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<h1 class="article-title">Bash: the special slash character in filename expansion</h1>
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<time class="article-timestamp" datetime="2015-05-19T18:33:51-07:00">May 19, 2015</time>
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<p>It is well-known and common sense that the slash character (<code>/</code>) serves a special role in Bash filename expansion. For instance, the asterisk <code>*</code> certainly won't match <code>/</code> or <code>.</code> when used in filename expansion; otherwise, a standalone <code>*</code> would match everything in the filesystem.</p>
<p>However, it is less clear how a literal slash character<a class="footnoteRef" href="#fn1" id="fnref1"><sup>1</sup></a> is treated in extended glob patterns. Naively one would expect it to just match a literal slash, but the real situtation is more complicated than that. Consider the following examples:</p>
<div class="sourceCode"><pre class="sourceCode bash"><code class="sourceCode bash"><span class="ex">bash-4.3</span>$ shopt -s extglob nullglob
<span class="ex">bash-4.3</span>$ echo /usr/@(bin<span class="kw">|</span><span class="ex">lib</span>)
<span class="ex">/usr/bin</span> /usr/lib
<span class="ex">bash-4.3</span>$ echo /usr@(/bin<span class="kw">|</span><span class="ex">/lib</span>)
<span class="ex">bash-4.3</span>$ [[ /usr/bin == /usr@(/bin<span class="kw">|</span><span class="ex">/lib</span>) ]] <span class="kw">&&</span> <span class="bu">echo</span> matching
<span class="ex">matching</span></code></pre></div>
<p>As seen from this example, patterns with slash simply doesn't work (in filename expansion) when placed in an extended glob pattern list, and there's no error whatsoever. I looked up the <a href="https://www.gnu.org/software/bash/manual/html_node/Pattern-Matching.html#Pattern-Matching">Bash Reference Manual</a> and the <a href="http://mywiki.wooledge.org/BashGuide/Patterns">Bash Guide</a> but neither mentioned this behavior. One might need to delve into the source code to say for sure what exactly is going on.</p>
<p>In comparison, Zsh and its <a href="http://zsh.sourceforge.net/Doc/Release/Expansion.html#Filename-Generation">docs</a> are much more up front about this issue:</p>
<blockquote>
<p>Note that grouping cannot extend over multiple directories: it is an error to have a â/â within a group (this only applies for patterns used in filename generation). ...</p>
</blockquote>
<p>And when we run equivalent code in Zsh:</p>
<div class="sourceCode"><pre class="sourceCode zsh"><code class="sourceCode zsh">zsh-5.0.5$ <span class="kw">setopt</span> NULL_GLOB
zsh-5.0.5$ <span class="kw">echo</span> /usr/<span class="kw">(</span>bin<span class="kw">|</span>lib<span class="kw">)</span>
/usr/bin /usr/lib
zsh-5.0.5$ <span class="kw">echo</span> /usr<span class="kw">(</span>/bin<span class="kw">|</span>/lib<span class="kw">)</span>
zsh: bad pattern: /usr<span class="kw">(</span>/bin<span class="kw">|</span>/lib<span class="kw">)</span>
zsh-5.0.5$<span class="kw"> [[</span> /usr/bin <span class="ot">==</span> /usr(/bin|/lib)<span class="kw"> ]]</span> <span class="kw">&&</span> <span class="kw">echo</span> matching
matching</code></pre></div>
<p>The lesson? Be careful not to use a pattern like <code>@(path1|path2|path3)</code> in Bash when the paths are absolute, or relative but contain the slash. Unlike Zsh, Bash just silently fails on a pattern like this, which is rather dangerous in scripts.</p>
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<li id="fn1"><p>Here, "a literal slash character" also applies to one that comes from tilde expansion, parameter expansion or command substitution, since they are performed before filename expansion in Bash.<a class="footnotes-backlink" href="#fnref1">âŠī¸</a></p></li>
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