </li> </ul> </nav> </div> </div> </div> <div class="md-sidebar md-sidebar--secondary" data-md-component=sidebar data-md-type=toc> <div class=md-sidebar__scrollwrap> <div class=md-sidebar__inner> <nav class="md-nav md-nav--secondary" aria-label="Table of contents"> <label class=md-nav__title for=__toc> <span class="md-nav__icon md-icon"></span> Table of contents </label> <ul class=md-nav__list data-md-component=toc data-md-scrollfix> <li class=md-nav__item> <a href=#synopsis class=md-nav__link> <span class=md-ellipsis> Synopsis </span> </a> </li> <li class=md-nav__item> <a href=#description class=md-nav__link> <span class=md-ellipsis> Description </span> </a> <nav class=md-nav aria-label=Description> <ul class=md-nav__list> <li class=md-nav__item> <a href=#return-status class=md-nav__link> <span class=md-ellipsis> Return status </span> </a> </li> <li class=md-nav__item> <a href=#redirections class=md-nav__link> <span class=md-ellipsis> Redirections </span> </a> </li> <li class=md-nav__item> <a href=#pitfalls class=md-nav__link> <span class=md-ellipsis> Pitfalls </span> </a> <nav class=md-nav aria-label=Pitfalls> <ul class=md-nav__list> <li class=md-nav__item> <a href=#avoid-the-final-pipeline-subshell class=md-nav__link> <span class=md-ellipsis> Avoid the final pipeline subshell </span> </a> </li> <li class=md-nav__item> <a href=#buffering class=md-nav__link> <span class=md-ellipsis> Buffering </span> </a> </li> <li class=md-nav__item> <a href=#background-processes class=md-nav__link> <span class=md-ellipsis> background processes </span> </a> </li> </ul> </nav> </li> </ul> </nav> </li> <li class=md-nav__item> <a href=#examples class=md-nav__link> <span class=md-ellipsis> Examples </span> </a> <nav class=md-nav aria-label=Examples> <ul class=md-nav__list> <li class=md-nav__item> <a href=#anonymous-coprocess class=md-nav__link> <span class=md-ellipsis> Anonymous Coprocess </span> </a> </li> <li class=md-nav__item> <a href=#named-coprocess class=md-nav__link> <span class=md-ellipsis> Named Coprocess </span> </a> </li> <li class=md-nav__item> <a href=#redirecting-the-output-of-a-script-to-a-file-and-to-the-screen class=md-nav__link> <span class=md-ellipsis> Redirecting the output of a script to a file and to the screen </span> </a> </li> </ul> </nav> </li> <li class=md-nav__item> <a href=#portability-considerations class=md-nav__link> <span class=md-ellipsis> Portability considerations </span> </a> <nav class=md-nav aria-label="Portability considerations"> <ul class=md-nav__list> <li class=md-nav__item> <a href=#other-shells class=md-nav__link> <span class=md-ellipsis> Other shells </span> </a> </li> <li class=md-nav__item> <a href=#only-one-coprocess-at-a-time class=md-nav__link> <span class=md-ellipsis> Only one coprocess at a time </span> </a> </li> </ul> </nav> </li> <li class=md-nav__item> <a href=#see-also class=md-nav__link> <span class=md-ellipsis> See also </span> </a> </li> </ul> </nav> </div> </div> </div> <div class=md-content data-md-component=content> <article class="md-content__inner md-typeset"> <a href=https://github.com/flokoe/bash-hackers-wiki/edit/main/docs/syntax/keywords/coproc.md title="Edit this page" class="md-content__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M10 20H6V4h7v5h5v3.1l2-2V8l-6-6H6c-1.1 0-2 .9-2 2v16c0 1.1.9 2 2 2h4zm10.2-7c.1 0 .3.1.4.2l1.3 1.3c.2.2.2.6 0 .8l-1 1-2.1-2.1 1-1c.1-.1.2-.2.4-.2m0 3.9L14.1 23H12v-2.1l6.1-6.1z"/></svg> </a> <a href=https://github.com/flokoe/bash-hackers-wiki/raw/main/docs/syntax/keywords/coproc.md title="View source of this page" class="md-content__button md-icon"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M17 18c.56 0 1 .44 1 1s-.44 1-1 1-1-.44-1-1 .44-1 1-1m0-3c-2.73 0-5.06 1.66-6 4 .94 2.34 3.27 4 6 4s5.06-1.66 6-4c-.94-2.34-3.27-4-6-4m0 6.5a2.5 2.5 0 0 1-2.5-2.5 2.5 2.5 0 0 1 2.5-2.5 2.5 2.5 0 0 1 2.5 2.5 2.5 2.5 0 0 1-2.5 2.5M9.27 20H6V4h7v5h5v4.07c.7.08 1.36.25 2 .49V8l-6-6H6a2 2 0 0 0-2 2v16a2 2 0 0 0 2 2h4.5a8.2 8.2 0 0 1-1.23-2"/></svg> </a> <h1 id=the-coproc-keyword>The coproc keyword<a class=headerlink href=#the-coproc-keyword title="Permanent link">&para;</a></h1> <h2 id=synopsis>Synopsis<a class=headerlink href=#synopsis title="Permanent link">&para;</a></h2> <div class=highlight><pre><span></span><code> coproc [NAME] command [redirections]
</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 summary of everything around the topic</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 7, 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 7, 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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