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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/newbie_traps/ rel=canonical><link href=../debuggingtips/ rel=prev><link href=../nonportable/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.5.44"><title>Beginner Mistakes - The Bash Hackers Wiki</title><link rel=stylesheet href=../../assets/stylesheets/main.0253249f.min.css><link rel=stylesheet href=../../assets/stylesheets/palette.06af60db.min.css><link rel=preconnect href=https://fonts.gstatic.com crossorigin><link rel=stylesheet href="https://fonts.googleapis.com/css?family=Roboto:300,300i,400,400i,700,700i%7CRoboto+Mono:400,400i,700,700i&display=fallback"><style>:root{--md-text-font:"Roboto";--md-code-font:"Roboto Mono"}</style><script>__md_scope=new URL("../..",location),__md_hash=e=>[...e].reduce(((e,_)=>(e<<5)-e+_.charCodeAt(0)),0),__md_get=(e,_=localStorage,t=__md_scope)=>JSON.parse(_.getItem(t.pathname+"."+e)),__md_set=(e,_,t=localStorage,a=__md_scope)=>{try{t.setItem(a.pathname+"."+e,JSON.stringify(_))}catch(e){}}</script></head> <body dir=ltr data-md-color-scheme=default data-md-color-primary=indigo data-md-color-accent=indigo> <input class=md-toggle data-md-toggle=drawer type=checkbox id=__drawer autocomplete=off> <input class=md-toggle data-md-toggle=search type=checkbox id=__search autocomplete=off> <label class=md-overlay for=__drawer></label> <div data-md-component=skip> <a href=#beginner-mistakes class=md-skip> Skip to content </a> </div> <div data-md-component=announce> </div> <header class=md-header data-md-component=header> <nav class="md-header__inner md-grid" aria-label=Header> <a href=../.. title="The Bash Hackers Wiki" class="md-header__button md-logo" aria-label="The Bash Hackers Wiki" data-md-component=logo> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a3 3 0 0 0 3-3 3 3 0 0 0-3-3 3 3 0 0 0-3 3 3 3 0 0 0 3 3m0 3.54C9.64 9.35 6.5 8 3 8v11c3.5 0 6.64 1.35 9 3.54 2.36-2.19 5.5-3.54 9-3.54V8c-3.5 0-6.64 1.35-9 3.54"/></svg> </a> <label class="md-header__button md-icon" for=__drawer> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M3 6h18v2H3zm0 5h18v2H3zm0 5h18v2H3z"/></svg> </label> <div class=md-header__title data-md-component=header-title> <div class=md-header__ellipsis> <div class=md-header__topic> <span class=md-ellipsis> The Bash Hackers Wiki </span> </div> <div class=md-header__topic data-md-component=header-topic> <span class=md-ellipsis> Beginner Mistakes </span> </div> </div> </div> <form class=md-header__option data-md-component=palette> <input class=md-option data-md-color-media data-md-color-scheme=default data-md-color-primary=indigo data-md-color-accent=indigo aria-label="Switch to dark mode" type=radio name=__palette id=__palette_0> <label class="md-header__button md-icon" title="Switch to dark mode" for=__palette_1 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 8a4 4 0 0 0-4 4 4 4 0 0 0 4 4 4 4 0 0 0 4-4 4 4 0 0 0-4-4m0 10a6 6 0 0 1-6-6 6 6 0 0 1 6-6 6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> <input class=md-option data-md-color-media data-md-color-scheme=slate data-md-color-primary=indigo data-md-color-accent=indigo aria-label="Switch to light mode" type=radio name=__palette id=__palette_1> <label class="md-header__button md-icon" title="Switch to light mode" for=__palette_0 hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M12 18c-.89 0-1.74-.2-2.5-.55C11.56 16.5 13 14.42 13 12s-1.44-4.5-3.5-5.45C10.26 6.2 11.11 6 12 6a6 6 0 0 1 6 6 6 6 0 0 1-6 6m8-9.31V4h-4.69L12 .69 8.69 4H4v4.69L.69 12 4 15.31V20h4.69L12 23.31 15.31 20H20v-4.69L23.31 12z"/></svg> </label> </form> <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers
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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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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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