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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/scripting/basics/ rel=canonical><link href=../bashchanges/ rel=prev><link href=../debuggingtips/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.5.40"><title>The basics of shell scripting - The Bash Hackers Wiki</title><link rel=stylesheet href=../../assets/stylesheets/main.8c3ca2c6.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.06af60db.min.css><link rel=preconnect href=https://fonts.gstatic.com crossorigin><link rel=stylesheet href="https://fonts.googleapis.com/css?family=Roboto:300,300i,400,400i,700,700i%7CRoboto+Mono:400,400i,700,700i&display=fallback"><style>:root{--md-text-font:"Roboto";--md-code-font:"Roboto Mono"}</style><script>__md_scope=new 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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 md-tabs__item--active"> <a href=../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> <a href=../../syntax/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 class=md-sidebar__inner> <nav class="md-nav md-nav--primary 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3.6-3.3.3-5.6-1.3-5.6-3.6 0-2 2.3-3.6 5.2-3.6 3-.3 5.6 1.3 5.6 3.6m-31.1-4.5c-.7 2 1.3 4.3 4.3 4.9 2.6 1 5.6 0 6.2-2s-1.3-4.3-4.3-5.2c-2.6-.7-5.5.3-6.2 2.3m44.2-1.7c-2.9.7-4.9 2.6-4.6 4.9.3 2 2.9 3.3 5.9 2.6 2.9-.7 4.9-2.6 4.6-4.6-.3-1.9-3-3.2-5.9-2.9M244.8 8C106.1 8 0 113.3 0 252c0 110.9 69.8 205.8 169.5 239.2 12.8 2.3 17.3-5.6 17.3-12.1 0-6.2-.3-40.4-.3-61.4 0 0-70 15-84.7-29.8 0 0-11.4-29.1-27.8-36.6 0 0-22.9-15.7 1.6-15.4 0 0 24.9 2 38.6 25.8 21.9 38.6 58.6 27.5 72.9 20.9 2.3-16 8.8-27.1 16-33.7-55.9-6.2-112.3-14.3-112.3-110.5 0-27.5 7.6-41.3 23.6-58.9-2.6-6.5-11.1-33.3 2.6-67.9 20.9-6.5 69 27 69 27 20-5.6 41.5-8.5 62.8-8.5s42.8 2.9 62.8 8.5c0 0 48.1-33.6 69-27 13.7 34.7 5.2 61.4 2.6 67.9 16 17.7 25.8 31.5 25.8 58.9 0 96.5-58.9 104.2-114.8 110.5 9.2 7.9 17 22.9 17 46.4 0 33.7-.3 75.4-.3 83.6 0 6.5 4.6 14.4 17.3 12.1C428.2 457.8 496 362.9 496 252 496 113.3 383.5 8 244.8 8M97.2 352.9c-1.3 1-1 3.3.7 5.2 1.6 1.6 3.9 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--active md-nav__item--section md-nav__item--nested"> <input class="md-nav__toggle md-toggle " type=checkbox id=__nav_7 checked> <label class=md-nav__link for=__nav_7 id=__nav_7_label tabindex> <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=true> <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=../bashbehaviour/ class=md-nav__link> <span class=md-ellipsis> Bash's behaviour </span> </a> </li> <li class=md-nav__item> <a href=../bashchanges/ class=md-nav__link> <span class=md-ellipsis> Bash changes </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> The basics of shell scripting </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 basics of shell scripting </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=#script-files class=md-nav__link> <span class=md-ellipsis> Script files </span> </a> </li> <li class=md-nav__item> <a href=#the-shebang class=md-nav__link> <span class=md-ellipsis> The Shebang </span> </a> </li> <li class=md-nav__item> <a href=#the-standard-filedescriptors class=md-nav__link> <span class=md-ellipsis> The standard filedescriptors </span> </a> </li> <li class=md-nav__item> <a href=#variable-names class=md-nav__link> <span class=md-ellipsis> Variable names </span> </a> </li> <li class=md-nav__item> <a href=#exit-codes class=md-nav__link> <span class=md-ellipsis> Exit codes </span> </a> </li> <li class=md-nav__item> <a href=#comments class=md-nav__link> <span class=md-ellipsis> Comments </span> </a> <nav class=md-nav aria-label=Comments> <ul class=md-nav__list> <li class=md-nav__item> <a href=#block-commenting class=md-nav__link> <span class=md-ellipsis> Block commenting </span> </a> </li> </ul> </nav> </li> <li class=md-nav__item> <a href=#variable-scope class=md-nav__link> <span class=md-ellipsis> Variable scope </span> </a> <nav class=md-nav aria-label="Variable scope"> <ul class=md-nav__list> <li class=md-nav__item> <a href=#local-variables class=md-nav__link> <span class=md-ellipsis> Local variables </span> </a> </li> <li class=md-nav__item> <a href=#environment-variables class=md-nav__link> <span class=md-ellipsis> Environment variables </span> </a> </li> </ul> </nav> </li> </ul> </nav> </li> <li class=md-nav__item> <a href=../debuggingtips/ class=md-nav__link> <span class=md-ellipsis> Debugging a script </span> </a> </li> <li class=md-nav__item> <a href=../newbie_traps/ class=md-nav__link> <span class=md-ellipsis> Beginner Mistakes </span> </a> </li> <li class=md-nav__item> <a href=../nonportable/ class=md-nav__link> <span class=md-ellipsis> Portability