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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. |
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发表于 2004-4-13 22:11:58
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