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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/howto/dissectabadoneliner/ rel=canonical><link href=../conffile/ rel=prev><link href=../edit-ed/ rel=next><link rel=icon href=../../assets/images/favicon.png><meta name=generator content="mkdocs-1.6.1, mkdocs-material-9.5.44"><title>Dissect a bad oneliner - 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=#dissect-a-bad-oneliner 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> Dissect a bad oneliner </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(
</code></pre></div> <p>This is an actual one-liner someone asked about in <code>#bash</code>. <strong>There are several things wrong with it. Let's break it down!</strong></p> <div class=highlight><pre><span></span><code><a id=__codelineno-1-1 name=__codelineno-1-1 href=#__codelineno-1-1></a>$<span class=w> </span>ls<span class=w> </span>*.zip<span class=w> </span><span class=p>|</span><span class=w> </span><span class=k>while</span><span class=w> </span><span class=nb>read</span><span class=w> </span>i<span class=p>;</span><span class=w> </span><span class=k>do</span><span class=w> </span>...<span class=p>;</span><span class=w> </span><span class=k>done</span>
</code></pre></div> <p>(Please read <a href=http://mywiki.wooledge.org/ParsingLs>http://mywiki.wooledge.org/ParsingLs</a>.) This command executes <code>ls</code> on the expansion of <code>*.zip</code>. Assuming there are filenames in the current directory that end in '.zip', ls will give a human-readable list of those names. The output of ls is not for parsing. But in sh and bash alike, we can loop safely over the glob itself:</p> <div class=highlight><pre><span></span><code><a id=__codelineno-2-1 name=__codelineno-2-1 href=#__codelineno-2-1></a>$<span class=w> </span><span class=k>for</span><span class=w> </span>i<span class=w> </span><span class=k>in</span><span class=w> </span>*.zip<span class=p>;</span><span class=w> </span><span class=k>do</span><span class=w> </span><span class=nv>j</span><span class=o>=</span><span class=sb>`</span><span class=nb>echo</span><span class=w> </span><span class=nv>$i</span><span class=w> </span><span class=p>|</span><span class=w> </span>sed<span class=w> </span><span class=s1>&#39;s/.zip//g&#39;</span><span class=sb>`</span><span class=p>;</span><span class=w> </span>mkdir<span class=w> </span><span class=nv>$j</span><span class=p>;</span><span class=w> </span><span class=nb>cd</span><span class=w> </span><span class=nv>$j</span><span class=p>;</span><span class=w> </span>unzip<span class=w> </span>../<span class=nv>$i</span><span class=p>;</span><span class=w> </span><span class=nb>cd</span><span class=w> </span>..<span class=p>;</span><span class=w> </span><span class=k>done</span>
</code></pre></div> <p>Let's break it down some more!</p> <div class=highlight><pre><span></span><code><a id=__codelineno-3-1 name=__codelineno-3-1 href=#__codelineno-3-1></a><span class=nv>j</span><span class=o>=</span><span class=sb>`</span><span class=nb>echo</span><span class=w> </span><span class=nv>$i</span><span class=w> </span><span class=p>|</span><span class=w> </span>sed<span class=w> </span><span class=s1>&#39;s/.zip//g&#39;</span><span class=sb>`</span><span class=w> </span><span class=c1># where $i is some name ending in &#39;.zip&#39;</span>
</code></pre></div> <p>The goal here seems to be get the filename without its <code>.zip</code> extension. In fact, there is a POSIX&reg;-compliant command to do this: <code>basename</code> The implementation here is suboptimal in several ways, but the only thing that's genuinely error-prone with this is "<code>echo $i</code>". Echoing an <em>unquoted</em> variable means <a href=../../syntax/expansion/wordsplit/ >wordsplitting</a> will take place, so any whitespace in <code>$i</code> will essentially be normalized. In <code>sh</code> it is necessary to use an external command and a subshell to achieve the goal, but we can eliminate the pipe (subshells, external commands, and pipes carry extra overhead when they launch, so they can really hurt performance in a loop). Just for good measure, let's use the more readable, <a href=../../syntax/expansion/cmdsubst/ >modern</a> <code>$()</code> construct instead of the old style backticks:</p> <div class=highlight><pre><span></span><code><a id=__codelineno-4-1 name=__codelineno-4-1 href=#__codelineno-4-1></a>sh<span class=w> </span>$<span class=w> </span><span class=k>for</span><span class=w> </span>i<span class=w> </span><span class=k>in</span><span class=w> </span>*.zip<span class=p>;</span><span class=w> </span><span class=k>do</span><span class=w> </span><span class=nv>j</span><span class=o>=</span><span class=k>$(</span>basename<span class=w> </span><span class=s2>&quot;</span><span class=nv>$i</span><span class=s2>&quot;</span><span class=w> </span><span class=s2>&quot;.zip&quot;</span><span class=k>)</span><span class=p>;</span><span class=w> </span>mkdir<span class=w> </span><span class=nv>$j</span><span class=p>;</span><span class=w> </span><span class=nb>cd</span><span class=w> </span><span class=nv>$j</span><span class=p>;</span><span class=w> </span>unzip<span class=w> </span>../<span class=nv>$i</span><span class=p>;</span><span class=w> </span><span class=nb>cd</span><span class=w> </span>..<span class=p>;</span><span class=w> </span><span class=k>done</span>
</code></pre></div> <p>In Bash we don't need the subshell or the external basename command. See <a href=../../syntax/pe/#substring_removal>Substring removal with parameter expansion</a>:</p> <div class=highlight><pre><span></span><code><a id=__codelineno-5-1 name=__codelineno-5-1 href=#__codelineno-5-1></a>bash<span class=w> </span>$<span class=w> </span><span class=k>for</span><span class=w> </span>i<span class=w> </span><span class=k>in</span><span class=w> </span>*.zip<span class=p>;</span><span class=w> </span><span class=k>do</span><span class=w> </span><span class=nv>j</span><span class=o>=</span><span class=s2>&quot;</span><span class=si>${</span><span class=nv>i</span><span class=p>%.zip</span><span class=si>}</span><span class=s2>&quot;</span><span class=p>;</span><span class=w> </span>mkdir<span class=w> </span><span class=nv>$j</span><span class=p>;</span><span class=w> </span><span class=nb>cd</span><span class=w> </span><span class=nv>$j</span><span class=p>;</span><span class=w> </span>unzip<span class=w> </span>../<span class=nv>$i</span><span class=p>;</span><span class=w> </span><span class=nb>cd</span><span class=w> </span>..<span class=p>;</span><span class=w> </span><span class=k>done</span>
</code></pre></div> <p>Let's keep going:</p> <div class=highlight><pre><span></span><code><a id=__codelineno-6-1 name=__codelineno-6-1 href=#__codelineno-6-1></a>$<span class=w> </span>mkdir<span class=w> </span><span class=nv>$j</span><span class=p>;</span><span class=w> </span><span class=nb>cd</span><span class=w> </span><span class=nv>$j</span><span class=p>;</span><span class=w> </span>...<span class=p>;</span><span class=w> </span><span class=nb>cd</span><span class=w> </span>..
</code></pre></div> <p>As a programmer, you <strong>never</strong> know the situation under which your program will run. Even if you do, the following best practice will never hurt: When a following command depends on the success of a previous command(s), check for success! You can do this with the "<code>&amp;&amp;</code>" conjunction, that way, if the previous command fails, bash will not try to execute the following command(s). It's fully POSIX&reg;. Oh, and remember what I said about <a href=../../syntax/expansion/wordsplit/ >wordsplitting</a> in the previous step? Well, if you don't quote <code>$j</code>, wordsplitting can happen again.</p> <div class=highlight><pre><span></span><code><a id=__codelineno-7-1 name=__codelineno-7-1 href=#__codelineno-7-1></a>$<span class=w> </span>mkdir<span class=w> </span><span class=s2>&quot;</span><span class=nv>$j</span><span class=s2>&quot;</span><span class=w> </span><span class=o>&amp;&amp;</span><span class=w> </span><span class=nb>cd</span><span class=w> </span><span class=s2>&quot;</span><span class=nv>$j</span><span class=s2>&quot;</span><span class=w> </span><span class=o>&amp;&amp;</span><span class=w> </span>...<span class=w> </span><span class=o>&amp;&amp;</span><span class=w> </span><span class=nb>cd</span><span class=w> </span>..