talk </span> </a> </li> <li class=md-nav__item> <a href=../obsolete/ class=md-nav__link> <span class=md-ellipsis> Obsolete and deprecated syntax </span> </a> </li> <li class=md-nav__item> <a href=../posparams/ class=md-nav__link> <span class=md-ellipsis> Handling positional parameters </span> </a> </li> <li class=md-nav__item> <a href=../processtree/ class=md-nav__link> <span class=md-ellipsis> Bash and the process tree </span> </a> </li> <li class=md-nav__item> <a href=../style/ class=md-nav__link> <span class=md-ellipsis> Scripting with style </span> </a> </li> <li class=md-nav__item> <a href=../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--nested"> <input class="md-nav__toggle md-toggle " type=checkbox id=__nav_9> <label class=md-nav__link for=__nav_9 id=__nav_9_label tabindex=0> <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=false> <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=../../syntax/arith_expr/ class=md-nav__link> <span class=md-ellipsis> Arithmetic expressions </span> </a> </li> <li class=md-nav__item> <a href=../../syntax/arrays/ class=md-nav__link> <span class=md-ellipsis> Arrays </span> </a> </li> <li class=md-nav__item> <a href=../../syntax/basicgrammar/ class=md-nav__link> <span class=md-ellipsis> Basic grammar rules of Bash </span> </a> </li> <li class=md-nav__item> <a href=../../syntax/pattern/ class=md-nav__link> <span class=md-ellipsis> Patterns and pattern matching </span> </a> </li> <li class=md-nav__item> <a href=../../syntax/pe/ class=md-nav__link> <span class=md-ellipsis> Parameter expansion </span> </a> </li> <li class=md-nav__item> <a href=../../syntax/quoting/ class=md-nav__link> <span class=md-ellipsis> Quotes and escaping </span> </a> </li> <li class=md-nav__item> <a href=../../syntax/redirection/ class=md-nav__link> <span class=md-ellipsis> Redirection </span> </a> </li> <li class=md-nav__item> <a href=../../syntax/shellvars/ class=md-nav__link> <span class=md-ellipsis> Special parameters and shell variables </span> </a> </li> <li class=md-nav__item> <a href=../../syntax/words/ class=md-nav__link> <span class=md-ellipsis> Words... </span> </a> </li> <li class="md-nav__item 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=0> <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=../../syntax/ccmd/arithmetic_eval/ class=md-nav__link> <span class=md-ellipsis> Arithmetic evaluation (command) </span> </a> </li> <li class=md-nav__item> <a href=../../syntax/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=../../syntax/ccmd/case/ class=md-nav__link> <span class=md-ellipsis> The case statement </span> </a> </li> <li class=md-nav__item> <a href=../../syntax/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=../../syntax/ccmd/conditional_expression/ class=md-nav__link> <span class=md-ellipsis> The conditional expression </span> </a> </li> <li class=md-nav__item> <a href=../../syntax/ccmd/grouping_plain/ class=md-nav__link> <span class=md-ellipsis> Grouping commands </span> </a> </li> <li class=md-nav__item> <a href=../../syntax/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=../../syntax/ccmd/if_clause/ class=md-nav__link> <span class=md-ellipsis> The if-clause </span> </a> </li> <li class=md-nav__item> <a href=../../syntax/ccmd/intro/ class=md-nav__link> <span class=md-ellipsis> Bash compound commands </span> </a> </li> <li class=md-nav__item> <a href=../../syntax/ccmd/until_loop/ class=md-nav__link> <span class=md-ellipsis> The until loop </span> </a> </li> <li class=md-nav__item> <a href=../../syntax/ccmd/user_select/ class=md-nav__link> <span class=md-ellipsis> User selections </span> </a> </li> <li class=md-nav__item> <a href=../../syntax/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--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=0> <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=../../syntax/expansion/arith/ class=md-nav__link> <span class=md-ellipsis> Arithmetic expansion </span> </a> </li> <li class=md-nav__item> <a href=../../syntax/expansion/brace/ class=md-nav__link> <span class=md-ellipsis> Brace expansion </span> </a> </li> <li class=md-nav__item> <a href=../../syntax/expansion/cmdsubst/ class=md-nav__link> <span class=md-ellipsis> Command substitution </span> </a> </li> <li class=md-nav__item> <a href=../../syntax/expansion/globs/ class=md-nav__link> <span class=md-ellipsis> Pathname expansion (globbing) </span> </a> </li> <li class=md-nav__item> <a href=../../syntax/expansion/intro/ class=md-nav__link> <span class=md-ellipsis> Expansions and substitutions </span> </a> </li> <li class=md-nav__item> <a href=../../syntax/expansion/proc_subst/ class=md-nav__link> <span class=md-ellipsis> Process substitution </span> </a> </li> <li class=md-nav__item> <a href=../../syntax/expansion/tilde/ class=md-nav__link> <span class=md-ellipsis> Tilde expansion </span> </a> </li> <li class=md-nav__item> <a href=../../syntax/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--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=0> <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=../../syntax/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--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=0> <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=../../syntax/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=#script-files class=md-nav__link> <span class=md-ellipsis> Script files </span> </a> </li> <li class=md-nav__item> <a href=#the-shebang class=md-nav__link> <span class=md-ellipsis> The Shebang </span> </a> </li> <li class=md-nav__item> <a href=#the-standard-filedescriptors class=md-nav__link> <span class=md-ellipsis> The standard filedescriptors </span> </a> </li> <li class=md-nav__item> <a href=#variable-names class=md-nav__link> <span class=md-ellipsis> Variable names </span> </a> </li> <li class=md-nav__item> <a href=#exit-codes class=md-nav__link> <span class=md-ellipsis> Exit codes </span> </a> </li> <li class=md-nav__item> <a href=#comments class=md-nav__link> <span class=md-ellipsis> Comments </span> </a> <nav class=md-nav aria-label=Comments> <ul class=md-nav__list> <li class=md-nav__item> <a href=#block-commenting class=md-nav__link> <span class=md-ellipsis> Block commenting </span> </a> </li> </ul> </nav> </li> <li class=md-nav__item> <a href=#variable-scope class=md-nav__link> <span class=md-ellipsis> Variable scope </span> </a> <nav class=md-nav aria-label="Variable scope"> <ul class=md-nav__list> <li class=md-nav__item> <a href=#local-variables class=md-nav__link> <span class=md-ellipsis> Local variables </span> </a> </li> <li class=md-nav__item> <a href=#environment-variables class=md-nav__link> <span class=md-ellipsis> Environment variables </span> </a> </li> </ul> </nav> </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/#basics class=md-tag>basics</a> <a href=../../tags/#learning class=md-tag>learning</a> <a href=../../tags/#tutorial class=md-tag>tutorial</a> </nav> <a href=https://github.com/flokoe/bash-hackers-wiki/edit/main/docs/scripting/basics.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/scripting/basics.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-basics-of-shell-scripting>The basics of shell scripting<a class=headerlink href=#the-basics-of-shell-scripting title="Permanent link">&para;</a></h1> <h2 id=script-files>Script files<a class=headerlink href=#script-files title="Permanent link">&para;</a></h2> <p>A shell script usually resides inside a file. The file can be executable, but you can call a Bash script with that filename as a parameter:</p> <div class=highlight><pre><span></span><code>bash ./myfile
</code></pre></div> <p>There is <strong>no need to add a boring filename extension</strong> like <code>.bash</code> or <code>.sh</code>. That is a holdover from UNIX&reg;, where executables are not tagged by the extension, but by <strong>permissions</strong> (filemode). The file name can be any combination of legal filename characters. Adding a proper filename extension is a convention, nothing else.</p> <div class=highlight><pre><span></span><code>chmod +x ./myfile
</code></pre></div> <p>If the file is executable, and you want to use it by calling only the script name, the shebang must be included in the file.</p> <h2 id=the-shebang>The Shebang<a class=headerlink href=#the-shebang title="Permanent link">&para;</a></h2> <p>The in-file specification of the interpreter of that file, for example:</p> <div class=highlight><pre><span></span><code><a id=__codelineno-0-1 name=__codelineno-0-1 href=#__codelineno-0-1></a><span class=ch>#!/bin/bash</span>
<a id=__codelineno-0-2 name=__codelineno-0-2 href=#__codelineno-0-2></a><span class=nb>echo</span><span class=w> </span><span class=s2>&quot;Hello world...&quot;</span>
</code></pre></div> <p>This is interpreted by the kernel <sup id=fnref:1><a class=footnote-ref href=#fn:1>1</a></sup> of your system. In general, if a file is executable, but not an executable (binary) program, and such a line is present, the program specified after <code>#!</code> is started with the scriptname and all its arguments. These two characters <code>#</code> and <code>!</code> must be <strong>the first two bytes</strong> in the file!</p> <p>You can follow the process by using <code>echo</code> as a fake interpreter:</p> <div class=highlight><pre><span></span><code>#!/bin/echo
