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‎README.md

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## Tips
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* [CLI tip 12: squeeze empty lines](https://learnbyexample.github.io/tips/cli-tip-12/)
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* [Python tip 12: negate a regex grouping](https://learnbyexample.github.io/tips/python-tip-12/)
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* [Vim tip 10: Undo and Redo](https://learnbyexample.github.io/tips/vim-tip-10/)
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* [CLI tip 11: longest line length](https://learnbyexample.github.io/tips/cli-tip-11/)

‎atom.xml

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<link href="https://learnbyexample.github.io/atom.xml" rel="self" type="application/atom+xml"/>
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<link href="https://learnbyexample.github.io"/>
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<generator uri="https://www.getzola.org/">Zola</generator>
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<updated>2022-06-22T00:00:00+00:00</updated>
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<updated>2022-06-29T00:00:00+00:00</updated>
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<id>https://learnbyexample.github.io/atom.xml</id>
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<entry xml:lang="en">
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<title>CLI tip 12: squeeze empty lines</title>
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<published>2022-06-29T00:00:00+00:00</published>
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<updated>2022-06-29T00:00:00+00:00</updated>
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<link href="https://learnbyexample.github.io/tips/cli-tip-12/" type="text/html"/>
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<id>https://learnbyexample.github.io/tips/cli-tip-12/</id>
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<content type="html">&lt;p&gt;&lt;code&gt;awk&lt;&#x2F;code&gt; has a builtin feature to process input content paragraph wise (by setting &lt;code&gt;RS&lt;&#x2F;code&gt; to an empty string). But, did you know that &lt;code&gt;cat&lt;&#x2F;code&gt;, &lt;code&gt;less&lt;&#x2F;code&gt; and &lt;code&gt;grep&lt;&#x2F;code&gt; can also be used to squeeze empty lines?&lt;&#x2F;p&gt;
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&lt;p&gt;&lt;code&gt;cat -s&lt;&#x2F;code&gt; (and &lt;code&gt;less -s&lt;&#x2F;code&gt;) will squeeze multiple empty lines in the input to a single empty line in the output. Here&#x27;s an example:&lt;&#x2F;p&gt;
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&lt;pre style=&quot;background-color:#f5f5f5;&quot;&gt;
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&lt;code&gt;&lt;span style=&quot;color:#1f1f1f;&quot;&gt;$ cat ip.txt
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hello
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world
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apple
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banana
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cherry
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tea coffee
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chocolate
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$ cat &lt;&#x2F;span&gt;&lt;span style=&quot;color:#72ab00;&quot;&gt;-&lt;&#x2F;span&gt;&lt;span style=&quot;color:#1f1f1f;&quot;&gt;s ip.txt
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hello
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world
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apple
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banana
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cherry
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tea coffee
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&lt;&#x2F;span&gt;&lt;&#x2F;code&gt;&lt;&#x2F;pre&gt;
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&lt;p&gt;Here&#x27;s an example with empty lines at the start&#x2F;end of the input:&lt;&#x2F;p&gt;
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&lt;pre style=&quot;background-color:#f5f5f5;&quot;&gt;
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&lt;code&gt;&lt;span style=&quot;color:#1f1f1f;&quot;&gt;$ &lt;&#x2F;span&gt;&lt;span style=&quot;color:#b39f04;&quot;&gt;printf &lt;&#x2F;span&gt;&lt;span style=&quot;color:#d07711;&quot;&gt;&amp;#39;\n\n\ndragon\n\n\nunicorn\n\n\n&amp;#39;&lt;&#x2F;span&gt;&lt;span style=&quot;color:#1f1f1f;&quot;&gt;
