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HTML
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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> </nav> </header> <div class=md-container data-md-component=container> <nav class=md-tabs aria-label=Tabs data-md-component=tabs> <div class=md-grid> <ul class=md-tabs__list> <li class=md-tabs__item> <a href=../.. class=md-tabs__link> Start </a> </li> <li class=md-tabs__item> <a href=../../commands/classictest/ class=md-tabs__link> Commands </a> </li> <li class=md-tabs__item> <a href=../../dict/directory/ class=md-tabs__link> Dict </a> </li> <li class=md-tabs__item> <a href=../../howto/calculate-dc/ class=md-tabs__link> Howto </a> </li> <li class=md-tabs__item> <a href=../../internals/shell_options/ class=md-tabs__link> Internals </a> </li> <li class=md-tabs__item> <a href=../../misc/bashphorisms/ class=md-tabs__link> Misc </a> </li> <li class="md-tabs__item 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 md-nav--lifted" aria-label=Navigation 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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> <a href=../basics/ class=md-nav__link> <span class=md-ellipsis> The basics of shell scripting </span> </a> </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 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> Beginner Mistakes </span> <span class="md-nav__icon md-icon"></span> </label> <a href=./ class="md-nav__link md-nav__link--active"> <span class=md-ellipsis> Beginner Mistakes </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-execution class=md-nav__link> <span class=md-ellipsis> Script execution </span> </a> <nav class=md-nav aria-label="Script execution"> <ul class=md-nav__list> <li class=md-nav__item> <a href=#your-perfect-bash-script-executes-with-syntax-errors class=md-nav__link> <span class=md-ellipsis> Your perfect Bash script executes with syntax errors </span> </a> </li> <li class=md-nav__item> <a href=#your-script-named-test-doesnt-execute class=md-nav__link> <span class=md-ellipsis> Your script named "test" doesn't execute </span> </a> </li> </ul> </nav> </li> <li class=md-nav__item> <a href=#globbing class=md-nav__link> <span class=md-ellipsis> Globbing </span> </a> <nav class=md-nav aria-label=Globbing> <ul class=md-nav__list> <li class=md-nav__item> <a href=#brace-expansion-is-not-globbing class=md-nav__link> <span class=md-ellipsis> Brace expansion is not globbing </span> </a> </li> </ul> </nav> </li> <li class=md-nav__item> <a href=#test-command class=md-nav__link> <span class=md-ellipsis> Test-command </span> </a> </li> <li class=md-nav__item> <a href=#variables class=md-nav__link> <span class=md-ellipsis> Variables </span> </a> <nav class=md-nav aria-label=Variables> <ul class=md-nav__list> <li class=md-nav__item> <a href=#setting-variables class=md-nav__link> <span class=md-ellipsis> Setting variables </span> </a> <nav class=md-nav aria-label="Setting variables"> <ul class=md-nav__list> <li class=md-nav__item> <a href=#the-dollar-sign class=md-nav__link> <span class=md-ellipsis> The Dollar-Sign </span> </a> </li> <li class=md-nav__item> <a href=#whitespace class=md-nav__link> <span class=md-ellipsis> Whitespace </span> </a> </li> </ul> </nav> </li> <li class=md-nav__item> <a href=#expanding-using-variables class=md-nav__link> <span class=md-ellipsis> Expanding (using) variables </span> </a> </li> <li class=md-nav__item> <a href=#exporting class=md-nav__link> <span class=md-ellipsis> Exporting </span> </a> </li> </ul> </nav> </li> <li class=md-nav__item> <a href=#exit-codes class=md-nav__link> <span class=md-ellipsis> Exit codes </span> </a> <nav class=md-nav aria-label="Exit codes"> <ul class=md-nav__list> <li class=md-nav__item> <a href=#reacting-to-exit-codes class=md-nav__link> <span class=md-ellipsis> Reacting to exit codes </span> </a> </li> <li class=md-nav__item> <a href=#output-vs-return-value class=md-nav__link> <span class=md-ellipsis> Output vs. Return Value </span> </a> </li> </ul> </nav> </li> </ul> </nav> </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-execution class=md-nav__link> <span class=md-ellipsis> Script execution </span> </a> <nav class=md-nav aria-label="Script execution"> <ul class=md-nav__list> <li class=md-nav__item> <a href=#your-perfect-bash-script-executes-with-syntax-errors class=md-nav__link> <span class=md-ellipsis> Your perfect Bash script executes with syntax errors </span> </a> </li> <li class=md-nav__item> <a href=#your-script-named-test-doesnt-execute class=md-nav__link> <span class=md-ellipsis> Your script named "test" doesn't execute </span> </a> </li> </ul> </nav> </li> <li class=md-nav__item> <a href=#globbing class=md-nav__link> <span class=md-ellipsis> Globbing </span> </a> <nav class=md-nav aria-label=Globbing> <ul class=md-nav__list> <li class=md-nav__item> <a href=#brace-expansion-is-not-globbing class=md-nav__link> <span class=md-ellipsis> Brace expansion is not globbing </span> </a> </li> </ul> </nav> </li> <li class=md-nav__item> <a href=#test-command class=md-nav__link> <span class=md-ellipsis> Test-command </span> </a> </li> <li class=md-nav__item> <a href=#variables class=md-nav__link> <span class=md-ellipsis> Variables </span> </a> <nav class=md-nav aria-label=Variables> <ul class=md-nav__list> <li class=md-nav__item> <a href=#setting-variables class=md-nav__link> <span class=md-ellipsis> Setting variables </span> </a> <nav class=md-nav aria-label="Setting variables"> <ul class=md-nav__list> <li class=md-nav__item> <a href=#the-dollar-sign class=md-nav__link> <span class=md-ellipsis> The Dollar-Sign </span> </a> </li> <li class=md-nav__item> <a href=#whitespace class=md-nav__link> <span class=md-ellipsis> Whitespace </span> </a> </li> </ul> </nav> </li> <li class=md-nav__item> <a href=#expanding-using-variables class=md-nav__link> <span class=md-ellipsis> Expanding (using) variables </span> </a> </li> <li class=md-nav__item> <a href=#exporting class=md-nav__link> <span class=md-ellipsis> Exporting </span> </a> </li> </ul> </nav> </li> <li class=md-nav__item> <a href=#exit-codes class=md-nav__link> <span class=md-ellipsis> Exit codes </span> </a> <nav class=md-nav aria-label="Exit codes"> <ul class=md-nav__list> <li class=md-nav__item> <a href=#reacting-to-exit-codes class=md-nav__link> <span class=md-ellipsis> Reacting to exit codes </span> </a> </li> <li class=md-nav__item> <a href=#output-vs-return-value class=md-nav__link> <span class=md-ellipsis> Output vs. Return Value </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/#pitfalls class=md-tag>pitfalls</a> <a href=../../tags/#traps class=md-tag>traps</a> <a href=../../tags/#beginners class=md-tag>beginners</a> </nav> <a href=https://github.com/flokoe/bash-hackers-wiki/edit/main/docs/scripting/newbie_traps.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/newbie_traps.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=beginner-mistakes>Beginner Mistakes<a class=headerlink href=#beginner-mistakes title="Permanent link">¶</a></h1> <p>Here are some typical traps:</p> <h2 id=script-execution>Script execution<a class=headerlink href=#script-execution title="Permanent link">¶</a></h2> <h3 id=your-perfect-bash-script-executes-with-syntax-errors>Your perfect Bash script executes with syntax errors<a class=headerlink href=#your-perfect-bash-script-executes-with-syntax-errors title="Permanent link">¶</a></h3> <p>If you write Bash scripts with Bash specific syntax and features, run them with <u>Bash</u>, and run them with Bash in <u>native mode</u>.</p> <p><strong>Wrong</strong>:</p> <ul> <li>no shebang<ul> <li>the interpreter used depends on the OS implementation and current shell</li> <li><strong>can</strong> be run by calling bash with the script name as an argument, e.g. <code>bash myscript</code></li> </ul> </li> <li><code>#!/bin/sh</code> shebang<ul> <li>depends on what <code>/bin/sh</code> actually is, for a Bash it means compatiblity mode, <strong>not</strong> native mode</li> </ul> </li> </ul> <p>See also:</p> <ul> <li><a href=../bashbehaviour/#sh_mode>Bash startup mode: SH mode</a></li> <li><a href=../bashbehaviour/#posix_run_mode>Bash run mode: POSIX mode</a></li> </ul> <h3 id=your-script-named-test-doesnt-execute>Your script named "test" doesn't execute<a class=headerlink href=#your-script-named-test-doesnt-execute title="Permanent link">¶</a></h3> <p>Give it another name. The executable <code>test</code> already exists.</p> <p>In Bash it's a builtin. With other shells, it might be an executable file. Either way, it's bad name choice!</p> <p>Workaround: You can call it using the pathname:</p> <div class=highlight><pre><span></span><code>/home/user/bin/test
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</code></pre></div> <h2 id=globbing>Globbing<a class=headerlink href=#globbing title="Permanent link">¶</a></h2> <h3 id=brace-expansion-is-not-globbing>Brace expansion is not globbing<a class=headerlink href=#brace-expansion-is-not-globbing title="Permanent link">¶</a></h3> <p>The following command line is not related to globbing (filename expansion):</p> <div class=highlight><pre><span></span><code># YOU EXPECT
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# -i1.vob -i2.vob -i3.vob ....
