Prevent grep returning an error when input doesn't match

Question

I want to write in a bash script a piece of code that checks if a program is already running. I have the following in order to search whether bar is running

Answer 1

Short answer

Write

ps -ef | grep bar | { grep -v grep || test $? = 1; }

if you are using set -e.

If you use bash's pipefail option (set -o pipefail), remember to apply the exception handling (||test) to every grep in the pipeline:

ps -ef | { grep bar || test $? = 1; } | { grep -v grep || test $? = 1; }

In shell scripts I suggest you to use the ”catch-1-grep“ (c1grep) utility function:

c1grep() { grep "$@" || test $? = 1; }

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Explained

grep's exit status is either 0, 1 or 2: [1]

• 0 means a line is selected
• 1 means no lines were selected
• 2 means an error occurred

grep can also return other codes if it's interrupted by a signal (e.g. 130 for SIGINT).

Since we only want to ignore exit status 1, we use test to suppress that specific exit status.

• If grep returns 0, test is not run.
• If grep returns 1, test is run and returns 0.
• If grep returns any other value, test is run and returns 1.

In the last case, the script will exit immediately due to set -e or set -o pipefail. However, if you don't care about grep errors at all, you can of course write

ps -ef | grep bar | { grep -v grep || true; }

as suggested by Sean.

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[additional] usage in shell scripts

In shell scripts, if you are using grep a lot, I suggest you to define an utility function:

# "catch exit status 1" grep wrapper
c1grep() { grep "$@" || test $? = 1; }

This way your pipe will get short & simple again, without losing the features of set -e and set -o pipefail:

ps -ef | c1grep bar | c1grep -v grep

FYI:

• I called it c1grep to emphasize it's simply catching exit status 1, nothing else.
• I could have called the function grep instead (grep() { env grep "$@" ...; }), but I prefer a less confusing and more explicit name, c1grep.

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[additional] ps + grep

So if you want to know how to avoid grep -v grep or even the | grep part of ps|grep at all, take a look at some of the other answers; but this is somewhat off-topic imho.

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[1] grep manpage