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<!--replace-end-7--><!--replace-end-4--><!--replace-end-1--></head><body><div class="ui fluid container universe"><!--replace-start-2--><!--replace-start-3--><!--replace-start-6--><div class="ui text container" id="zettel-container" style="position: relative"><div class="zettel-view"><article class="ui raised attached segment zettel-content"><div class="pandoc"><h1 id="title-h1">Expansions and substitutions</h1><p>Bash is weird in that parentheses, braces and brackets are used not just as markers for different code blocks but as the designators of commands in their own right. The type of bracket you use effects how your input is interpreted.</p><p>Below are the main forms of expansion and substitution:</p><table class="ui table"><thead><tr><th>Representation</th><th>Name</th></tr></thead><tbody><tr><td><code>~</code> .</td><td>Tilde expansion</td></tr><tr><td><code>{..}</code></td><td>Brace expansion</td></tr><tr><td><code>${...}</code></td><td>Parameter expansion</td></tr><tr><td><code>$(...)</code></td><td>Command substitution</td></tr><tr><td><code>$((...))</code></td><td>Arithmetic expansion</td></tr></tbody></table><h2 id="brace-expansion-">Brace expansion: <code>{..}</code></h2><p>Brace expansion is for changing a smaller part of a greater whole programmatically. This is best understood by looking at examples:</p><pre><code class="language-none">echo c{a,o,u}t
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cat cot cut</code></pre><pre><code class="language-none">echo /tmp/{1..3}/file.txt
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/tmp/1/file.txt /tmp/2/file.txt /tmp/3/file.txt</code></pre><pre><code class="bash language-bash">echo {1..5}
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1 2 3 4 5
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echo {5..1}
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5 4 3 2 1
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echo {a...c}
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a b c</code></pre><p>We can also set sequences. If we wanted to count to twenty in intervals of two</p><pre><code class="bash language-bash">echo {1..20..2}
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1 3 5 7 9 11 13 15 17 19</code></pre><blockquote><p>Note that we type <em>two</em> dots <strong>not</strong> an elipsis</p></blockquote><h3 id="example-use-case">Example use case</h3><p>We might use brace expansion to generate sequential file names using a pre-defined naming scheme, eg.</p><pre><code class="language-none">touch file_{01..12}{a..d}
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ls
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file_01a
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file_01b
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file_01c
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file_01d
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file_02a
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file_02b
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...
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file_12d</code></pre><p>The syntax here basically means: for each of the elements in the first list, run the second list against them.</p><h2 id="parameter-expansion-">Parameter expansion: <code>${...}</code></h2><p>We use most frequently for returning the value of stored <span class="zettel-link-container cf"><span class="zettel-link" title="Zettel: Variables and datatypes in Bash"><a href="Variables_and_data_types_in_Bash.html">variables</a></span></span>. Techically we do not have to use the braces, we can retrieve with just <code>$var</code> however it’s better to use them to minimise interpretation fuck-ups which happen a lot.</p><p>When the braces are used, this allows us to transform the values before they are returned such as only returning from the 6th character: <code>${var:6}</code>.</p><h2 id="command-substition-">Command substition: <code>$(...)</code></h2><p>Command substitution (circle-brackets) allows us to put the output of one command inside another. Bash runs the bracketed command in a <span class="zettel-link-container cf"><span class="zettel-link" title="Zettel: Shell sessions"><a href="Shell_sessions.html">sub-shell</a></span></span> and then returns it to the main user shell.</p><p>For example:</p><pre><code class="bash language-bash">echo "The current directory is $(pwd)."</code></pre><h2 id="arithemtic-expansion-">Arithemtic expansion: <code>$((...))</code></h2><p>We use arithmetic expansion when we want to calculate numerical values</p><p>See <span class="zettel-link-container cf"><span class="zettel-link" title="Zettel: Working with numbers in Bash"><a href="Working_with_numbers_in_Bash.html">Working with numbers in Bash</a></span></span> for more.</p></div></article><nav class="ui attached segment deemphasized backlinksPane" id="neuron-backlinks-pane"><h3 class="ui header">Backlinks</h3><ul class="backlinks"><li><span class="zettel-link-container cf"><span class="zettel-link"><a href="Working_with_numbers_in_Bash.html">Working with numbers in Bash</a></span></span><ul class="context-list" style="zoom: 85%;"><li class="item"><div class="pandoc"><p>Note: we do not use a dollar-sign when referring to variables within arithmetic evaluation, there is no need. If we do, we get an error. This is because we are using an <span class="zettel-link-container cf"><span class="zettel-link" title="Zettel: Expansions and substitutions"><a href="Expansions_and_substitutions.html">expansion</a></span></span>, therefore the variables are already being interpreted as variables.</p></div></li></ul></li></ul></nav><nav class="ui attached segment deemphasized bottomPane" id="neuron-tags-pane"><div><span class="ui basic label zettel-tag" title="Tag">shell</span></div></nav><nav class="ui bottom attached icon compact inverted menu blue" id="neuron-nav-bar"><!--replace-start-9--><!--replace-end-9--><a class="right item" href="impulse.html" title="Open Impulse"><i class="wave square icon"></i></a></nav></div></div><!--replace-end-6--><!--replace-end-3--><!--replace-end-2--><div class="ui center aligned container footer-version"><div class="ui tiny image"><a href="https://neuron.zettel.page"><img alt="logo" src="https://raw.githubusercontent.com/srid/neuron/master/assets/neuron.svg" title="Generated by Neuron 1.9.35.3" /></a></div></div></div></body></html> |