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echo -i{*.vob,}
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# YOU GET
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# -i*.vob -i
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</code></pre></div> <p><strong>Why?</strong> The brace expansion is simple text substitution. All possible text formed by the prefix, the postfix and the braces themselves are generated. In the example, these are only two: <code>-i*.vob</code> and <code>-i</code>. The filename expansion happens <strong>after</strong> that, so there is a chance that <code>-i*.vob</code> is expanded to a filename - if you have files like <code>-ihello.vob</code>. But it definitely doesn't do what you expected.</p> <p>Please see:</p> <ul> <li><a href=../../syntax/expansion/brace/ >brace</a></li> </ul> <h2 id=test-command>Test-command<a class=headerlink href=#test-command title="Permanent link">¶</a></h2> <ul> <li><code>if [ $foo ] ...</code></li> <li><code>if [-d $dir] ...</code></li> <li>...</li> </ul> <p>Please see:</p> <ul> <li><a href=../../commands/classictest/#pitfalls_summarized>The classic test command - pitfalls</a></li> </ul> <h2 id=variables>Variables<a class=headerlink href=#variables title="Permanent link">¶</a></h2> <h3 id=setting-variables>Setting variables<a class=headerlink href=#setting-variables title="Permanent link">¶</a></h3> <h4 id=the-dollar-sign>The Dollar-Sign<a class=headerlink href=#the-dollar-sign title="Permanent link">¶</a></h4> <p>There is no <code>$</code> (dollar-sign) when you reference the <strong>name</strong> of a variable! Bash is not PHP!</p> <div class=highlight><pre><span></span><code># THIS IS WRONG!
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$myvar="Hello world!"
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</code></pre></div> <p>A variable name preceeded with a dollar-sign always means that the variable gets <strong>expanded</strong>. In the example above, it might expand to nothing (because it wasn't set), effectively resulting in...</p> <div class=highlight><pre><span></span><code>="Hello world!"
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</code></pre></div> <p>...which <strong>definitely is wrong</strong>!</p> <p>When you need the <strong>name</strong> of a variable, you write <strong>only the name</strong>, for example</p> <ul> <li>(as shown above) to set variables: <code>picture=/usr/share/images/foo.png</code></li> <li>to name variables to be used by the <code>read</code> builtin command: <code>read picture</code></li> <li>to name variables to be unset: <code>unset picture</code></li> </ul> <p>When you need the <strong>content</strong> of a variable, you prefix its name with <strong>a dollar-sign</strong>, like</p> <ul> <li>echo \"The used picture is: \$picture\"</li> </ul> <h4 id=whitespace>Whitespace<a class=headerlink href=#whitespace title="Permanent link">¶</a></h4> <p>Putting spaces on either or both sides of the equal-sign (<code>=</code>) when assigning a value to a variable <strong>will</strong> fail.</p> <div class=highlight><pre><span></span><code># INCORRECT 1
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example = Hello
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# INCORRECT 2
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example= Hello
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# INCORRECT 3
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example =Hello
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</code></pre></div> <p>The only valid form is <strong>no spaces between the variable name and assigned value</strong>:</p> <div class=highlight><pre><span></span><code># CORRECT 1
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example=Hello
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# CORRECT 2
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example=" Hello"
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</code></pre></div> <h3 id=expanding-using-variables>Expanding (using) variables<a class=headerlink href=#expanding-using-variables title="Permanent link">¶</a></h3> <p>A typical beginner's trap is quoting.</p> <p>As noted above, when you want to <strong>expand</strong> a variable i.e. \"get the content\", the variable name needs to be prefixed with a dollar-sign. But, since Bash knows various ways to quote and does word-splitting, the result isn't always the same.</p> <p>Let's define an example variable containing text with spaces:</p> <div class=highlight><pre><span></span><code>example="Hello world"
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</code></pre></div> <p>Used form result number of words</p> <hr> <p><code>$example</code> <code>Hello world</code> 2 <code>"$example"</code> <code>Hello world</code> 1 <code>\$example</code> <code>$example</code> 1 <code>'$example'</code> <code>$example</code> 1</p> <p>If you use parameter expansion, you <strong>must</strong> use the <strong>name</strong> (<code>PATH</code>) of the referenced variables/parameters. i.e. <strong>not</strong> (<code>$PATH</code>):</p> <div class=highlight><pre><span></span><code># WRONG!
