ispmail-workaround-org/dist/linuxtip/pipes/index.html
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</script> <ul class="top-level astro-3ii7xxms"> <li class="astro-3ii7xxms"> <details open class="astro-3ii7xxms"> <sl-sidebar-restore data-index="0"></sl-sidebar-restore> <summary class="astro-3ii7xxms"> <span class="group-label astro-3ii7xxms"> <span class="large astro-3ii7xxms">ISPmail for Debian 13</span> </span> <svg aria-hidden="true" class="caret astro-3ii7xxms astro-c6vsoqas" width="16" height="16" viewBox="0 0 24 24" fill="currentColor" style="--sl-icon-size: 1.25rem;"><path d="m14.83 11.29-4.24-4.24a1 1 0 1 0-1.42 1.41L12.71 12l-3.54 3.54a1 1 0 0 0 0 1.41 1 1 0 0 0 .71.29 1 1 0 0 0 .71-.29l4.24-4.24a1.002 1.002 0 0 0 0-1.42Z"/></svg> </summary> <ul class="astro-3ii7xxms"> <li class="astro-3ii7xxms"> <a href="/ispmail-trixie/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Start here</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-trixie/upgrading/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Upgrading from Debian Bookworm</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-trixie/install-debian/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Installing Debian</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-trixie/install-the-software-packages/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Install software packages</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-trixie/virtual-domains/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Virtual domains</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-trixie/database/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Database setup</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-trixie/overview/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Big picture</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-trixie/dns-records/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">DNS records</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-trixie/tls-certificate/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">TLS certificate</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-trixie/postfix/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Postfix</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-trixie/dovecot/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Dovecot</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-trixie/lmtp/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">LMTP</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-trixie/webmail/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Webmail using Roundcube</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-trixie/relaying/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Sending / Relaying</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-trixie/imap/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">IMAP</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-trixie/managing-users-aliases-and-domains/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Managing users, aliases and domains</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-trixie/catching-spam-with-rspamd/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Catching spam with rspamd</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-trixie/anti-spoofing-dkim-spf/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Prevent spoofing using DKIM and SPF</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-trixie/catch-all/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Catch-all addresses</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-trixie/quotas/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Quotas</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-trixie/going-live/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Going