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In a project, I have a test setup where I read the exit status $? of a command.

In my test, I want to ensure that the string foobar is not present in the output.

When I run the command:

./program | grep foobar

It returns 1, indicating a failure.

I need it to return 0.

Is there a way to negate the return value?

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1 Answer 1

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! inverts the exit code of a pipeline.

! ./program | grep -q foobar
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    This inverts all non-zero exit codes into 0 and 0 to 1. But what most want in this scenario is to only invert 0 <-> 1. Using ! doesn't just change the result of exit code 1 (grep for not found) but also other codes like 2. A important thing to note about this answer is that it uses grep -q which changes the exit-code behavior to not exit with non-zero exit code even on fatal errors. Commented Sep 3, 2024 at 8:08
  • @CervEd Your opinion what most people might want in this situation should be a comment on the question, not on an answer. Why do you put all this in a comment anyway instead of providing an additional answer...? Commented Sep 3, 2024 at 20:18
  • I'm providing additional context which I believe is important to understand why this works for this particular problem formulation and invocation of grep (grep -q) but not grep without the quiet flag or other commands which use exit code 1 for something that is not an error and other exit codes for errors (like diff) Commented Sep 4, 2024 at 8:04

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