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authorjohannst <johannst@users.noreply.github.com>2020-09-16 22:23:09 +0000
committerjohannst <johannst@users.noreply.github.com>2020-09-16 22:23:09 +0000
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+ <h1><a class="header" href="#awk1" id="awk1">awk(1)</a></h1>
+<pre><code class="language-markdown">awk [opt] program [input]
+ -F &lt;sepstr&gt; field separator string (can be regex)
+ program awk program
+ input file or stdin if not file given
+</code></pre>
+<h2><a class="header" href="#input-processing" id="input-processing">Input processing</a></h2>
+<p>Input is processed in two stages:</p>
+<ol>
+<li>Splitting input into a sequence of <code>records</code>.
+By default split at <code>newline</code> character, but can be changed via the
+builtin <code>RS</code> variable.</li>
+<li>Splitting a <code>record</code> into <code>fields</code>. By default strings without <code>whitespace</code>,
+but can be changed via the builtin variable <code>FS</code> or command line option
+<code>-F</code>.</li>
+</ol>
+<p>Fields are accessed as follows:</p>
+<ul>
+<li><code>$0</code> whole <code>record</code></li>
+<li><code>$1</code> field one</li>
+<li><code>$2</code> field two</li>
+<li>...</li>
+</ul>
+<h2><a class="header" href="#program" id="program">Program</a></h2>
+<p>An <code>awk</code> program is composed of pairs of the form:</p>
+<pre><code class="language-markdown">pattern { action }
+</code></pre>
+<p>The program is run against each <code>record</code> in the input stream. If a <code>pattern</code>
+matches a <code>record</code> the corresponding <code>action</code> is executed and can access the
+<code>fields</code>.</p>
+<pre><code class="language-markdown">INPUT
+ |
+ v
+record ----&gt; ∀ pattern matched
+ | |
+ v v
+fields ----&gt; run associated action
+</code></pre>
+<p>Any valid awk <code>expr</code> can be a <code>pattern</code>.</p>
+<h3><a class="header" href="#special-pattern" id="special-pattern">Special pattern</a></h3>
+<p>awk provides two special patterns, <code>BEGIN</code> and <code>END</code>, which can be used
+multiple times. Actions with those patterns are <strong>executed exactly once</strong>.</p>
+<ul>
+<li><code>BEGIN</code> actions are run before processing the first record</li>
+<li><code>END</code> actions are run after processing the last record</li>
+</ul>
+<h3><a class="header" href="#special-variables" id="special-variables">Special variables</a></h3>
+<ul>
+<li><code>RS</code> <em>record separator</em>: first char is the record separator, by default
+<newline></li>
+<li><code>FS</code> <em>field separator</em>: regex to split records into fields, by default
+<space></li>
+<li><code>NR</code> <em>number record</em>: number of current record</li>
+</ul>
+<h3><a class="header" href="#special-statements--functions" id="special-statements--functions">Special statements &amp; functions</a></h3>
+<ul>
+<li>
+<p><code>printf &quot;fmt&quot;, args...</code></p>
+<p>Print format string, args are comma separated.</p>
+<ul>
+<li><code>%s</code> string</li>
+<li><code>%d</code> decimal</li>
+<li><code>%x</code> hex</li>
+<li><code>%f</code> float</li>
+</ul>
+<p>Width can be specified as <code>%Ns</code>, this reserves <code>N</code> chars for a string.
+For floats one can use <code>%N.Mf</code>, <code>N</code> is the total number including <code>.</code> and
+<code>M</code>.</p>
+</li>
+<li>
+<p><code>strftime(&quot;fmt&quot;)</code></p>
+<p>Print time stamp formatted by <code>fmt</code>.</p>
+<ul>
+<li><code>%Y</code> full year (eg 2020)</li>
+<li><code>%m</code> month (01-12)</li>
+<li><code>%d</code> day (01-31)</li>
+<li><code>%F</code> alias for <code>%Y-%m-%d</code></li>
+<li><code>%H</code> hour (00-23)</li>
+<li><code>%M</code> minute (00-59)</li>
+<li><code>%S</code> second (00-59)</li>
+<li><code>%T</code> alias for <code>%H:%M:%S</code></li>
+</ul>
+</li>
+</ul>
+<h2><a class="header" href="#examples" id="examples">Examples</a></h2>
+<h3><a class="header" href="#filter-records" id="filter-records">Filter records</a></h3>
+<pre><code class="language-bash">awk 'NR%2 == 0 { print $0 }' &lt;file&gt;
+</code></pre>
+<p>The pattern <code>NR%2 == 0</code> matches every second record and the action <code>{ print $0 }</code>
+prints the whole record.</p>
+<h3><a class="header" href="#capture-in-variables" id="capture-in-variables">Capture in variables</a></h3>
+<pre><code class="language-bash"># /proc/&lt;pid&gt;/status
+# Name: cat
+# ...
+# VmRSS: 516 kB
+# ...
+
+for f in /proc/*/status; do
+ cat $f | awk '
+ /^VmRSS/ { rss = $2/1024 }
+ /^Name/ { name = $2 }
+ END { printf &quot;%16s %6d MB\n&quot;, name, rss }';
+done | sort -k2 -n
+</code></pre>
+<p>We capture values from <code>VmRSS</code> and <code>Name</code> into variables and print them at the
+<code>END</code> once processing all records is done.</p>
+<h3><a class="header" href="#run-shell-command-and-capture-output" id="run-shell-command-and-capture-output">Run shell command and capture output</a></h3>
+<pre><code class="language-bash">cat /proc/1/status | awk '
+ /^Pid/ {
+ &quot;ps --no-header -o user &quot; $2 | getline user;
+ print user
+ }'
+</code></pre>
+<p>We build a <code>ps</code> command line and capture the first line of the processes output
+in the <code>user</code> variable and then print it.</p>
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