live</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-trixie/automated-installation/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Automated installation</span> </a> </li> </ul> </details> </li><li class="astro-3ii7xxms"> <details open class="astro-3ii7xxms"> <sl-sidebar-restore data-index="1"></sl-sidebar-restore> <summary class="astro-3ii7xxms"> <span class="group-label astro-3ii7xxms"> <span class="large astro-3ii7xxms">ISPmail for Debian 12</span> </span> <svg aria-hidden="true" class="caret astro-3ii7xxms astro-c6vsoqas" width="16" height="16" viewBox="0 0 24 24" fill="currentColor" style="--sl-icon-size: 1.25rem;"><path d="m14.83 11.29-4.24-4.24a1 1 0 1 0-1.42 1.41L12.71 12l-3.54 3.54a1 1 0 0 0 0 1.41 1 1 0 0 0 .71.29 1 1 0 0 0 .71-.29l4.24-4.24a1.002 1.002 0 0 0 0-1.42Z"/></svg> </summary> <ul class="astro-3ii7xxms"> <li class="astro-3ii7xxms"> <a href="/ispmail-bookworm/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Start here</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-bookworm/whats-new/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">What&#39;s new</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-bookworm/migrating-from-a-bullseye-to-a-bookworm-server/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Migrating from your old (Bullseye) server</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-bookworm/big-picture/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">The big picture</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-bookworm/types-of-email-domains/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Types of email domains</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-bookworm/install-debian-bookworm-on-your-server/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Installing Debian</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-bookworm/install-the-software-packages/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Install software packages</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-bookworm/creating-a-tls-encryption-key-and-certificate/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">TLS certificate</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-bookworm/prepare-the-database/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Preparing the database</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-bookworm/making-postfix-get-its-information-from-the-mariadb-database/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Let Postfix access MariaDB</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-bookworm/catchall-aliases/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Catch-all aliases</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-bookworm/setting-up-dovecot/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Setting up Dovecot</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-bookworm/let-postfix-send-emails-to-dovecot/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Let Postfix send emails to Dovecot</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-bookworm/quotas/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Quotas</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-bookworm/testing-imap/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Testing IMAP</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-bookworm/webmail-using-roundcube/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Webmail using Roundcube</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-bookworm/testing-email-delivery/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Testing email