</code></pre></div> <p>We don't need a script body here, as the file will never be interpreted and executed by "<code>echo</code>". You can see what the Operating System does, it calls "<code>/bin/echo</code>" with the name of the executable file and following arguments.</p> <div class=highlight><pre><span></span><code>$ /home/bash/bin/test testword hello
/home/bash/bin/test testword hello
</code></pre></div> <p>The same way, with <code>#!/bin/bash</code> the shell "<code>/bin/bash</code>" is called with the script filename as an argument. It's the same as executing "<code>/bin/bash /home/bash/bin/test testword hello</code>"</p> <p>If the interpreter can be specified with arguments and how long it can be is system-specific (see <a href=http://www.in-ulm.de/~mascheck/various/shebang/ >#!-magic</a>). When Bash executes a file with a #!/bin/bash shebang, the shebang itself is ignored, since the first character is a hashmark "<code>#</code>", which indicates a comment. The shebang is for the operating system, not for the shell. Programs that don't ignore such lines, may not work as shebang driven interpreters.</p> <p><wrap center round important 60%> <u><strong>Attention:</strong></u>When the specified interpreter is unavailable or not executable (permissions), you usually get a "<code>bad interpreter</code>" error message., If you get nothing and it fails, check the shebang. Older Bash versions will respond with a "<code>no such file or directory</code>" error for a nonexistant interpreter specified by the shebang. </wrap></p> <p><strong>Additional note:</strong> When you specify <code>#!/bin/sh</code> as shebang and that's a link to a Bash, then Bash will run in POSIX&reg; mode! See:</p> <ul> <li><a href=../bashbehaviour/ >Bash behaviour</a>.</li> </ul> <p>A common method is to specify a shebang like</p> <div class=highlight><pre><span></span><code>#!/usr/bin/env bash
</code></pre></div> <p>...which just moves the location of the potential problem to</p> <ul> <li>the <code>env</code> utility must be located in /usr/bin/</li> <li>the needed <code>bash</code> binary must be located in <code>PATH</code></li> </ul> <p>Which one you need, or whether you think which one is good, or bad, is up to you. There is no bulletproof portable way to specify an interpreter. <strong>It's a common misconception that it solves all problems. Period.</strong></p> <h2 id=the-standard-filedescriptors>The standard filedescriptors<a class=headerlink href=#the-standard-filedescriptors title="Permanent link">&para;</a></h2> <p>Once Initialized, every normal UNIX&reg;-program has <em>at least 3 open files</em>:</p> <ul> <li><strong>stdin</strong>: standard input</li> <li><strong>stdout</strong>: standard output</li> <li><strong>stderr</strong>: standard error output</li> </ul> <p>Usually, they're all connected to your terminal, stdin as input file (keyboard), stdout and stderr as output files (screen). When calling such a program, the invoking shell can change these filedescriptor connections away from the terminal to any other file (see redirection). Why two different output filedescriptors? It's convention to send error messages and warnings to stderr and only program output to stdout. This enables the user to decide if they want to see nothing, only the data, only the errors, or both - and where they want to see them.</p> <p>When you write a script:</p> <ul> <li>always read user-input from <code>stdin</code></li> <li>always write diagnostic/error/warning messages to <code>stderr</code></li> </ul> <p>To learn more about the standard filedescriptors, especially about redirection and piping, see:</p> <ul> <li><a href=../../howto/redirection_tutorial/ >An illustrated redirection tutorial</a></li> </ul> <h2 id=variable-names>Variable names<a class=headerlink href=#variable-names title="Permanent link">&para;</a></h2> <p>It's good practice to use lowercase names for your variables, as shell and system-variable names are usually all in UPPERCASE. However, you should avoid naming your variables any of the following (incomplete list!):</p> <hr> <p><code>BASH</code> <code>BASH_ARGC</code> <code>BASH_ARGV</code> <code>BASH_LINENO</code> <code>BASH_SOURCE</code> <code>BASH_VERSINFO</code> <code>BASH_VERSION</code> <code>COLUMNS</code> <code>DIRSTACK</code> <code>DISPLAY</code> <code>EDITOR</code> <code>EUID</code> <code>GROUPS</code> <code>HISTFILE</code> <code>HISTFILESIZE</code> <code>HISTSIZE</code> <code>HOME</code> <code>HOSTNAME</code> <code>IFS</code> <code>LANG</code> <code>LANGUAGE</code> <code>LC_ALL</code> <code>LINES</code> <code>LOGNAME</code> <code>LS_COLORS</code> <code>MACHTYPE</code> <code>MAILCHECK</code> <code>OLDPWD</code> <code>OPTERR</code> <code>OPTIND</code> <code>OSTYPE</code> <code>PATH</code> <code>PIPESTATUS</code> <code>PPID</code> <code>PROMPT_COMMAND</code> <code>PS1</code> <code>PS2</code> <code>PS4</code> <code>PS3</code> <code>PWD</code> <code>SHELL</code> <code>SHELLOPTS</code> <code>SHLVL</code> <code>TERM</code> <code>UID</code> <code>USER</code> <code>USERNAME</code> <code>XAUTHORITY</code></p> <hr> <p>This list is incomplete. <strong>The safest way is to use all-lowercase variable names.