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    <h1>What is sh</h1> <p><code>sh</code> (or the Shell Command Language) is a programming language described by the <a href="http://pubs.opengroup.org/onlinepubs/009695399/utilities/xcu_chap02.html" rel="noreferrer">POSIX standard</a>. It has many implementations (<code>ksh88</code>, <code>dash</code>, ...). <code>bash</code> can also be considered an implementation of <code>sh</code> (see below).</p> <p>Because <code>sh</code> is a specification, not an implementation, <code>/bin/sh</code> is a symlink (or a hard link) to an actual implementation on most POSIX systems.</p> <h1>What is bash</h1> <p><code>bash</code> started as an <code>sh</code>-compatible implementation (although it predates the POSIX standard by a few years), but as time passed it has acquired many extensions. Many of these extensions may change the behavior of valid POSIX shell scripts, so by itself <code>bash</code> is not a valid POSIX shell. Rather, it is a dialect of the POSIX shell language.</p> <p><code>bash</code> supports a <code>--posix</code> switch, which makes it more POSIX-compliant. It also tries to mimic POSIX if invoked as <code>sh</code>.</p> <h1>sh = bash?</h1> <p>For a long time, <code>/bin/sh</code> used to point to <code>/bin/bash</code> on most GNU/Linux systems. As a result, it had almost become safe to ignore the difference between the two. But that started to change recently.</p> <p>Some popular examples of systems where <code>/bin/sh</code> does not point to <code>/bin/bash</code> (and on some of which <code>/bin/bash</code> may not even exist) are:</p> <ol> <li>Modern Debian and Ubuntu systems, which symlink <code>sh</code> to <code>dash</code> by default;</li> <li><a href="https://en.wikipedia.org/wiki/BusyBox" rel="noreferrer">Busybox</a>, which is usually run during the Linux system boot time as part of <code>initramfs</code>. It uses the <code>ash</code> shell implementation.</li> <li>BSDs, and in general any non-Linux systems. OpenBSD uses <code>pdksh</code>, a descendant of the Korn shell. FreeBSD's <code>sh</code> is a descendant of the original UNIX Bourne shell. Solaris has its own <code>sh</code> which for a long time was not POSIX-compliant; a free implementation is available from the <a href="http://heirloom.sourceforge.net/sh.html" rel="noreferrer">Heirloom project</a>.</li> </ol> <p>How can you find out what <code>/bin/sh</code> points to on your system?</p> <p>The complication is that <code>/bin/sh</code> could be a symbolic link or a hard link. If it's a symbolic link, a <a href="http://pubs.opengroup.org/onlinepubs/9699919799/utilities/file.html" rel="noreferrer">portable</a> way to resolve it is:</p> <pre><code>% file -h /bin/sh /bin/sh: symbolic link to bash </code></pre> <p>If it's a hard link, try</p> <pre><code>% find -L /bin -samefile /bin/sh /bin/sh /bin/bash </code></pre> <p>In fact, the <code>-L</code> flag covers both symlinks and hardlinks, but the disadvantage of this method is that it is not portable — POSIX <a href="http://pubs.opengroup.org/onlinepubs/9699919799/utilities/find.html" rel="noreferrer">does not require</a> <code>find</code> to support the <code>-samefile</code> option, although both <a href="https://www.gnu.org/software/findutils/manual/html_mono/find.html#Hard-Links" rel="noreferrer">GNU find</a> and <a href="https://www.freebsd.org/cgi/man.cgi?find(1)" rel="noreferrer">FreeBSD find</a> support it.</p> <h1>Shebang line</h1> <p>Ultimately, it's up to you to decide which one to use, by writing the «shebang» line.</p> <p>E.g.</p> <pre><code>#!/bin/sh </code></pre> <p>will use <code>sh</code> (and whatever that happens to point to),</p> <pre><code>#!/bin/bash </code></pre> <p>will use <code>/bin/bash</code> if it's available (and fail with an error message if it's not). Of course, you can also specify another implementation, e.g.</p> <pre><code>#!/bin/dash </code></pre> <h1>Which one to use</h1> <p>For my own scripts, I prefer <code>sh</code> for the following reasons:</p> <ul> <li>it is standardized</li> <li>it is much simpler and easier to learn</li> <li>it is portable across POSIX systems — even if they happen not to have <code>bash</code>, they are required to have <code>sh</code></li> </ul> <p>There are advantages to using <code>bash</code> as well. Its features make programming more convenient and similar to programming in other modern programming languages. These include things like scoped local variables and arrays. Plain <code>sh</code> is a very minimalistic programming language.</p>
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