delivery</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-bookworm/relaying-outgoing-emails-through-postfix/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Relaying outgoing emails through Postfix</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-bookworm/catching-spam-with-rspamd/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Catching spam with rspamd</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-bookworm/setting-dns-records/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Setting DNS records</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-bookworm/prevent-spoofing-using-dkim/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Prevent spoofing using DKIM</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-bookworm/managing-users-aliases-and-domains/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Managing users, aliases and domains</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-bookworm/monitoring-and-backup/" aria-current="false" class="astro-3ii7xxms"> <span class="astro-3ii7xxms">Monitoring and Backup</span> </a> </li><li class="astro-3ii7xxms"> <a href="/ispmail-bookworm/automatic-installation-with-ansible/" aria-current="false" 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<main data-pagefind-body class="astro-bguv2lll" lang="en" dir="ltr"> <div class="content-panel astro-7nkwcw3z"> <div class="sl-container astro-7nkwcw3z"> <h1 id="_top" class="astro-j6tvhyss">Pipes and redirection</h1> </div> </div> <div class="content-panel astro-7nkwcw3z"> <div class="sl-container astro-7nkwcw3z"> <div class="sl-markdown-content"> <p>Many system administrators seem to have problems with the concepts of pipes and redirection in a shell. A coworker recently asked me how to deal with log files. How to find the information he was looking for. This article tries to shed some light on it.</p>
<div class="sl-heading-wrapper level-h1"><h1 id="input--output-of-shell-commands">Input / Output of shell commands</h1><a class="sl-anchor-link" href="#input--output-of-shell-commands"><span aria-hidden="true" class="sl-anchor-icon"><svg width="16" height="16" viewBox="0 0 24 24"><path fill="currentcolor" d="m12.11 15.39-3.88 3.88a2.52 2.52 0 0 1-3.5 0 2.47 2.47 0 0 1 0-3.5l3.88-3.88a1 1 0 0 0-1.42-1.42l-3.88 3.89a4.48 4.48 0 0 0 6.33 6.33l3.89-3.88a1 1 0 1 0-1.42-1.42Zm8.58-12.08a4.49 4.49 0 0 0-6.33 0l-3.89 3.88a1 1 0 0 0 1.42 1.42l3.88-3.88a2.52 2.52 0 0 1 3.5 0 2.47 2.47 0 0 1 0 3.5l-3.88 3.88a1 1 0 1 0 1.42 1.42l3.88-3.89a4.49 4.49 0 0 0 0-6.33ZM8.83 15.17a1 1 0 0 0 1.1.22 1 1 0 0 0 .32-.22l4.92-4.92a1 1 0 0 0-1.42-1.42l-4.92 4.92a1 1 0 0 0 0 1.42Z"></path></svg></span><span class="sr-only">Section titled “Input / Output of shell commands”</span></a></div>
<p>Many of the basic Linux/UNIX shell commands work in a similar way. Every command that you start from the shell gets three <em>channels</em> assigned:</p>
<ul>
<li>STDIN (channel 0):<br>
Where your command draws the input from. If you dont specify anything special this will be your keyboard input.</li>
<li>STDOUT (channel 1):<br>
Where your commands output is sent to. If you dont specify anything special the output is displayed in your shell.</li>
<li>STDERR (channel 2):<br>
If anything wrong happens the command will send error message here. By default the output is also displayed in your shell.</li>
</ul>
<p>Try it yourself. The most basic command that just passes everything through from STDIN to STDOUT is the cat command. Just open a shell and type cat and press Enter. Nothing seems to happen. But actually cat is waiting for input. Type something like “hello world”. Every time you press Enter after a line cat will output your input. So you will get an echo of everything you type. To let cat know that you are done with the input send it an end-of-file (EOF) signal by pressing Ctrl-D on an empty line.</p>