</code></pre></div> <p>That's almost right, but there's one problem -- what happens if <code>$j</code> contains a slash? Then <code>cd ..</code> will not return to the original directory. That's wrong! <code>cd -</code> causes cd to return to the previous working directory, so it's a much better choice:</p> <div class=highlight><pre><span></span><code><a id=__codelineno-8-1 name=__codelineno-8-1 href=#__codelineno-8-1></a>$<span class=w> </span>mkdir<span class=w> </span><span class=s2>&quot;</span><span class=nv>$j</span><span class=s2>&quot;</span><span class=w> </span><span class=o>&amp;&amp;</span><span class=w> </span><span class=nb>cd</span><span class=w> </span><span class=s2>&quot;</span><span class=nv>$j</span><span class=s2>&quot;</span><span class=w> </span><span class=o>&amp;&amp;</span><span class=w> </span>...<span class=w> </span><span class=o>&amp;&amp;</span><span class=w> </span><span class=nb>cd</span><span class=w> </span>-
</code></pre></div> <p>(If it occurred to you that I forgot to check for success after cd -, good job! You could do this with <code>{ cd - || break; }</code>, but I'm going to leave that out because it's verbose and I think it's likely that we will be able to get back to our original working directory without a problem.)</p> <p>So now we have:</p> <div class=highlight><pre><span></span><code><a id=__codelineno-9-1 name=__codelineno-9-1 href=#__codelineno-9-1></a>sh<span class=w> </span>$<span class=w> </span><span class=k>for</span><span class=w> </span>i<span class=w> </span><span class=k>in</span><span class=w> </span>*.zip<span class=p>;</span><span class=w> </span><span class=k>do</span><span class=w> </span><span class=nv>j</span><span class=o>=</span><span class=k>$(</span>basename<span class=w> </span><span class=s2>&quot;</span><span class=nv>$i</span><span class=s2>&quot;</span><span class=w> </span><span class=s2>&quot;.zip&quot;</span><span class=k>)</span><span class=p>;</span><span class=w> </span>mkdir<span class=w> </span><span class=s2>&quot;</span><span class=nv>$j</span><span class=s2>&quot;</span><span class=w> </span><span class=o>&amp;&amp;</span><span class=w> </span><span class=nb>cd</span><span class=w> </span><span class=s2>&quot;</span><span class=nv>$j</span><span class=s2>&quot;</span><span class=w> </span><span class=o>&amp;&amp;</span><span class=w> </span>unzip<span class=w> </span>../<span class=nv>$i</span><span class=w> </span><span class=o>&amp;&amp;</span><span class=w> </span><span class=nb>cd</span><span class=w> </span>-<span class=p>;</span><span class=w> </span><span class=k>done</span>
</code></pre></div> <div class=highlight><pre><span></span><code><a id=__codelineno-10-1 name=__codelineno-10-1 href=#__codelineno-10-1></a>bash<span class=w> </span>$<span class=w> </span><span class=k>for</span><span class=w> </span>i<span class=w> </span><span class=k>in</span><span class=w> </span>*.zip<span class=p>;</span><span class=w> </span><span class=k>do</span><span class=w> </span><span class=nv>j</span><span class=o>=</span><span class=s2>&quot;</span><span class=si>${</span><span class=nv>i</span><span class=p>%.zip</span><span class=si>}</span><span class=s2>&quot;</span><span class=p>;</span><span class=w> </span>mkdir<span class=w> </span><span class=s2>&quot;</span><span class=nv>$j</span><span class=s2>&quot;</span><span class=w> </span><span class=o>&amp;&amp;</span><span class=w> </span><span class=nb>cd</span><span class=w> </span><span class=s2>&quot;</span><span class=nv>$j</span><span class=s2>&quot;</span><span class=w> </span><span class=o>&amp;&amp;</span><span class=w> </span>unzip<span class=w> </span>../<span class=nv>$i</span><span class=w> </span><span class=o>&amp;&amp;</span><span class=w> </span><span class=nb>cd</span><span class=w> </span>-<span class=p>;</span><span class=w> </span><span class=k>done</span>
</code></pre></div> <p>Let's throw the <code>unzip</code> command back in the mix:</p> <div class=highlight><pre><span></span><code><a id=__codelineno-11-1 name=__codelineno-11-1 href=#__codelineno-11-1></a>mkdir<span class=w> </span><span class=s2>&quot;</span><span class=nv>$j</span><span class=s2>&quot;</span><span class=w> </span><span class=o>&amp;&amp;</span><span class=w> </span><span class=nb>cd</span><span class=w> </span><span class=s2>&quot;</span><span class=nv>$j</span><span class=s2>&quot;</span><span class=w> </span><span class=o>&amp;&amp;</span><span class=w> </span>unzip<span class=w> </span>../<span class=nv>$i</span><span class=w> </span><span class=o>&amp;&amp;</span><span class=w> </span><span class=nb>cd</span><span class=w> </span>-