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echo "The first character of PATH is ${$PATH:0:1}"
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# CORRECT
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echo "The first character of PATH is ${PATH:0:1}"
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</code></pre></div> <p>Note that if you are using variables in <a href=../../syntax/arith_expr/ >arithmetic expressions</a>, then the bare <strong>name</strong> is allowed:</p> <div class=highlight><pre><span></span><code>((a=$a+7)) # Add 7 to a
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((a = a + 7)) # Add 7 to a. Identical to the previous command.
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((a += 7)) # Add 7 to a. Identical to the previous command.
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a=$((a+7)) # POSIX-compatible version of previous code.
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</code></pre></div> <p>Please see:</p> <ul> <li><a href=../../syntax/words/ >words</a></li> <li><a href=../../syntax/quoting/ >quoting</a></li> <li><a href=../../syntax/expansion/wordsplit/ >wordsplit</a></li> <li><a href=../../syntax/pe/ >pe</a></li> </ul> <h3 id=exporting>Exporting<a class=headerlink href=#exporting title="Permanent link">¶</a></h3> <p>Exporting a variable means giving <strong>newly created</strong> (child-)processes a copy of that variable. It does <strong>not</strong> copy a variable created in a child process back to the parent process. The following example does <strong>not</strong> work, since the variable <code>hello</code> is set in a child process (the process you execute to start that script <code>./script.sh</code>):</p> <div class=highlight><pre><span></span><code>$ cat script.sh
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export hello=world
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$ ./script.sh
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$ echo $hello
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$
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</code></pre></div> <p>Exporting is one-way. The direction is from parent process to child process, not the reverse. The above example <strong>will</strong> work, when you don't execute the script, but include (\"source\") it:</p> <div class=highlight><pre><span></span><code>$ source ./script.sh
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$ echo $hello
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world
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$
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</code></pre></div> <p>In this case, the export command is of no use.</p> <p>Please see:</p> <ul> <li><a href=../processtree/ >processtree</a></li> </ul> <h2 id=exit-codes>Exit codes<a class=headerlink href=#exit-codes title="Permanent link">¶</a></h2> <h3 id=reacting-to-exit-codes>Reacting to exit codes<a class=headerlink href=#reacting-to-exit-codes title="Permanent link">¶</a></h3> <p>If you just want to react to an exit code, regardless of its specific value, you <strong>don't need</strong> to use <code>$?</code> in a test command like this:</p> <div class=highlight><pre><span></span><code><a id=__codelineno-0-1 name=__codelineno-0-1 href=#__codelineno-0-1></a>grep<span class=w> </span>^root:<span class=w> </span>/etc/passwd<span class=w> </span>>/dev/null<span class=w> </span><span class=m>2</span>><span class=p>&</span><span class=m>1</span>
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<a id=__codelineno-0-2 name=__codelineno-0-2 href=#__codelineno-0-2></a>
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<a id=__codelineno-0-3 name=__codelineno-0-3 href=#__codelineno-0-3></a><span class=k>if</span><span class=w> </span><span class=o>[</span><span class=w> </span><span class=nv>$?</span><span class=w> </span>-ne<span class=w> </span><span class=m>0</span><span class=w> </span><span class=o>]</span><span class=p>;</span><span class=w> </span><span class=k>then</span>
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<a id=__codelineno-0-4 name=__codelineno-0-4 href=#__codelineno-0-4></a><span class=w> </span><span class=nb>echo</span><span class=w> </span><span class=s2>"root was not found - check the pub at the corner"</span>
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<a id=__codelineno-0-5 name=__codelineno-0-5 href=#__codelineno-0-5></a><span class=k>fi</span>