<div class="sl-heading-wrapper level-h1"><h1 id="the-pipeline">The pipe(line)</h1><a class="sl-anchor-link" href="#the-pipeline"><span aria-hidden="true" class="sl-anchor-icon"><svg width="16" height="16" viewBox="0 0 24 24"><path fill="currentcolor" d="m12.11 15.39-3.88 3.88a2.52 2.52 0 0 1-3.5 0 2.47 2.47 0 0 1 0-3.5l3.88-3.88a1 1 0 0 0-1.42-1.42l-3.88 3.89a4.48 4.48 0 0 0 6.33 6.33l3.89-3.88a1 1 0 1 0-1.42-1.42Zm8.58-12.08a4.49 4.49 0 0 0-6.33 0l-3.89 3.88a1 1 0 0 0 1.42 1.42l3.88-3.88a2.52 2.52 0 0 1 3.5 0 2.47 2.47 0 0 1 0 3.5l-3.88 3.88a1 1 0 1 0 1.42 1.42l3.88-3.89a4.49 4.49 0 0 0 0-6.33ZM8.83 15.17a1 1 0 0 0 1.1.22 1 1 0 0 0 .32-.22l4.92-4.92a1 1 0 0 0-1.42-1.42l-4.92 4.92a1 1 0 0 0 0 1.42Z"></path></svg></span><span class="sr-only">Section titled “The pipe(line)”</span></a></div>
<p>A more interesting application of the STDIN/STDOUT is to chain commands together. The output of the first command becomes the input of the second command. Imagine the following chain:</p>
<p><img src="/_astro/stdinouterr.D_fTYlSf_ZgmOAW.webp" alt="Diagram of STDIN, STDOUT and STDERR" loading="lazy" decoding="async" fetchpriority="auto" width="500" height="105"></p>
<p>The contents of the file /var/log/syslog are sent (as input) to the grep command. grep will filter the stream for lines containing the word postfix and output that. Now the next grep picks up what was filtered and filter it further for the word removed. So now we have only lines containing both postfix and removed. And finally these lines are sent to wc -l which is a shell command counting the lines of some input. In my case it found 27 of such lines and printed that number to my shell. In shell syntax this reads:</p>
<div class="expressive-code"><link rel="stylesheet" href="/_astro/ec.v4551.css"><script type="module" src="/_astro/ec.0vx5m.js"></script><figure class="frame not-content"><figcaption class="header"></figcaption><pre data-language="plaintext"><code><div class="ec-line"><div class="code"><span style="--0:#d6deeb;--1:#403f53">cat /var/log/syslog | grep 'postfix' | grep 'removed' | wc -l</span></div></div></code></pre><div class="copy"><div aria-live="polite"></div><button title="Copy to clipboard" data-copied="Copied!" data-code="cat /var/log/syslog | grep &#x27;postfix&#x27; | grep &#x27;removed&#x27; | wc -l"><div></div></button></div></figure></div>
<p>The | character is called <em>pipe</em>. A sequence of such commands joined together with pipes are called <em>pipeline</em>.</p>
<div class="sl-heading-wrapper level-h1"><h1 id="useless-use-of-cat">Useless use of cat</h1><a class="sl-anchor-link" href="#useless-use-of-cat"><span aria-hidden="true" class="sl-anchor-icon"><svg width="16" height="16" viewBox="0 0 24 24"><path fill="currentcolor" d="m12.11 15.39-3.88 3.88a2.52 2.52 0 0 1-3.5 0 2.47 2.47 0 0 1 0-3.5l3.88-3.88a1 1 0 0 0-1.42-1.42l-3.88 3.89a4.48 4.48 0 0 0 6.33 6.33l3.89-3.88a1 1 0 1 0-1.42-1.42Zm8.58-12.08a4.49 4.49 0 0 0-6.33 0l-3.89 3.88a1 1 0 0 0 1.42 1.42l3.88-3.88a2.52 2.52 0 0 1 3.5 0 2.47 2.47 0 0 1 0 3.5l-3.88 3.88a1 1 0 1 0 1.42 1.42l3.88-3.89a4.49 4.49 0 0 0 0-6.33ZM8.83 15.17a1 1 0 0 0 1.1.22 1 1 0 0 0 .32-.22l4.92-4.92a1 1 0 0 0-1.42-1.42l-4.92 4.92a1 1 0 0 0 0 1.42Z"></path></svg></span><span class="sr-only">Section titled “Useless use of cat</span></a></div>
<p>Actually cat is supposed to be used for con<strong>cat</strong>enating files. Like “cat file1 file2”. But some administrators abuse the command to put something into a pipeline. Thats bad style and the reason why Randal L. Schwartz (a seasoned programmer) used to hand out virtual <a href="http://partmaps.org/era/unix/award.html">“Useless use of cat” awards</a>. Shell commands usually can take a filename as the last argument as an input. So this would be right:</p>