</strong></p> <h2 id=exit-codes>Exit codes<a class=headerlink href=#exit-codes title="Permanent link">&para;</a></h2> <p>Every program you start terminates with an exit code and reports it to the operating system. This exit code can be utilized by Bash. You can show it, you can act on it, you can control script flow with it. The code is a number between 0 and 255. Values from 126 to 255 are reserved for use by the shell directly, or for special purposes, like reporting a termination by a signal:</p> <ul> <li><strong>126</strong>: the requested command (file) was found, but can't be executed</li> <li><strong>127</strong>: command (file) not found</li> <li><strong>128</strong>: according to ABS it's used to report an invalid argument to the exit builtin, but I wasn't able to verify that in the source code of Bash (see code 255)</li> <li><strong>128 + N</strong>: the shell was terminated by the signal N</li> <li><strong>255</strong>: wrong argument to the exit builtin (see code 128)</li> </ul> <p>The lower codes 0 to 125 are not reserved and may be used for whatever the program likes to report. A value of 0 means <strong>successful</strong> termination, a value not 0 means <strong>unsuccessful</strong> termination. This behavior (== 0, != 0) is also what Bash reacts to in some flow control statements.</p> <p>An example of using the exit code of the program <code>grep</code> to check if a specific user is present in /etc/passwd:</p> <div class=highlight><pre><span></span><code><a id=__codelineno-1-1 name=__codelineno-1-1 href=#__codelineno-1-1></a><span class=k>if</span><span class=w> </span>grep<span class=w> </span>^root<span class=w> </span>/etc/passwd<span class=p>;</span><span class=w> </span><span class=k>then</span>
<a id=__codelineno-1-2 name=__codelineno-1-2 href=#__codelineno-1-2></a><span class=w> </span><span class=nb>echo</span><span class=w> </span><span class=s2>&quot;The user root was found&quot;</span>
<a id=__codelineno-1-3 name=__codelineno-1-3 href=#__codelineno-1-3></a><span class=k>else</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><span class=s2>&quot;The user root was not found&quot;</span>
<a id=__codelineno-1-5 name=__codelineno-1-5 href=#__codelineno-1-5></a><span class=k>fi</span>
</code></pre></div> <p>A common decision making command is "<code>test</code>" or its equivalent "<code>[</code>". But note that, when calling test with the name "<code>[</code>", the square brackets are not part of the shell syntax, the left bracket <strong>is</strong> the test command!</p> <div class=highlight><pre><span></span><code><a id=__codelineno-2-1 name=__codelineno-2-1 href=#__codelineno-2-1></a><span class=k>if</span><span class=w> </span><span class=o>[</span><span class=w> </span><span class=s2>&quot;</span><span class=nv>$mystring</span><span class=s2>&quot;</span><span class=w> </span><span class=o>=</span><span class=w> </span><span class=s2>&quot;Hello world&quot;</span><span class=w> </span><span class=o>]</span><span class=p>;</span><span class=w> </span><span class=k>then</span>
<a id=__codelineno-2-2 name=__codelineno-2-2 href=#__codelineno-2-2></a><span class=w> </span><span class=nb>echo</span><span class=w> </span><span class=s2>&quot;Yeah dude, you entered the right words...&quot;</span>
<a id=__codelineno-2-3 name=__codelineno-2-3 href=#__codelineno-2-3></a><span class=k>else</span>
<a id=__codelineno-2-4 name=__codelineno-2-4 href=#__codelineno-2-4></a><span class=w> </span><span class=nb>echo</span><span class=w> </span><span class=s2>&quot;Eeeek - go away...&quot;</span>
<a id=__codelineno-2-5 name=__codelineno-2-5 href=#__codelineno-2-5></a><span class=k>fi</span>
</code></pre></div> <p>Read more about <a href=../../commands/classictest/ >the test command</a></p> <p>A common exit code check method uses the "<code>||</code>" or "<code>&amp;&amp;</code>" operators. This lets you execute a command based on whether or not the previous command completed successfully:</p> <div class=highlight><pre><span></span><code><a id=__codelineno-3-1 name=__codelineno-3-1 href=#__codelineno-3-1></a>grep<span class=w> </span>^root:<span class=w> </span>/etc/passwd<span class=w> </span>&gt;/dev/null<span class=w> </span><span class=o>||</span><span class=w> </span><span class=nb>echo</span><span class=w> </span><span class=s2>&quot;root was not found - check the pub at the corner.&quot;</span>
