Difference between pgrep in sh and bash

Question

Here is a test:

$ bash -c "pgrep -f novalidname"
$ sh -c "pgrep -f novalidname"
11202

Why is pgrep giving output when run from sh? (As far as I can see, there are no processes on my computer that is named novalidname)

Answer 1

It's probably a timing issue and pgrep finds itself, as you're issuing it with -f and novalidname is present in the command line. Try with -l to confirm.

Answer 2

The actual explanation:

1. Regardless of flags, pgrep never returns its own PID.

2. If you execute bash -c with a simple command, then bash will exec the command rather than creating a redundant subshell to execute it in. Consequently, bash -c "pgrep -f blah" will replace the bash process with a pgrep process. If that pgrep process is the only process whose command line includes blah, then pgrep will not display any PIDs (as per 1).

3. dash does not perform the above optimization. (zsh and ksh do.) So if on your system, sh is implemented with dash, then sh -c "pgrep -f blah" will result in two processes being executed -- the sh process and the pgrep child -- both of which contain blah in their command lines. pgrep will not report itself, but it will report its parent.