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
/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,
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
if $var; then
: 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'
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.