<a id=__codelineno-3-2 name=__codelineno-3-2 href=#__codelineno-3-2></a>which<span class=w> </span>vi<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;Your favourite editor is installed.&quot;</span>
</code></pre></div> <p>Please, when your script exits on errors, provide a "<code>FALSE</code>" exit code, so others can check the script execution.</p> <h2 id=comments>Comments<a class=headerlink href=#comments title="Permanent link">&para;</a></h2> <p>In a larger, or complex script, it's wise to comment the code. Comments can help with debugging or tests. Comments start with the <code>#</code> character (hashmark) and continue to the end of the line:</p> <div class=highlight><pre><span></span><code><a id=__codelineno-4-1 name=__codelineno-4-1 href=#__codelineno-4-1></a><span class=ch>#!/bin/bash</span>
<a id=__codelineno-4-2 name=__codelineno-4-2 href=#__codelineno-4-2></a><span class=c1># This is a small script to say something.</span>
<a id=__codelineno-4-3 name=__codelineno-4-3 href=#__codelineno-4-3></a><span class=nb>echo</span><span class=w> </span><span class=s2>&quot;Be liberal in what you accept, and conservative in what you send&quot;</span><span class=w> </span><span class=c1># say something</span>
</code></pre></div> <p>The first thing was already explained, it's the so-called shebang, for the shell, <strong>only a comment</strong>. The second one is a comment from the beginning of the line, the third comment starts after a valid command. All three syntactically correct.</p> <h3 id=block-commenting>Block commenting<a class=headerlink href=#block-commenting title="Permanent link">&para;</a></h3> <p>To temporarily disable complete blocks of code you would normally have to prefix every line of that block with a <code>#</code> (hashmark) to make it a comment. There's a little trick, using the pseudo command <code>:</code> (colon) and input redirection. The <code>:</code> does nothing, it's a pseudo command, so it does not care about standard input. In the following code example, you want to test mail and logging, but not dump the database, or execute a shutdown:</p> <div class=highlight><pre><span></span><code><a id=__codelineno-5-1 name=__codelineno-5-1 href=#__codelineno-5-1></a><span class=ch>#!/bin/bash</span>
<a id=__codelineno-5-2 name=__codelineno-5-2 href=#__codelineno-5-2></a><span class=c1># Write info mails, do some tasks and bring down the system in a safe way</span>
<a id=__codelineno-5-3 name=__codelineno-5-3 href=#__codelineno-5-3></a><span class=nb>echo</span><span class=w> </span><span class=s2>&quot;System halt requested&quot;</span><span class=w> </span><span class=p>|</span><span class=w> </span>mail<span class=w> </span>-s<span class=w> </span><span class=s2>&quot;System halt&quot;</span><span class=w> </span>netadmin@example.com
<a id=__codelineno-5-4 name=__codelineno-5-4 href=#__codelineno-5-4></a>logger<span class=w> </span>-t<span class=w> </span>SYSHALT<span class=w> </span><span class=s2>&quot;System halt requested&quot;</span>
<a id=__codelineno-5-5 name=__codelineno-5-5 href=#__codelineno-5-5></a>
<a id=__codelineno-5-6 name=__codelineno-5-6 href=#__codelineno-5-6></a><span class=c1>##### The following &quot;code block&quot; is effectively ignored</span>
<a id=__codelineno-5-7 name=__codelineno-5-7 href=#__codelineno-5-7></a>:<span class=w> </span>&lt;&lt;<span class=s2>&quot;SOMEWORD&quot;</span>
<a id=__codelineno-5-8 name=__codelineno-5-8 href=#__codelineno-5-8></a>/etc/init.d/mydatabase<span class=w> </span>clean_stop
<a id=__codelineno-5-9 name=__codelineno-5-9 href=#__codelineno-5-9></a>mydatabase_dump<span class=w> </span>/var/db/db1<span class=w> </span>/mnt/fsrv0/backups/db1
<a id=__codelineno-5-10 name=__codelineno-5-10 href=#__codelineno-5-10></a>logger<span class=w> </span>-t<span class=w> </span>SYSHALT<span class=w> </span><span class=s2>&quot;System halt: pre-shutdown actions done, now shutting down the system&quot;</span>
<a id=__codelineno-5-11 name=__codelineno-5-11 href=#__codelineno-5-11></a>shutdown<span class=w> </span>-h<span class=w> </span>NOW
<a id=__codelineno-5-12 name=__codelineno-5-12 href=#__codelineno-5-12></a>SOMEWORD
<a id=__codelineno-5-13 name=__codelineno-5-13 href=#__codelineno-5-13></a><span class=c1>##### The ignored codeblock ends here</span>
