I'm running Puppet, and I need to run one of the defined exec commands only if a line does not exist in the output from a different command. I can set up a contingency with a unless clause (which will run the command unless another command returns exit code 0) or with a onlyif clause (which will run a command only if another command returns exit code 0).
Is there a standard Linux command which will verify a particular substring exists in the input, and return an exit code otherwise?
Would grep do what you want ?
In particular
grep -q something somefile
will exit with status 0 if something was in somefile and 1 if not.
The somefile can be a pipe too
somecommand | grep -q something
will exit with status 0 if the output from somecommand contains something.
if somecommand | grep -q expected ; then it-was-there ; else scream-it-was-not-there ; fi
grep -q conforms to POSIX. (A little late, but just in case someone else stumbles across this.)
grep. Everything else seems to support it. Thanks! Erroneous comment baleeted.