<div class="expressive-code"><figure class="frame not-content"><figcaption class="header"></figcaption><pre data-language="plaintext"><code><div class="ec-line"><div class="code"><span style="--0:#d6deeb;--1:#403f53">grep something /var/log/syslog | wc -l</span></div></div></code></pre><div class="copy"><div aria-live="polite"></div><button title="Copy to clipboard" data-copied="Copied!" data-code="grep something /var/log/syslog | wc -l"><div></div></button></div></figure></div>
<p>While this works but is considered bad style:</p>
<div class="expressive-code"><figure class="frame not-content"><figcaption class="header"></figcaption><pre data-language="plaintext"><code><div class="ec-line"><div class="code"><span style="--0:#d6deeb;--1:#403f53">cat /var/log/syslog | grep something | wc</span></div></div></code></pre><div class="copy"><div aria-live="polite"></div><button title="Copy to clipboard" data-copied="Copied!" data-code="cat /var/log/syslog | grep something | wc"><div></div></button></div></figure></div>
<p>Or if you knew that grep even has a “-c” option to count lines the whole task could be done with just grep:</p>
<div class="expressive-code"><figure class="frame not-content"><figcaption class="header"></figcaption><pre data-language="plaintext"><code><div class="ec-line"><div class="code"><span style="--0:#d6deeb;--1:#403f53">grep -c something /var/log/syslog</span></div></div></code></pre><div class="copy"><div aria-live="polite"></div><button title="Copy to clipboard" data-copied="Copied!" data-code="grep -c something /var/log/syslog"><div></div></button></div></figure></div>
<div class="sl-heading-wrapper level-h1"><h1 id="using-files-as-input-and-output">Using files as input and output</h1><a class="sl-anchor-link" href="#using-files-as-input-and-output"><span aria-hidden="true" class="sl-anchor-icon"><svg width="16" height="16" viewBox="0 0 24 24"><path fill="currentcolor" d="m12.11 15.39-3.88 3.88a2.52 2.52 0 0 1-3.5 0 2.47 2.47 0 0 1 0-3.5l3.88-3.88a1 1 0 0 0-1.42-1.42l-3.88 3.89a4.48 4.48 0 0 0 6.33 6.33l3.89-3.88a1 1 0 1 0-1.42-1.42Zm8.58-12.08a4.49 4.49 0 0 0-6.33 0l-3.89 3.88a1 1 0 0 0 1.42 1.42l3.88-3.88a2.52 2.52 0 0 1 3.5 0 2.47 2.47 0 0 1 0 3.5l-3.88 3.88a1 1 0 1 0 1.42 1.42l3.88-3.89a4.49 4.49 0 0 0 0-6.33ZM8.83 15.17a1 1 0 0 0 1.1.22 1 1 0 0 0 .32-.22l4.92-4.92a1 1 0 0 0-1.42-1.42l-4.92 4.92a1 1 0 0 0 0 1.42Z"></path></svg></span><span class="sr-only">Section titled “Using files as input and output”</span></a></div>
<div class="sl-heading-wrapper level-h2"><h2 id="output-stdout">Output (STDOUT)</h2><a class="sl-anchor-link" href="#output-stdout"><span aria-hidden="true" class="sl-anchor-icon"><svg width="16" height="16" viewBox="0 0 24 24"><path fill="currentcolor" d="m12.11 15.39-3.88 3.88a2.52 2.52 0 0 1-3.5 0 2.47 2.47 0 0 1 0-3.5l3.88-3.88a1 1 0 0 0-1.42-1.42l-3.88 3.89a4.48 4.48 0 0 0 6.33 6.33l3.89-3.88a1 1 0 1 0-1.42-1.42Zm8.58-12.08a4.49 4.49 0 0 0-6.33 0l-3.89 3.88a1 1 0 0 0 1.42 1.42l3.88-3.88a2.52 2.52 0 0 1 3.5 0 2.47 2.47 0 0 1 0 3.5l-3.88 3.88a1 1 0 1 0 1.42 1.42l3.88-3.89a4.49 4.49 0 0 0 0-6.33ZM8.83 15.17a1 1 0 0 0 1.1.22 1 1 0 0 0 .32-.22l4.92-4.92a1 1 0 0 0-1.42-1.42l-4.92 4.92a1 1 0 0 0 0 1.42Z"></path></svg></span><span class="sr-only">Section titled “Output (STDOUT)”</span></a></div>
<p>Instead of using the console for input and the screen for output you can use files instead. While</p>
