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    <p>Wow! That's brings backs lots of memories!</p> <p>Back in the 1980s and early 1990s, there were two basic shells, Bourne shell (<code>/bin/sh</code>) and C shell (<code>/bin/csh</code>).</p> <p>Bourne shell had very few user friendly things. There were no aliases or command substitutions. Therefore, most people liked using C Shell as their default shell.</p> <p>However, Csh was a terrible scripting language. (See <a href="http://www.faqs.org/faqs/unix-faq/shell/csh-whynot/">http://www.faqs.org/faqs/unix-faq/shell/csh-whynot/</a>). Therefore, you used C shell as your shell, but wrote your scripts in Bourne shell which had a much better syntax.</p> <p>However, there was a tiny little problem: Since your default shell is C Shell, typing a shell script name at the command line, and the C Shell would pick it up and try to execute it.</p> <p>To get around this, you put <code>:</code> as the first line in your program. This was a Bourne shell comment, but was an invalid C Shell command. Thus the script would fail to run if you forgot to put <code>sh</code> in front of it.</p> <p>Later on, systems would know if the first line was <code>:</code>, it should be a Bourne shell script. And, even later, you could put <code>#:</code> so it would be a comment and not a command. Some people put the name of the shell, <code>/bin/sh</code> next to it to help remind people were suppose to run this as a Bourne shell script.</p> <p>C shell started dying out after Kornshell started becoming popular. It was about this time when the shebang (<code>#!</code>) came out, but that was only for AT&amp;T and not the Berkeley derived systems. BSD systems didn't get the shebang until the late 1980s. And, Sun people used C Shell as their default shell until Solaris came out.</p> <p>I hadn't seen a program begin with <code>#: /bin/sh</code> in ages.</p> <p>BTW, it is common to start your scripts this way:</p> <pre><code>#! /usr/bin/env perl </code></pre> <p>This way, you use the version of Perl that's in your path and don't have to worry what directory it is in. For example, if you begin your script with:</p> <pre><code>#! /usr/local/bin/perl </code></pre> <p>And, Perl is actually in <code>/usr/bin</code>, your script won't run. The <code>env</code> program is always in <code>/usr/bin</code> and is guaranteed to work. Of course, if you want to use a specific version of Perl and not the one that is in the path, you'd go with the first method.</p>
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    1. PODifference between #!/bin/sh and #:/bin/sh
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    1. USDavid W.
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    1. PODifference between #!/bin/sh and #:/bin/sh
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    1. COThere are [some odd systems](http://en.wikipedia.org/wiki/Shebang_(Unix) that have `env` elsewhere, I believe.
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      1. USDennis Williamson
    2. COThanks a lot for this relevant complete answer !!
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      1. USVlagorce
    3. CO@Dennis: I certainly didn't know there were systems with /bin/env instead of /usr/bin/env. That makes things trickier. However, the two systems pointed out were SCO's OpenServer and the Cray UNICO (or something like that). I haven't seen a Cray in years, and I don't think SCO will be around much longer. Okay, /usr/bin/env isn't universal, but it's pretty damn close. Heck, even the universal <a href="http://en.wikipedia.org/wiki/Fine-structure_constant">fine-structure constant</a>, Alpha, may not be universal.
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