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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--active md-nav__item--section md-nav__item--nested"> <input class="md-nav__toggle md-toggle " type=checkbox id=__nav_9_11 checked> <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=true> <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=../arith/ class=md-nav__link> <span class=md-ellipsis> Arithmetic expansion </span> </a> </li> <li class=md-nav__item> <a href=../brace/ class=md-nav__link> <span class=md-ellipsis> Brace expansion </span> </a> </li> <li class=md-nav__item> <a href=../cmdsubst/ class=md-nav__link> <span class=md-ellipsis> Command substitution </span> </a> </li> <li class=md-nav__item> <a href=../globs/ class=md-nav__link> <span class=md-ellipsis> Pathname expansion (globbing) </span> </a> </li> <li class=md-nav__item> <a href=../intro/ class=md-nav__link> <span class=md-ellipsis> Expansions and substitutions </span> </a> </li> <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> Process substitution </span> <span class="md-nav__icon md-icon"></span> </label> <a href=./ class="md-nav__link md-nav__link--active"> <span class=md-ellipsis> Process substitution </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=#scope class=md-nav__link> <span class=md-ellipsis> Scope </span> </a> </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=#avoiding-subshells class=md-nav__link> <span class=md-ellipsis> Avoiding subshells </span> </a> </li> <li class=md-nav__item> <a href=#process-substitution-assigned-to-a-parameter class=md-nav__link> <span class=md-ellipsis> Process substitution assigned to a parameter </span> </a> </li> </ul> </nav> </li> <li class=md-nav__item> <a href=#bugs-and-portability-considerations class=md-nav__link> <span class=md-ellipsis> Bugs and Portability Considerations </span> </a> </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> <li class=md-nav__item> <a href=../tilde/ class=md-nav__link> <span class=md-ellipsis> Tilde expansion </span> </a> </li> <li class=md-nav__item> <a href=../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--section md-nav__item--nested"> <input class="md-nav__toggle md-toggle " type=checkbox id=__nav_9_13> <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=false> <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> <a href=../../keywords/coproc/ class=md-nav__link> <span class=md-ellipsis> The coproc keyword </span> </a> </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=#scope class=md-nav__link> <span class=md-ellipsis> Scope </span> </a> </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=#avoiding-subshells class=md-nav__link> <span class=md-ellipsis> Avoiding subshells </span> </a> </li> <li class=md-nav__item> <a href=#process-substitution-assigned-to-a-parameter class=md-nav__link> <span class=md-ellipsis> Process substitution assigned to a parameter </span> </a> </li> </ul> </nav> </li> <li class=md-nav__item> <a href=#bugs-and-portability-considerations class=md-nav__link> <span class=md-ellipsis> Bugs and Portability Considerations </span> </a> </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"> <nav class=md-tags> <a href=../../../tags/#bash class=md-tag>bash</a> <a href=../../../tags/#shell class=md-tag>shell</a> <a href=../../../tags/#scripting class=md-tag>scripting</a> <a href=../../../tags/#expansion class=md-tag>expansion</a> <a href=../../../tags/#substitution class=md-tag>substitution</a> <a href=../../../tags/#text class=md-tag>text</a> <a href=../../../tags/#stdin class=md-tag>stdin</a> <a href=../../../tags/#stdout class=md-tag>stdout</a> <a href=../../../tags/#save class=md-tag>save</a> <a href=../../../tags/#capture class=md-tag>capture</a> </nav> <a href=https://github.com/flokoe/bash-hackers-wiki/edit/main/docs/syntax/expansion/proc_subst.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/expansion/proc_subst.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=process-substitution>Process substitution<a class=headerlink href=#process-substitution title="Permanent link">¶</a></h1> <p>Process substitution is a form of redirection where the input or output of a process (some sequence of commands) appear as a temporary file.</p> <div class=highlight><pre><span></span><code><( <LIST> )
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>( <LIST> )