</code></pre></div> <p>What happened? The <code>:</code> pseudo command was given some input by redirection (a here-document) - the pseudo command didn't care about it, effectively, the entire block was ignored.</p> <p>The here-document-tag was quoted here <strong>to avoid substitutions</strong> in the "commented" text! Check <a href=../../syntax/redirection/#tag_heredoc>redirection with here-documents</a> for more</p> <h2 id=variable-scope>Variable scope<a class=headerlink href=#variable-scope title="Permanent link">&para;</a></h2> <p>In Bash, the scope of user variables is generally <em>global</em>. That means, it does <strong>not</strong> matter whether a variable is set in the "main program" or in a "function", the variable is defined everywhere.</p> <p>Compare the following <em>equivalent</em> code snippets:</p> <div class=highlight><pre><span></span><code><a id=__codelineno-6-1 name=__codelineno-6-1 href=#__codelineno-6-1></a><span class=nv>myvariable</span><span class=o>=</span><span class=nb>test</span>
<a id=__codelineno-6-2 name=__codelineno-6-2 href=#__codelineno-6-2></a><span class=nb>echo</span><span class=w> </span><span class=nv>$myvariable</span>
</code></pre></div> <div class=highlight><pre><span></span><code><a id=__codelineno-7-1 name=__codelineno-7-1 href=#__codelineno-7-1></a>myfunction<span class=o>()</span><span class=w> </span><span class=o>{</span>
<a id=__codelineno-7-2 name=__codelineno-7-2 href=#__codelineno-7-2></a><span class=w> </span><span class=nv>myvariable</span><span class=o>=</span><span class=nb>test</span>
<a id=__codelineno-7-3 name=__codelineno-7-3 href=#__codelineno-7-3></a><span class=o>}</span>
<a id=__codelineno-7-4 name=__codelineno-7-4 href=#__codelineno-7-4></a>
<a id=__codelineno-7-5 name=__codelineno-7-5 href=#__codelineno-7-5></a>myfunction
<a id=__codelineno-7-6 name=__codelineno-7-6 href=#__codelineno-7-6></a><span class=nb>echo</span><span class=w> </span><span class=nv>$myvariable</span>
</code></pre></div> <p>In both cases, the variable <code>myvariable</code> is set and accessible from everywhere in that script, both in functions and in the "main program".</p> <p><strong><u>Attention:</u></strong> When you set variables in a child process, for example a <em>subshell</em>, they will be set there, but you will <strong>never</strong> have access to them outside of that subshell. One way to create a subshell is the pipe. It's all mentioned in a small article about <a href=../processtree/ >Bash in the processtree</a>!</p> <h3 id=local-variables>Local variables<a class=headerlink href=#local-variables title="Permanent link">&para;</a></h3> <p>Bash provides ways to make a variable's scope <em>local</em> to a function:</p> <ul> <li>Using the <code>local</code> keyword, or</li> <li>Using <code>declare</code> (which will <em>detect</em> when it was called from within a function and make the variable(s) local).</li> </ul> <div class=highlight><pre><span></span><code><a id=__codelineno-8-1 name=__codelineno-8-1 href=#__codelineno-8-1></a>myfunc<span class=o>()</span><span class=w> </span><span class=o>{</span>
<a id=__codelineno-8-2 name=__codelineno-8-2 href=#__codelineno-8-2></a><span class=nb>local</span><span class=w> </span><span class=nv>var</span><span class=o>=</span>VALUE
<a id=__codelineno-8-3 name=__codelineno-8-3 href=#__codelineno-8-3></a>
<a id=__codelineno-8-4 name=__codelineno-8-4 href=#__codelineno-8-4></a><span class=c1># alternative, only when used INSIDE a function</span>
<a id=__codelineno-8-5 name=__codelineno-8-5 href=#__codelineno-8-5></a><span class=nb>declare</span><span class=w> </span><span class=nv>var</span><span class=o>=</span>VALUE
<a id=__codelineno-8-6 name=__codelineno-8-6 href=#__codelineno-8-6></a>
<a id=__codelineno-8-7 name=__codelineno-8-7 href=#__codelineno-8-7></a>...
<a id=__codelineno-8-8 name=__codelineno-8-8 href=#__codelineno-8-8></a><span class=o>}</span>
</code></pre></div> <p>The <em>local</em> keyword (or declaring a variable using the <code>declare</code> command) tags a variable to be treated <em>completely local and separate</em> inside the function where it was declared:</p> <div class=highlight><pre><span></span><code><a id=__codelineno-9-1 name=__codelineno-9-1 href=#__codelineno-9-1></a><span class=nv>foo</span><span class=o>=</span>external
<a id=__codelineno-9-2 name=__codelineno-9-2 href=#__codelineno-9-2></a>
<a id=__codelineno-9-3 name=__codelineno-9-3 href=#__codelineno-9-3></a>printvalue<span class=o>()</span><span class=w> </span><span class=o>{</span>
<a id=__codelineno-9-4 name=__codelineno-9-4 href=#__codelineno-9-4></a><span class=nb>local</span><span class=w> </span><span class=nv>foo</span><span class=o>=</span>internal
<a id=__codelineno-9-5 name=__codelineno-9-5 href=#__codelineno-9-5></a>
<a id=__codelineno-9-6 name=__codelineno-9-6 href=#__codelineno-9-6></a><span class=nb>echo</span><span class=w> </span><span class=nv>$foo</span>