<div class="expressive-code"><figure class="frame not-content"><figcaption class="header"></figcaption><pre data-language="plaintext"><code><div class="ec-line"><div class="code"><span style="--0:#d6deeb;--1:#403f53">date</span></div></div></code></pre><div class="copy"><div aria-live="polite"></div><button title="Copy to clipboard" data-copied="Copied!" data-code="date"><div></div></button></div></figure></div>
<p>shows you the current date on the console you can use</p>
<div class="expressive-code"><figure class="frame not-content"><figcaption class="header"></figcaption><pre data-language="plaintext"><code><div class="ec-line"><div class="code"><span style="--0:#d6deeb;--1:#403f53">date >currentdatefile</span></div></div></code></pre><div class="copy"><div aria-live="polite"></div><button title="Copy to clipboard" data-copied="Copied!" data-code="date >currentdatefile"><div></div></button></div></figure></div>
<p>to redirect the <em>output</em> of the command (STDOUT) to the file named currentdatefile.</p>
<div class="sl-heading-wrapper level-h2"><h2 id="input-stdin">Input (STDIN)</h2><a class="sl-anchor-link" href="#input-stdin"><span aria-hidden="true" class="sl-anchor-icon"><svg width="16" height="16" viewBox="0 0 24 24"><path fill="currentcolor" d="m12.11 15.39-3.88 3.88a2.52 2.52 0 0 1-3.5 0 2.47 2.47 0 0 1 0-3.5l3.88-3.88a1 1 0 0 0-1.42-1.42l-3.88 3.89a4.48 4.48 0 0 0 6.33 6.33l3.89-3.88a1 1 0 1 0-1.42-1.42Zm8.58-12.08a4.49 4.49 0 0 0-6.33 0l-3.89 3.88a1 1 0 0 0 1.42 1.42l3.88-3.88a2.52 2.52 0 0 1 3.5 0 2.47 2.47 0 0 1 0 3.5l-3.88 3.88a1 1 0 1 0 1.42 1.42l3.88-3.89a4.49 4.49 0 0 0 0-6.33ZM8.83 15.17a1 1 0 0 0 1.1.22 1 1 0 0 0 .32-.22l4.92-4.92a1 1 0 0 0-1.42-1.42l-4.92 4.92a1 1 0 0 0 0 1.42Z"></path></svg></span><span class="sr-only">Section titled “Input (STDIN)”</span></a></div>
<p>This also works as <em>input</em>. The command</p>
<div class="expressive-code"><figure class="frame not-content"><figcaption class="header"></figcaption><pre data-language="plaintext"><code><div class="ec-line"><div class="code"><span style="--0:#d6deeb;--1:#403f53">grep something</span></div></div></code></pre><div class="copy"><div aria-live="polite"></div><button title="Copy to clipboard" data-copied="Copied!" data-code="grep something"><div></div></button></div></figure></div>
<p>will search for the word something in what you type on your keyboard. But if you want to look for something in a file called somefile you could run</p>
<div class="expressive-code"><figure class="frame not-content"><figcaption class="header"></figcaption><pre data-language="plaintext"><code><div class="ec-line"><div class="code"><span style="--0:#d6deeb;--1:#403f53">grep something &#x3C;somefile</span></div></div></code></pre><div class="copy"><div aria-live="polite"></div><button title="Copy to clipboard" data-copied="Copied!" data-code="grep something <somefile"><div></div></button></div></figure></div>
<div class="sl-heading-wrapper level-h2"><h2 id="input-and-output">Input and output</h2><a class="sl-anchor-link" href="#input-and-output"><span aria-hidden="true" class="sl-anchor-icon"><svg width="16" height="16" viewBox="0 0 24 24"><path fill="currentcolor" d="m12.11 15.39-3.88 3.88a2.52 2.52 0 0 1-3.5 0 2.47 2.47 0 0 1 0-3.5l3.88-3.88a1 1 0 0 0-1.42-1.42l-3.88 3.89a4.48 4.48 0 0 0 6.33 6.33l3.89-3.88a1 1 0 1 0-1.42-1.42Zm8.58-12.08a4.49 4.49 0 0 0-6.33 0l-3.89 3.88a1 1 0 0 0 1.42 1.42l3.88-3.88a2.52 2.52 0 0 1 3.5 0 2.47 2.47 0 0 1 0 3.5l-3.88 3.88a1 1 0 1 0 1.42 1.42l3.88-3.89a4.49 4.49 0 0 0 0-6.33ZM8.83 15.17a1 1 0 0 0 1.1.22 1 1 0 0 0 .32-.22l4.92-4.92a1 1 0 0 0-1.42-1.42l-4.92 4.92a1 1 0 0 0 0 1.42Z"></path></svg></span><span class="sr-only">Section titled “Input and output”</span></a></div>
<p>You can also redirect both <em>input and output</em> in the same command. A politically incorrect way to copy a file would be</p>