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</code></pre></div> <p>Process substitution is performed <strong>simultaneously</strong> with <a href=../../pe/ >parameter expansion</a>, <a href=../cmdsubst/ >command substitution</a> and <a href=../arith/ >arithmetic expansion</a>.</p> <p>The <a href=../../basicgrammar/#lists>command list</a> <code><LIST></code> is executed and its</p> <ul> <li>standard output filedescriptor in the <code><( ... )</code> form or</li> <li>standard input filedescriptor in the <code>>( ... )</code> form</li> </ul> <p>is connected to a FIFO or a file in <code>/dev/fd/</code>. The filename (where the filedescriptor is connected) is then used as a substitution for the <code><(...)</code>-construct.</p> <p>That, for example, allows to give data to a command that can't be reached by pipelining (that doesn't expect its data from <code>stdin</code> but from a file).</p> <h3 id=scope>Scope<a class=headerlink href=#scope title="Permanent link">¶</a></h3> <div class="admonition note"> <p class=admonition-title>Note</p> <p>According to multiple comments and sources, the scope of process substitution file descriptors is <strong>not</strong> stable, guaranteed, or specified by bash. Newer versions of bash (5.0+) seem to have shorter scope, and substitutions scope seems to be shorter than function scope. See <a href=https://unix.stackexchange.com/questions/425456/conditional-process-substitution>stackexchange</a> and <a href=https://stackoverflow.com/questions/46660020/bash-what-is-the-scope-of-the-process-substitution>stackoverflow</a>; the latter discussion contains a script that can test the scoping behavior case-by-case</p> </div> <p>If a process substitution is expanded as an argument to a function, expanded to an environment variable during calling of a function, or expanded to any assignment within a function, the process substitution will be "held open" for use by any command within the function or its callees, until the function in which it was set returns. If the same variable is set again within a callee, unless the new variable is local, the previous process substitution is closed and will be unavailable to the caller when the callee returns.</p> <p>In essence, process substitutions expanded to variables within functions remain open until the function in which the process substitution occured returns - even when assigned to locals that were set by a function's caller. Dynamic scope doesn't protect them from closing.</p> <h2 id=examples>Examples<a class=headerlink href=#examples title="Permanent link">¶</a></h2> <p>This code is useless, but it demonstrates how it works:</p> <div class=highlight><pre><span></span><code><a id=__codelineno-0-1 name=__codelineno-0-1 href=#__codelineno-0-1></a>$<span class=w> </span><span class=nb>echo</span><span class=w> </span><<span class=o>(</span>ls<span class=o>)</span>
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<a id=__codelineno-0-2 name=__codelineno-0-2 href=#__codelineno-0-2></a>/dev/fd/63
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</code></pre></div> <p>The <strong>output</strong> of the <code>ls</code>-program can then be accessed by reading the file <code>/dev/fd/63</code>.</p> <p>Consider the following:</p> <div class=highlight><pre><span></span><code><a id=__codelineno-1-1 name=__codelineno-1-1 href=#__codelineno-1-1></a>diff<span class=w> </span><<span class=o>(</span>ls<span class=w> </span><span class=s2>"</span><span class=nv>$first_directory</span><span class=s2>"</span><span class=o>)</span><span class=w> </span><<span class=o>(</span>ls<span class=w> </span><span class=s2>"</span><span class=nv>$second_directory</span><span class=s2>"</span><span class=o>)</span>
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</code></pre></div> <p>This will compare the contents of each directory. In this command, each <em>process</em> is <em>substituted</em> for a <em>file</em>, and diff doesn't see <(bla), it sees two files, so the effective command is something like</p> <div class=highlight><pre><span></span><code><a id=__codelineno-2-1 name=__codelineno-2-1 href=#__codelineno-2-1></a>diff<span class=w> </span>/dev/fd/63<span class=w> </span>/dev/fd/64
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</code></pre></div> <p>where those files are written to and destroyed automatically.</p> <h3 id=avoiding-subshells>Avoiding subshells<a class=headerlink href=#avoiding-subshells title="Permanent link">¶</a></h3> <div class="admonition info"> <p class=admonition-title>See Also</p> <p>See Also: <a href=http://mywiki.wooledge.org/BashFAQ/024>BashFAQ/024</a> -- <em>I set variables in a loop that's in a pipeline. Why do they disappear after the loop terminates? Or, why can't I pipe data to read?</em></p> </div> <p>One of the most common uses for process substitutions is to avoid the final subshell that results from executing a pipeline. The following is a <strong>wrong</strong> piece of code to count all files in <code>/etc</code> is:</p> <div class=highlight><pre><span></span><code><a id=__codelineno-3-1 name=__codelineno-3-1 href=#__codelineno-3-1></a><span class=nv>counter</span><span class=o>=</span><span class=m>0</span>