<a id=__codelineno-9-7 name=__codelineno-9-7 href=#__codelineno-9-7></a><span class=o>}</span>
<a id=__codelineno-9-8 name=__codelineno-9-8 href=#__codelineno-9-8></a>
<a id=__codelineno-9-9 name=__codelineno-9-9 href=#__codelineno-9-9></a>
<a id=__codelineno-9-10 name=__codelineno-9-10 href=#__codelineno-9-10></a><span class=c1># this will print &quot;external&quot;</span>
<a id=__codelineno-9-11 name=__codelineno-9-11 href=#__codelineno-9-11></a><span class=nb>echo</span><span class=w> </span><span class=nv>$foo</span>
<a id=__codelineno-9-12 name=__codelineno-9-12 href=#__codelineno-9-12></a>
<a id=__codelineno-9-13 name=__codelineno-9-13 href=#__codelineno-9-13></a><span class=c1># this will print &quot;internal&quot;</span>
<a id=__codelineno-9-14 name=__codelineno-9-14 href=#__codelineno-9-14></a>printvalue
<a id=__codelineno-9-15 name=__codelineno-9-15 href=#__codelineno-9-15></a>
<a id=__codelineno-9-16 name=__codelineno-9-16 href=#__codelineno-9-16></a><span class=c1># this will print - again - &quot;external&quot;</span>
<a id=__codelineno-9-17 name=__codelineno-9-17 href=#__codelineno-9-17></a><span class=nb>echo</span><span class=w> </span><span class=nv>$foo</span>
</code></pre></div> <h3 id=environment-variables>Environment variables<a class=headerlink href=#environment-variables title="Permanent link">&para;</a></h3> <p>The environment space is not directly related to the topic about scope, but it's worth mentioning.</p> <p>Every UNIX&reg; process has a so-called <em>environment</em>. Other items, in addition to variables, are saved there, the so-called <em>environment variables</em>. When a child process is created (in Bash e.g. by simply executing another program, say <code>ls</code> to list files), the whole environment <em>including the environment variables</em> is copied to the new process. Reading that from the other side means: <strong>Only variables that are part of the environment are available in the child process.</strong></p> <p>A variable can be tagged to be part of the environment using the <code>export</code> command:</p> <div class=highlight><pre><span></span><code><a id=__codelineno-10-1 name=__codelineno-10-1 href=#__codelineno-10-1></a><span class=c1># create a new variable and set it:</span>
<a id=__codelineno-10-2 name=__codelineno-10-2 href=#__codelineno-10-2></a><span class=c1># -&gt; This is a normal shell variable, not an environment variable!</span>
<a id=__codelineno-10-3 name=__codelineno-10-3 href=#__codelineno-10-3></a><span class=nv>myvariable</span><span class=o>=</span><span class=s2>&quot;Hello world.&quot;</span>
<a id=__codelineno-10-4 name=__codelineno-10-4 href=#__codelineno-10-4></a>
<a id=__codelineno-10-5 name=__codelineno-10-5 href=#__codelineno-10-5></a><span class=c1># make the variable visible to all child processes:</span>
<a id=__codelineno-10-6 name=__codelineno-10-6 href=#__codelineno-10-6></a><span class=c1># -&gt; Make it an environment variable: &quot;export&quot; it</span>
<a id=__codelineno-10-7 name=__codelineno-10-7 href=#__codelineno-10-7></a><span class=nb>export</span><span class=w> </span>myvariable
</code></pre></div> <p>Remember that the <em>exported</em> variable is a <strong>copy</strong>. There is no provision to "copy it back to the parent." See the article about <a href=../processtree/ >Bash in the process tree</a>!</p> <div class=footnote> <hr> <ol> <li id=fn:1> <p>under specific circumstances, also by the shell itself&#160;<a class=footnote-backref href=#fnref:1 title="Jump back to footnote 1 in the text">&#8617;</a></p> </li> </ol> </div> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date">October 12, 2024</span> </span> <span class=md-source-file__fact> <span class=md-icon title=Created> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M14.47 15.08 11 13V7h1.5v5.25l3.08 1.83c-.41.28-.79.62-1.11 1m-1.39 4.84c-.36.05-.71.08-1.08.08-4.42 0-8-3.58-8-8s3.58-8 8-8 8 3.58 8 8c0 .37-.03.72-.08 1.08.69.1 1.33.32 1.92.64.1-.56.16-1.13.16-1.72 0-5.5-4.5-10-10-10S2 6.5 2 12s4.47 10 10 10c.59 0 1.16-.06 1.72-.16-.32-.59-.54-1.23-.64-1.92M18 15v3h-3v2h3v3h2v-3h3v-2h-3v-3z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date">October 12, 2024</span> </span> </aside> <h2 id=__comments>Comments</h2> <script src=https://giscus.app/client.js data-repo=flokoe/bash-hackers-wiki data-repo-id=R_kgDOJ3Nr6Q data-category="Giscus Page Comments" data-category-id=DIC_kwDOJ3Nr6c4CXq9t data-mapping=pathname data-strict=1 data-reactions-enabled=1 data-emit-metadata=0 data-input-position=top data-theme=preferred_color_scheme data-lang=en data-loading=lazy crossorigin=anonymous async>
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