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dragon
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unicorn
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$ &lt;&#x2F;span&gt;&lt;span style=&quot;color:#b39f04;&quot;&gt;printf &lt;&#x2F;span&gt;&lt;span style=&quot;color:#d07711;&quot;&gt;&amp;#39;\n\n\ndragon\n\n\nunicorn\n\n\n&amp;#39; &lt;&#x2F;span&gt;&lt;span style=&quot;color:#72ab00;&quot;&gt;|&lt;&#x2F;span&gt;&lt;span style=&quot;color:#1f1f1f;&quot;&gt; cat &lt;&#x2F;span&gt;&lt;span style=&quot;color:#72ab00;&quot;&gt;-&lt;&#x2F;span&gt;&lt;span style=&quot;color:#1f1f1f;&quot;&gt;s
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unicorn
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&lt;&#x2F;span&gt;&lt;&#x2F;code&gt;&lt;&#x2F;pre&gt;
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&lt;p&gt;And here&#x27;s a solution with &lt;code&gt;awk&lt;&#x2F;code&gt;. Unlike the &lt;code&gt;-s&lt;&#x2F;code&gt; option, this will completely remove empty lines at the start&#x2F;end of the input.&lt;&#x2F;p&gt;
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&lt;pre style=&quot;background-color:#f5f5f5;&quot;&gt;
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&lt;code&gt;&lt;span style=&quot;color:#1f1f1f;&quot;&gt;$ awk &lt;&#x2F;span&gt;&lt;span style=&quot;color:#72ab00;&quot;&gt;-&lt;&#x2F;span&gt;&lt;span style=&quot;color:#1f1f1f;&quot;&gt;v &lt;&#x2F;span&gt;&lt;span style=&quot;color:#c23f31;&quot;&gt;RS&lt;&#x2F;span&gt;&lt;span style=&quot;color:#72ab00;&quot;&gt;= &lt;&#x2F;span&gt;&lt;span style=&quot;color:#d07711;&quot;&gt;&amp;#39;{print s $0; s=&amp;quot;\n&amp;quot;}&amp;#39;&lt;&#x2F;span&gt;&lt;span style=&quot;color:#1f1f1f;&quot;&gt; ip.txt
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hello
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apple
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$ &lt;&#x2F;span&gt;&lt;span style=&quot;color:#b39f04;&quot;&gt;printf &lt;&#x2F;span&gt;&lt;span style=&quot;color:#d07711;&quot;&gt;&amp;#39;\n\n\ndragon\n\n\nunicorn\n\n\n&amp;#39; &lt;&#x2F;span&gt;&lt;span style=&quot;color:#72ab00;&quot;&gt;|&lt;&#x2F;span&gt;&lt;span style=&quot;color:#1f1f1f;&quot;&gt; awk &lt;&#x2F;span&gt;&lt;span style=&quot;color:#72ab00;&quot;&gt;-&lt;&#x2F;span&gt;&lt;span style=&quot;color:#1f1f1f;&quot;&gt;v &lt;&#x2F;span&gt;&lt;span style=&quot;color:#c23f31;&quot;&gt;RS&lt;&#x2F;span&gt;&lt;span style=&quot;color:#72ab00;&quot;&gt;= &lt;&#x2F;span&gt;&lt;span style=&quot;color:#d07711;&quot;&gt;&amp;#39;{print s $0; s=&amp;quot;\n&amp;quot;}&amp;#39;&lt;&#x2F;span&gt;&lt;span style=&quot;color:#1f1f1f;&quot;&gt;
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dragon
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&lt;&#x2F;span&gt;&lt;&#x2F;code&gt;&lt;&#x2F;pre&gt;
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&lt;p&gt;The &lt;code&gt;awk&lt;&#x2F;code&gt; solution would be easier to extend, given its programmable features. For example, two empty lines between the groups:&lt;&#x2F;p&gt;
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&lt;pre style=&quot;background-color:#f5f5f5;&quot;&gt;
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&lt;code&gt;&lt;span style=&quot;color:#1f1f1f;&quot;&gt;$ awk &lt;&#x2F;span&gt;&lt;span style=&quot;color:#72ab00;&quot;&gt;-&lt;&#x2F;span&gt;&lt;span style=&quot;color:#1f1f1f;&quot;&gt;v &lt;&#x2F;span&gt;&lt;span style=&quot;color:#c23f31;&quot;&gt;RS&lt;&#x2F;span&gt;&lt;span style=&quot;color:#72ab00;&quot;&gt;= &lt;&#x2F;span&gt;&lt;span style=&quot;color:#d07711;&quot;&gt;&amp;#39;{print s $0; s=&amp;quot;\n\n&amp;quot;}&amp;#39;&lt;&#x2F;span&gt;&lt;span style=&quot;color:#1f1f1f;&quot;&gt; ip.txt
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hello
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world
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apple
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banana
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cherry
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&lt;&#x2F;span&gt;&lt;&#x2F;code&gt;&lt;&#x2F;pre&gt;
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&lt;p&gt;And here&#x27;s a surprising &lt;code&gt;GNU grep&lt;&#x2F;code&gt; solution, with a customizable group separator:&lt;&#x2F;p&gt;
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&lt;pre style=&quot;background-color:#f5f5f5;&quot;&gt;
