22.2.3 :

In the beginning, the magic number for Bourne shell scripts used to be a colon followed by a newline. Most Unices still support this, and will correctly pass a file with a single colon as its first line to `/bin/sh' for interpretation. Nobody uses this any more and I suspect some very new Unices may have forgotten about it entirely, so you should stick to the more usual `#! /bin/sh' syntax for your own scripts. You may occasionally come across a very old script that starts with a `:' though, and it is nice to know why!

In addition, all known Bourne compatible shells have a builtin command, `:' which always returns success. It is equivalent to the system command /bin/true, but can be used from a script without the overhead of starting another process. When setting a shell variable as a flag, it is good practice to use the commands, : and false as values, and choose the sense of the variable to be `:' in the common case: When you come to test the value of the variable, you will avoid the overhead of additional processes most of the time.

var=: if $var; then foo fi

The : command described above can take any number of arguments, which it will fastidiously ignore. This allows the `:' character to double up as a comment leader of sorts. Be aware that the characters that follow are not discarded, they are still interpreted by the shell, so metacharacters can have unexpected effects:

$ cat foo : : echo foo : `echo bar` : `echo baz >&2' $ ./foo baz

You may find very old shell scripts that are commented using `:', or new scripts that exploit this behavior in some esoteric fashion. My advice is, don't: It will bite you later.