<div class="expressive-code"><figure class="frame not-content"><figcaption class="header"></figcaption><pre data-language="plaintext"><code><div class="ec-line"><div class="code"><span style="--0:#d6deeb;--1:#403f53">cat &#x3C;oldfile >newfile</span></div></div></code></pre><div class="copy"><div aria-live="polite"></div><button title="Copy to clipboard" data-copied="Copied!" data-code="cat <oldfile >newfile"><div></div></button></div></figure></div>
<p>Of course you would use <code dir="auto">cp</code> for that purpose in real life.</p>
<div class="sl-heading-wrapper level-h2"><h2 id="errors-stderr">Errors (STDERR)</h2><a class="sl-anchor-link" href="#errors-stderr"><span aria-hidden="true" class="sl-anchor-icon"><svg width="16" height="16" viewBox="0 0 24 24"><path fill="currentcolor" d="m12.11 15.39-3.88 3.88a2.52 2.52 0 0 1-3.5 0 2.47 2.47 0 0 1 0-3.5l3.88-3.88a1 1 0 0 0-1.42-1.42l-3.88 3.89a4.48 4.48 0 0 0 6.33 6.33l3.89-3.88a1 1 0 1 0-1.42-1.42Zm8.58-12.08a4.49 4.49 0 0 0-6.33 0l-3.89 3.88a1 1 0 0 0 1.42 1.42l3.88-3.88a2.52 2.52 0 0 1 3.5 0 2.47 2.47 0 0 1 0 3.5l-3.88 3.88a1 1 0 1 0 1.42 1.42l3.88-3.89a4.49 4.49 0 0 0 0-6.33ZM8.83 15.17a1 1 0 0 0 1.1.22 1 1 0 0 0 .32-.22l4.92-4.92a1 1 0 0 0-1.42-1.42l-4.92 4.92a1 1 0 0 0 0 1.42Z"></path></svg></span><span class="sr-only">Section titled “Errors (STDERR)”</span></a></div>
<p>So far this covers STDIN (<code dir="auto">&#x3C;</code>) and STDOUT (<code dir="auto">></code>) but you also redirect the STDERR channel by using (<code dir="auto">2></code>). An example would be</p>
<div class="expressive-code"><figure class="frame not-content"><figcaption class="header"></figcaption><pre data-language="plaintext"><code><div class="ec-line"><div class="code"><span style="--0:#d6deeb;--1:#403f53">grep something &#x3C;somefile >resultfile 2>errorfile</span></div></div></code></pre><div class="copy"><div aria-live="polite"></div><button title="Copy to clipboard" data-copied="Copied!" data-code="grep something <somefile >resultfile 2>errorfile"><div></div></button></div></figure></div>
<div class="sl-heading-wrapper level-h2"><h2 id="21-magic">2>&#x26;1 magic</h2><a class="sl-anchor-link" href="#21-magic"><span aria-hidden="true" class="sl-anchor-icon"><svg width="16" height="16" viewBox="0 0 24 24"><path fill="currentcolor" d="m12.11 15.39-3.88 3.88a2.52 2.52 0 0 1-3.5 0 2.47 2.47 0 0 1 0-3.5l3.88-3.88a1 1 0 0 0-1.42-1.42l-3.88 3.89a4.48 4.48 0 0 0 6.33 6.33l3.89-3.88a1 1 0 1 0-1.42-1.42Zm8.58-12.08a4.49 4.49 0 0 0-6.33 0l-3.89 3.88a1 1 0 0 0 1.42 1.42l3.88-3.88a2.52 2.52 0 0 1 3.5 0 2.47 2.47 0 0 1 0 3.5l-3.88 3.88a1 1 0 1 0 1.42 1.42l3.88-3.89a4.49 4.49 0 0 0 0-6.33ZM8.83 15.17a1 1 0 0 0 1.1.22 1 1 0 0 0 .32-.22l4.92-4.92a1 1 0 0 0-1.42-1.42l-4.92 4.92a1 1 0 0 0 0 1.42Z"></path></svg></span><span class="sr-only">Section titled “2>&#x26;1 magic”</span></a></div>
<p>Many admins stumble when it comes to redirecting one channel to another. Say you want to redirect both STDOUT and STDERR to the same file. Then you cannot do</p>
<div class="expressive-code"><figure class="frame not-content"><figcaption class="header"></figcaption><pre data-language="plaintext"><code><div class="ec-line"><div class="code"><span style="--0:#d6deeb;--1:#403f53">grep something >resultfile 2>resultfile</span></div></div></code></pre><div class="copy"><div aria-live="polite"></div><button title="Copy to clipboard" data-copied="Copied!" data-code="grep something >resultfile 2>resultfile"><div></div></button></div></figure></div>
<p>It will only redirect the STDOUT (<code dir="auto">></code>) there and keep the resultfile open so “<code dir="auto">2></code>” fails to write to it. Instead you need to do</p>