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&lt;code&gt;&lt;span style=&quot;color:#7f8989;&quot;&gt;# single empty line
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&lt;&#x2F;span&gt;&lt;span style=&quot;color:#1f1f1f;&quot;&gt;$ grep &lt;&#x2F;span&gt;&lt;span style=&quot;color:#72ab00;&quot;&gt;--&lt;&#x2F;span&gt;&lt;span style=&quot;color:#1f1f1f;&quot;&gt;group&lt;&#x2F;span&gt;&lt;span style=&quot;color:#72ab00;&quot;&gt;-&lt;&#x2F;span&gt;&lt;span style=&quot;color:#1f1f1f;&quot;&gt;separator= &lt;&#x2F;span&gt;&lt;span style=&quot;color:#72ab00;&quot;&gt;-&lt;&#x2F;span&gt;&lt;span style=&quot;color:#5597d6;&quot;&gt;A0 &lt;&#x2F;span&gt;&lt;span style=&quot;color:#d07711;&quot;&gt;&amp;#39;.&amp;#39;&lt;&#x2F;span&gt;&lt;span style=&quot;color:#1f1f1f;&quot;&gt; ip.txt
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hello
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world
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apple
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banana
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cherry
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tea coffee
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chocolate
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&lt;&#x2F;span&gt;&lt;span style=&quot;color:#7f8989;&quot;&gt;# double empty line
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# empty lines at the start&#x2F;end of the input are removed too
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&lt;&#x2F;span&gt;&lt;span style=&quot;color:#1f1f1f;&quot;&gt;$ &lt;&#x2F;span&gt;&lt;span style=&quot;color:#b39f04;&quot;&gt;printf &lt;&#x2F;span&gt;&lt;span style=&quot;color:#d07711;&quot;&gt;&amp;#39;\n\n\ndragon\n\n\nunicorn\n\n\n&amp;#39; &lt;&#x2F;span&gt;&lt;span style=&quot;color:#72ab00;&quot;&gt;|&lt;&#x2F;span&gt;&lt;span style=&quot;color:#1f1f1f;&quot;&gt; grep &lt;&#x2F;span&gt;&lt;span style=&quot;color:#72ab00;&quot;&gt;--&lt;&#x2F;span&gt;&lt;span style=&quot;color:#1f1f1f;&quot;&gt;group&lt;&#x2F;span&gt;&lt;span style=&quot;color:#72ab00;&quot;&gt;-&lt;&#x2F;span&gt;&lt;span style=&quot;color:#1f1f1f;&quot;&gt;separator=&lt;&#x2F;span&gt;&lt;span style=&quot;color:#5597d6;&quot;&gt;$&amp;#39;&lt;&#x2F;span&gt;&lt;span style=&quot;color:#1f1f1f;&quot;&gt;\n&lt;&#x2F;span&gt;&lt;span style=&quot;color:#d07711;&quot;&gt;&amp;#39; -A0 &amp;#39;&lt;&#x2F;span&gt;&lt;span style=&quot;color:#1f1f1f;&quot;&gt;.&lt;&#x2F;span&gt;&lt;span style=&quot;color:#d07711;&quot;&gt;&amp;#39;
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&lt;&#x2F;span&gt;&lt;&#x2F;code&gt;&lt;&#x2F;pre&gt;
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&lt;p&gt;&lt;strong&gt;Video demo&lt;&#x2F;strong&gt;:&lt;&#x2F;p&gt;
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&lt;p align=&quot;center&quot;&gt;&lt;iframe width=&quot;560&quot; height=&quot;315&quot; loading=&quot;lazy&quot; src=&quot;https:&#x2F;&#x2F;www.youtube.com&#x2F;embed&#x2F;DjNF2Sbwyxk&quot; title=&quot;YouTube video player&quot; frameborder=&quot;0&quot; allow=&quot;accelerometer; clipboard-write; encrypted-media; gyroscope; picture-in-picture&quot; allowfullscreen&gt;&lt;&#x2F;iframe&gt;&lt;&#x2F;p&gt;
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&lt;br&gt;
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&lt;p&gt;&lt;img src=&quot;&#x2F;images&#x2F;info.svg&quot; alt=&quot;info&quot; &#x2F;&gt; See also my &lt;a href=&quot;https:&#x2F;&#x2F;github.com&#x2F;learnbyexample&#x2F;cli_text_processing_coreutils&quot;&gt;Command line text processing with GNU Coreutils&lt;&#x2F;a&gt;, &lt;a href=&quot;https:&#x2F;&#x2F;github.com&#x2F;learnbyexample&#x2F;learn_gnuawk&quot;&gt;GNU AWK&lt;&#x2F;a&gt; and &lt;a href=&quot;https:&#x2F;&#x2F;github.com&#x2F;learnbyexample&#x2F;learn_gnugrep_ripgrep&quot;&gt;GNU GREP and RIPGREP&lt;&#x2F;a&gt; ebooks.&lt;&#x2F;p&gt;
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</content>
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</entry>
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<title>Python tip 12: negate a regex grouping</title>
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<published>2022-06-22T00:00:00+00:00</published>

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<a href="https:&#x2F;&#x2F;learnbyexample.github.io&#x2F;tips&#x2F;cli-tip-12&#x2F;">CLI tip 12: squeeze empty lines</a>
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