</code></pre></div> <p>Well, besides word splitting, there's nothing terribly wrong with this. Still, did it occur to you that unzip might already be able to target a directory? There isn't a standard for the <code>unzip</code> command, but all the implementations I've seen can do it with the -d flag. So we can drop the cd commands entirely:</p> <div class=highlight><pre><span></span><code><a id=__codelineno-12-1 name=__codelineno-12-1 href=#__codelineno-12-1></a>$<span class=w> </span>mkdir<span class=w> </span><span class=s2>&quot;</span><span class=nv>$j</span><span class=s2>&quot;</span><span class=w> </span><span class=o>&amp;&amp;</span><span class=w> </span>unzip<span class=w> </span>-d<span class=w> </span><span class=s2>&quot;</span><span class=nv>$j</span><span class=s2>&quot;</span><span class=w> </span><span class=s2>&quot;</span><span class=nv>$i</span><span class=s2>&quot;</span>
</code></pre></div> <div class=highlight><pre><span></span><code><a id=__codelineno-13-1 name=__codelineno-13-1 href=#__codelineno-13-1></a>sh<span class=w> </span>$<span class=w> </span><span class=k>for</span><span class=w> </span>i<span class=w> </span><span class=k>in</span><span class=w> </span>*.zip<span class=p>;</span><span class=w> </span><span class=k>do</span><span class=w> </span><span class=nv>j</span><span class=o>=</span><span class=k>$(</span>basename<span class=w> </span><span class=s2>&quot;</span><span class=nv>$i</span><span class=s2>&quot;</span><span class=w> </span><span class=s2>&quot;.zip&quot;</span><span class=k>)</span><span class=p>;</span><span class=w> </span>mkdir<span class=w> </span><span class=s2>&quot;</span><span class=nv>$j</span><span class=s2>&quot;</span><span class=w> </span><span class=o>&amp;&amp;</span><span class=w> </span>unzip<span class=w> </span>-d<span class=w> </span><span class=s2>&quot;</span><span class=nv>$j</span><span class=s2>&quot;</span><span class=w> </span><span class=s2>&quot;</span><span class=nv>$i</span><span class=s2>&quot;</span><span class=p>;</span><span class=w> </span><span class=k>done</span>
</code></pre></div> <div class=highlight><pre><span></span><code><a id=__codelineno-14-1 name=__codelineno-14-1 href=#__codelineno-14-1></a>bash<span class=w> </span>$<span class=w> </span><span class=k>for</span><span class=w> </span>i<span class=w> </span><span class=k>in</span><span class=w> </span>*.zip<span class=p>;</span><span class=w> </span><span class=k>do</span><span class=w> </span><span class=nv>j</span><span class=o>=</span><span class=s2>&quot;</span><span class=si>${</span><span class=nv>i</span><span class=p>%.zip</span><span class=si>}</span><span class=s2>&quot;</span><span class=p>;</span><span class=w> </span>mkdir<span class=w> </span><span class=s2>&quot;</span><span class=nv>$j</span><span class=s2>&quot;</span><span class=w> </span><span class=o>&amp;&amp;</span><span class=w> </span>unzip<span class=w> </span>-d<span class=w> </span><span class=s2>&quot;</span><span class=nv>$j</span><span class=s2>&quot;</span><span class=w> </span><span class=s2>&quot;</span><span class=nv>$i</span><span class=s2>&quot;</span><span class=p>;</span><span class=w> </span><span class=k>done</span>
</code></pre></div> <p>There! That's as good as it gets.</p> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date">November 13, 2024</span> </span> <span class=md-source-file__fact> <span class=md-icon title=Created> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M14.47 15.08 11 13V7h1.5v5.25l3.08 1.83c-.41.28-.79.62-1.11 1m-1.39 4.84c-.36.05-.71.08-1.08.08-4.42 0-8-3.58-8-8s3.58-8 8-8 8 3.58 8 8c0 .37-.03.72-.08 1.08.69.1 1.33.32 1.92.64.1-.56.16-1.13.16-1.72 0-5.5-4.5-10-10-10S2 6.5 2 12s4.47 10 10 10c.59 0 1.16-.06 1.72-.16-.32-.59-.54-1.23-.64-1.92M18 15v3h-3v2h3v3h2v-3h3v-2h-3v-3z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date">November 13, 2024</span> </span> </aside> <h2 id=__comments>Comments</h2> <script src=https://giscus.app/client.js data-repo=flokoe/bash-hackers-wiki data-repo-id=R_kgDOJ3Nr6Q data-category="Giscus Page Comments" data-category-id=DIC_kwDOJ3Nr6c4CXq9t data-mapping=pathname data-strict=1 data-reactions-enabled=1 data-emit-metadata=0 data-input-position=top data-theme=preferred_color_scheme data-lang=en data-loading=lazy crossorigin=anonymous async>
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