<div class="expressive-code"><figure class="frame not-content"><figcaption class="header"></figcaption><pre data-language="plaintext"><code><div class="ec-line"><div class="code"><span style="--0:#d6deeb;--1:#403f53">grep something >resultfile 2>&#x26;1</span></div></div></code></pre><div class="copy"><div aria-live="polite"></div><button title="Copy to clipboard" data-copied="Copied!" data-code="grep something >resultfile 2>&#x26;1"><div></div></button></div></figure></div>
<p>This redirects STDOUT (1) to the resultfile and tells STDERR (2) to send the output to what STDOUT is set to (also resultfile).</p>
<p>What does <em>not</em> work is this order:</p>
<div class="expressive-code"><figure class="frame not-content"><figcaption class="header"></figcaption><pre data-language="plaintext"><code><div class="ec-line"><div class="code"><span style="--0:#d6deeb;--1:#403f53">grep something 2>&#x26;1 >resultfile</span></div></div></code></pre><div class="copy"><div aria-live="polite"></div><button title="Copy to clipboard" data-copied="Copied!" data-code="grep something 2>&#x26;1 >resultfile"><div></div></button></div></figure></div>
<p>It may look right to us humans but in fact does not redirect STDERR to the resultfile. The explanation: the shell interprets this line from left to right. So first the “2>&#x26;1” is evaluated which means “send STDERR to the same that STDOUT is <em>currently</em> set to”. As STDOUT is usually just printed to the shell it will send STDERR also to the shell. Next the shell finds “>resultfile” which sends STDOUT to the resultfile but does <em>not</em> touch the previous destination of STDERR. So STDERR output will still end up in the shell.</p>
<div class="sl-heading-wrapper level-h1"><h1 id="interesting-commands">Interesting commands</h1><a class="sl-anchor-link" href="#interesting-commands"><span aria-hidden="true" class="sl-anchor-icon"><svg width="16" height="16" viewBox="0 0 24 24"><path fill="currentcolor" d="m12.11 15.39-3.88 3.88a2.52 2.52 0 0 1-3.5 0 2.47 2.47 0 0 1 0-3.5l3.88-3.88a1 1 0 0 0-1.42-1.42l-3.88 3.89a4.48 4.48 0 0 0 6.33 6.33l3.89-3.88a1 1 0 1 0-1.42-1.42Zm8.58-12.08a4.49 4.49 0 0 0-6.33 0l-3.89 3.88a1 1 0 0 0 1.42 1.42l3.88-3.88a2.52 2.52 0 0 1 3.5 0 2.47 2.47 0 0 1 0 3.5l-3.88 3.88a1 1 0 1 0 1.42 1.42l3.88-3.89a4.49 4.49 0 0 0 0-6.33ZM8.83 15.17a1 1 0 0 0 1.1.22 1 1 0 0 0 .32-.22l4.92-4.92a1 1 0 0 0-1.42-1.42l-4.92 4.92a1 1 0 0 0 0 1.42Z"></path></svg></span><span class="sr-only">Section titled “Interesting commands”</span></a></div>
<ul>
<li>grep<br>
Filters out lines with certain search words. “grep -v” searches for all lines that do <em>not</em> contain the search word.</li>
<li>sort<br>
Sort the output alphabetically (needs to wait until EOF before doing its work). “sort -n” sorts numerically. “sort -u” filters out duplicate lines.usel</li>
<li>wc<br>
Word count. Counts the bytes, words and lines. “wc -l” just outputs how many lines were counted.</li>
<li><a href="http://linux.die.net/man/1/awk">awk</a><br>
A sophisticated language (similar to Perl) that can be used to do something with every line. <code dir="auto">awk {print $3}'</code> outputs the third column of every line.</li>
<li><a href="http://linux.die.net/man/1/sed">sed</a> (stream editor)<br>
A search/replace tool to change something in every line.</li>
<li>less<br>
Useful at the end of a pipe. Allows you to browse through the output one page at a time. (“less” refers to a similar but less capable tool called “more” that allowed you to see the first page and then press Space to view more.)</li>
<li>head<br>
Shows the first ten lines only. “head -50” shows the first 50 lines.</li>
<li>tail<br>
Shows the last ten lines only. “tail -50” shows the last 50 lines. “tail -f” follows a certain file.</li>
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