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</code></pre></div> <p>This can be simplified to:</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>!<span class=w> </span>grep<span class=w> </span>^root:<span class=w> </span>/etc/passwd<span class=w> </span>>/dev/null<span class=w> </span><span class=m>2</span>><span class=p>&</span><span class=m>1</span><span class=p>;</span><span class=w> </span><span class=k>then</span>
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<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>"root was not found - check the pub at the corner"</span>
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<a id=__codelineno-1-3 name=__codelineno-1-3 href=#__codelineno-1-3></a><span class=k>fi</span>
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|
</code></pre></div> <p>Or, simpler yet:</p> <div class=highlight><pre><span></span><code><a id=__codelineno-2-1 name=__codelineno-2-1 href=#__codelineno-2-1></a>grep<span class=w> </span>^root:<span class=w> </span>/etc/passwd<span class=w> </span>>/dev/null<span class=w> </span><span class=m>2</span>><span class=p>&</span><span class=m>1</span><span class=w> </span><span class=o>||</span><span class=w> </span><span class=nb>echo</span><span class=w> </span><span class=s2>"root was not found - check the pub at the corner"</span>
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|
</code></pre></div> <p>If you need the specific value of <code>$?</code>, there's no other choice. But if you need only a \"true/false\" exit indication, there's no need for <code>$?</code>.</p> <p>See also:</p> <ul> <li><a href=../basics/#exit_codes>Exit codes</a></li> </ul> <h3 id=output-vs-return-value>Output vs. Return Value<a class=headerlink href=#output-vs-return-value title="Permanent link">¶</a></h3> <p>It's important to remember the different ways to run a child command, and whether you want the output, the return value, or neither.</p> <p>When you want to run a command (or a pipeline) and save (or print) the <strong>output</strong>, whether as a string or an array, you use Bash's <code>$(command)</code> syntax:</p> <div class=highlight><pre><span></span><code>$(ls -l /tmp)
|
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newvariable=$(printf "foo")
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</code></pre></div> <p>When you want to use the <strong>return value</strong> of a command, just use the command, or add ( ) to run a command or pipeline in a subshell:</p> <div class=highlight><pre><span></span><code>if grep someuser /etc/passwd ; then
|
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# do something
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fi
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|
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if ( w | grep someuser | grep sqlplus ) ; then
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# someuser is logged in and running sqlplus
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fi
|
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</code></pre></div> <p>Make sure you\'re using the form you intended:</p> <div class=highlight><pre><span></span><code># WRONG!
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if $(grep ERROR /var/log/messages) ; then
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# send alerts
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fi
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</code></pre></div> <p>Please see:</p> <ul> <li><a href=../../syntax/ccmd/intro/ >intro</a></li> <li><a href=../../syntax/expansion/cmdsubst/ >cmdsubst</a></li> <li><a href=../../syntax/ccmd/grouping_subshell/ >grouping_subshell</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 7, 2024</span> </span> <span class=md-source-file__fact> <span class=md-icon title=Created> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M14.47 15.08 11 13V7h1.5v5.25l3.08 1.83c-.41.28-.79.62-1.11 1m-1.39 4.84c-.36.05-.71.08-1.08.08-4.42 0-8-3.58-8-8s3.58-8 8-8 8 3.58 8 8c0 .37-.03.72-.08 1.08.69.1 1.33.32 1.92.64.1-.56.16-1.13.16-1.72 0-5.5-4.5-10-10-10S2 6.5 2 12s4.47 10 10 10c.59 0 1.16-.06 1.72-.16-.32-.59-.54-1.23-.64-1.92M18 15v3h-3v2h3v3h2v-3h3v-2h-3v-3z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date">November 7, 2024</span> </span> </aside> <h2 id=__comments>Comments</h2> <script src=https://giscus.app/client.js data-repo=flokoe/bash-hackers-wiki data-repo-id=R_kgDOJ3Nr6Q data-category="Giscus Page Comments" data-category-id=DIC_kwDOJ3Nr6c4CXq9t data-mapping=pathname data-strict=1 data-reactions-enabled=1 data-emit-metadata=0 data-input-position=top data-theme=preferred_color_scheme data-lang=en data-loading=lazy crossorigin=anonymous async>
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