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<a id=__codelineno-3-2 name=__codelineno-3-2 href=#__codelineno-3-2></a>
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<a id=__codelineno-3-3 name=__codelineno-3-3 href=#__codelineno-3-3></a>find<span class=w> </span>/etc<span class=w> </span>-print0<span class=w> </span><span class=p>|</span><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>-rd<span class=w> </span><span class=s1>''</span><span class=w> </span>_<span class=p>;</span><span class=w> </span><span class=k>do</span>
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<a id=__codelineno-3-4 name=__codelineno-3-4 href=#__codelineno-3-4></a><span class=w> </span><span class=o>((</span>counter++<span class=o>))</span>
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<a id=__codelineno-3-5 name=__codelineno-3-5 href=#__codelineno-3-5></a><span class=k>done</span>
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<a id=__codelineno-3-6 name=__codelineno-3-6 href=#__codelineno-3-6></a>
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<a id=__codelineno-3-7 name=__codelineno-3-7 href=#__codelineno-3-7></a><span class=nb>echo</span><span class=w> </span><span class=s2>"</span><span class=nv>$counter</span><span class=s2> files"</span><span class=w> </span><span class=c1># prints "0 files"</span>
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</code></pre></div> <p>Due to the pipe, the <code>while read; do ... done</code> part is executed in a subshell (in Bash, by default), which means <code>counter</code> is only incremented within the subshell. When the pipeline finishes, the subshell is terminated, and the <code>counter</code> visible to <code>echo</code> is still at "0"!</p> <p>Process substitution helps us avoid the pipe operator (the reason for the subshell):</p> <div class=highlight><pre><span></span><code><a id=__codelineno-4-1 name=__codelineno-4-1 href=#__codelineno-4-1></a><span class=nv>counter</span><span class=o>=</span><span class=m>0</span>
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<a id=__codelineno-4-2 name=__codelineno-4-2 href=#__codelineno-4-2></a>
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<a id=__codelineno-4-3 name=__codelineno-4-3 href=#__codelineno-4-3></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>-rN1<span class=w> </span>_<span class=p>;</span><span class=w> </span><span class=k>do</span>
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<a id=__codelineno-4-4 name=__codelineno-4-4 href=#__codelineno-4-4></a><span class=w> </span><span class=o>((</span>counter++<span class=o>))</span>
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<a id=__codelineno-4-5 name=__codelineno-4-5 href=#__codelineno-4-5></a><span class=k>done</span><span class=w> </span><<span class=w> </span><<span class=o>(</span>find<span class=w> </span>/etc<span class=w> </span>-printf<span class=w> </span><span class=s1>' '</span><span class=o>)</span>
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<a id=__codelineno-4-6 name=__codelineno-4-6 href=#__codelineno-4-6></a>
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<a id=__codelineno-4-7 name=__codelineno-4-7 href=#__codelineno-4-7></a><span class=nb>echo</span><span class=w> </span><span class=s2>"</span><span class=nv>$counter</span><span class=s2> files"</span>
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</code></pre></div> <p>This is the normal input file redirection <code>< FILE</code>, just that the <code>FILE</code> in this case is the result of process substitution. It's important to note that the space is required in order to disambiguate the syntax from <a href=../../redirection/#here_documents>here documents</a>.</p> <div class=highlight><pre><span></span><code><a id=__codelineno-5-1 name=__codelineno-5-1 href=#__codelineno-5-1></a>:<span class=w> </span><<span class=w> </span><<span class=o>(</span>COMMAND<span class=o>)</span><span class=w> </span><span class=c1># Good.</span>
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<a id=__codelineno-5-2 name=__codelineno-5-2 href=#__codelineno-5-2></a>:<span class=w> </span><<<span class=o>(</span>...<span class=o>)</span><span class=w> </span><span class=c1># Wrong. Will be parsed as a heredoc. Bash fails when it comes across the unquoted metacharacter ''(''</span>
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<a id=__codelineno-5-3 name=__codelineno-5-3 href=#__codelineno-5-3></a>:<span class=w> </span>><<span class=o>(</span>...<span class=o>)</span><span class=w> </span><span class=c1># Technically valid but pointless syntax. Bash opens the pipe for writing, while the commands within the process substitution have their stdout connected to the pipe.</span>
