Purpose of the Colon Builtin ‘:’ in Shell Scripts – Understanding Shell Builtins

shell-builtinshell-script

I've hacked on a lot of shell scripts, and sometimes the simplest things baffle me. Today I ran across a script that made extensive use of the : (colon) bash builtin.

The documenation seems simple enough:

: (a colon)  
     : [arguments]

Do nothing beyond expanding arguments and performing redirections. The return status is zero.

However I have previously only seen this used in demonstrations of shell expansion. The use case in the script I ran across made extensive use of this structure:

if [ -f ${file} ]; then
    grep some_string ${file} >> otherfile || :
    grep other_string ${file} >> otherfile || :
fi

There were actually hundreds of greps, but they are just more of the same. No input/output redirects are present other than the simple structure above. No return values are checked later in the script.

I am reading this as a useless construct that says "or do nothing". What purpose could ending these greps with "or do nothing" serve? In what case would this construct cause a different outcome than simply leaving off the || : from all instances?

Best Answer

It appears the :s in your script are being used in lieu of true. If grep doesn't find a match in the file, it will return a nonzero exit code; as jw013 mentions in a comment, if errexit is set, probably by -e on the shebang line, the script would exit if any of the greps fail to find a match. Clearly, that's not what the author wanted, so (s)he added || : to make the exit status of that particular compound command always zero, like the more common (in my experience) || true/|| /bin/true.

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