Use of '#!' in *nix scripts

It's a way of defining of which shell should be invoked during execution of the script. It's called the Shebang.

Example, a file having the following Shebang line:

#!/bin/sh

would invoke the Bourne (sh) shell. So for example, your piece of script:

## Setting a default route 
#!/bin/sh

/sbin/route add SUBNET $5

would make /sbin/route add SUBNET $5 run in the Bourne shell interpreter. /sbin/route add SUBNET $5 is just a comment and doesn't do anything. However, the shebang should be on the first line, therefore that comment at the top will disallow the shebang to do its work, making it a useless addition.

There's a number of interpreters that are widely used and some typical interpreters for shebang lines are:

#!/bin/sh — Execute using sh, the Bourne shell (or a compatible shell)
#!/bin/csh — Execute using csh, the C shell (or a compatible shell)
#!/usr/bin/perl — Execute using Perl
#!/usr/bin/python — Execute using Python
#!/usr/bin/php — Execute using PHP's command line interpreter

As Olli mentioned in his comments, sometimes env (short for environment) is used to make sure the correct path to the interpreter is chosen.

BloodPhilia

Posted 2011-02-23T11:18:28.187

Reputation: 27 374

4Actually for non-standard things (for example perl, python and php), it's better idea to use env: #!/usr/bin/env perl, for example. That'll automatically choose correct path for perl, even if it's not /usr/bin. – Olli – 2011-02-23T11:42:15.057

@Olli it's just some examples! ;) But you're right of course... – BloodPhilia – 2011-02-23T11:43:04.980

@BloodPhilia: if so, it may be good idea to edit those examples :) – Olli – 2011-02-23T11:44:43.650

9The shebang has to be the first line so the initial commented line would prevent the second line from doing anything. Another style that's often used is #!/usr/bin/env python which will usually cause the script to be run by whatever python occurs first in the PATH. This is useful since sometimes executables are installed in different places so this works without having to modify the script. In a few rare cases, env is located somewhere else, so even this fails. – Paused until further notice. – 2011-02-23T11:46:04.023

1@Olli Happy now xD? – BloodPhilia – 2011-02-23T11:48:32.087

Well, and in a few rare cases there is no /bin/sh available. I would claim that /usr/bin/env is pretty much safe choice nowadays, especially compared to python/perl/php. – Olli – 2011-02-23T11:55:57.257

@Olli In a few rare cases, there's Windows installed... I can't cover every possiblity there ever can be... The OP just wants to know what the line means, not its best practices... – BloodPhilia – 2011-02-23T12:01:55.657

1@Olli: Under the Filesystem Hierarchy Standard, "essential" programs which must be available in single-user mode must be placed in /bin, while /usr is for non-essential programs, so /bin/sh is a safer bet than /usr/bin/env - it's always possible that /usr may not be mounted, after all. I agree that /usr/bin/env perl is definitely better than /usr/bin/perl, though. – Dave Sherohman – 2011-02-23T12:33:25.813

+1 for saying "the Bourne shell (or a compatible shell)". Some systems don't use Bourne shell, but still call it sh. – AndrejaKo – 2011-02-23T14:45:37.373

Thanks all foor the clear answers! Really helpful. Also thanks to BloodPhilia for the better formatting of my question. – Edgar Wieringa – 2011-02-24T04:40:12.903

Use of '#!' in *nix scripts

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