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</code></pre></div> <h3 id=process-substitution-assigned-to-a-parameter>Process substitution assigned to a parameter<a class=headerlink href=#process-substitution-assigned-to-a-parameter title="Permanent link">¶</a></h3> <p>This example demonstrates how process substitutions can be made to resemble "passable" objects. This results in converting the output of <code>f</code>'s argument to uppercase.</p> <div class=highlight><pre><span></span><code><a id=__codelineno-6-1 name=__codelineno-6-1 href=#__codelineno-6-1></a>f<span class=o>()</span><span class=w> </span><span class=o>{</span>
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<a id=__codelineno-6-2 name=__codelineno-6-2 href=#__codelineno-6-2></a><span class=w> </span>cat<span class=w> </span><span class=s2>"</span><span class=nv>$1</span><span class=s2>"</span><span class=w> </span>><span class=s2>"</span><span class=nv>$x</span><span class=s2>"</span>
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<a id=__codelineno-6-3 name=__codelineno-6-3 href=#__codelineno-6-3></a><span class=o>}</span>
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<a id=__codelineno-6-4 name=__codelineno-6-4 href=#__codelineno-6-4></a>
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<a id=__codelineno-6-5 name=__codelineno-6-5 href=#__codelineno-6-5></a><span class=nv>x</span><span class=o>=</span>><span class=o>(</span>tr<span class=w> </span><span class=s1>'[:lower:]'</span><span class=w> </span><span class=s1>'[:upper:]'</span><span class=o>)</span><span class=w> </span>f<span class=w> </span><<span class=o>(</span><span class=nb>echo</span><span class=w> </span><span class=s1>'hi there'</span><span class=o>)</span>
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</code></pre></div> <p>See the above section on <a href=#scope>#scope</a></p> <h2 id=bugs-and-portability-considerations>Bugs and Portability Considerations<a class=headerlink href=#bugs-and-portability-considerations title="Permanent link">¶</a></h2> <ul> <li>Process substitution is not specified by POSIX.</li> <li>Process substitution is disabled completely in Bash POSIX mode.</li> <li>Process substitution is implemented by Bash, Zsh, Ksh{88,93}, but not (yet) pdksh derivatives (mksh). Coprocesses may be used instead.</li> <li>Process substitution is supported only on systems that support either named pipes (FIFO - a <a href=../../../dict/special_file/ >special file</a>) or the <code>/dev/fd/*</code> method for accessing open files. If the system doesn't support <code>/dev/fd/*</code>, Bash falls back to creating named pipes. Note that not all shells that support process substitution have that fallback.</li> <li>Bash evaluates process substitutions within array indices, but not other arithmetic contexts. Ksh and Zsh do not. (Possible Bug)</li> </ul> <div class=highlight><pre><span></span><code><a id=__codelineno-7-1 name=__codelineno-7-1 href=#__codelineno-7-1></a><span class=c1># print "moo"</span>
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<a id=__codelineno-7-2 name=__codelineno-7-2 href=#__codelineno-7-2></a><span class=nv>dev</span><span class=o>=</span><span class=nv>fd</span><span class=o>=</span><span class=m>1</span><span class=w> </span>_<span class=o>[</span><span class=m>1</span><<span class=o>(</span><span class=nb>echo</span><span class=w> </span>moo<span class=w> </span>><span class=p>&</span><span class=m>2</span><span class=o>)]=</span>
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<a id=__codelineno-7-3 name=__codelineno-7-3 href=#__codelineno-7-3></a><span class=c1># fork bomb</span>
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<a id=__codelineno-7-4 name=__codelineno-7-4 href=#__codelineno-7-4></a><span class=si>${</span><span class=nv>dev</span><span class=p>[</span><span class=si>${</span><span class=nv>dev</span><span class=p>=</span><span class=s1>'dev[1>(${dev[dev]})]'</span><span class=si>}</span><span class=p>]</span><span class=si>}</span>
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</code></pre></div> <ul> <li>Issues with wait, race conditions, etc: <a href="https://groups.google.com/forum/?fromgroups=#!topic/comp.unix.shell/GqLNzUA4ulA">https://groups.google.com/forum/?fromgroups=#!topic/comp.unix.shell/GqLNzUA4ulA</a></li> </ul> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li>Internal: <a href=../intro/ >Introduction to expansion and substitution</a></li> <li>Internal: <a href=../../../scripting/processtree/ >Bash in the process tree</a> (subshells)</li> <li>Internal: <a href=../../redirection/ >Redirection</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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