just a : as first line in a shell

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# 1  
Old 05-26-2003
just a : as first line in a shell

I am working on porting some shells from reliant to solaris
Some of the shells which I have encountered have a single :
(colon) as the first line.What does this do?I have tried to type :
at the prompt ,it did not give any error though I am not sure if it did anything. Can anybody shed any light on this?
Thanks in advance

# 2  
Old 05-26-2003
The colon ( : ) in shell is treated as a NULL value. It will return an exit status of 0 (zero). I dont typically use this much, but it is useful for debugging.

Sometimes used on older systems as the first character in a file to denote a Bourne Shell script (according to O'Reilly, Unix in a Nutshell).

Last edited by google; 05-26-2003 at 08:08 PM..
# 3  
Old 05-27-2003
When the Bourne shell was introduced, it did not have any way to handle comments. It did have the : command which does nothing. So people used the : command as a poor man's comment. Like this:
: This is a comment.
This was better than nothing, but these "comments" have syntax restrictions. And they can have side effects that are persist past the statement.

Then Bill Joy introduced the c shell. It had the # syntax for comments. In these days, the shell had to run shell scripts. The kernel did not yet recognize the "#! /path/to/interpreter". Bill Joy wanted his shell to be able to run both sh scripts and csh scripts. So he had to tell them apart. His solution was to assume that all scripts will start with a comment. So if the first character was a ":", it was a sh script. And if the first character was a "#", it was a csh script.

But then Steve Bourne added in the # syntax to sh. That broke the csh solution. The fix to this problem was to let the kernel handle shell scripts. That is where the "#!" syntax came in.

So if you have an old unix system whose kernel cannot run shell scripts, and you want to run Bourne shell scripts, and you want to launch them from a csh instance, you must start them with a :.

Solaris supports the #! syntax. I would switch to the newer syntax. You are better off letting the kernel do the exec.
# 4  
Old 05-27-2003
I still find : occasionally useful as a non-header command. F'rinstance when I have a complex test condition that would be more awkward to negate, I sometimes write
if [ condition ]; then
    do stuff

The colon is probably not necessary, but it serves as a visual reminder that nothing is happening here (ref: Star Trek III: The Search For Spock) on purpose. I.e., maintenance time in the future is improved.
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