Quote from the man page of grep:

EXIT STATUS
Normally the exit status is 0 if a line is selected,
1 if no lines were selected, and 2 if an error occurred.
However, if the -q or --quiet or --silent is used and a line is selected,
the exit status is 0 even if an error occurred.

TL;DR

(grep $something || true)

To force grep exit 0 when no mathing could be found. This also works with grep -v when you want to find the invert matching result.

Details

If your grep command might be ends with no matching, and you need the output of grep to combine with awk, cut, xargs, ... you should use grep on CircleCI like this:

(grep ... || true)

(grep ... || :)

This () syntax works for bash, sh, zsh

For example:

${COMMAND_WITH_UNSURE_OUTPUT} | (grep ${TARGET_STRING} || true) | cut -d ' ' -f 1 | xargs ...

This will force grep return 0 even if no matching, and you could still pipe the output to another command. Unlike grep -q which returns 0 but no output could be used.

Alternatives

These commands do not have same effect. Might vary from different shells, use at your own risk and make sure you know what you are doing.

(grep $something || true)
{grep $something || true;}

(grep $something || test $? = 1)
{grep $something || test $? = 1;}

(grep $something || [[ $? == 1 ]])
{grep $something || [[ $? == 1 ]];}

(grep $something || :)
{grep $something || :;}

• : Do nothing beyond expanding arguments and performing redirections. The return status is zero.
  • https://www.gnu.org/software/bash/manual/html_node/Bourne-Shell-Builtins.html#Bourne-Shell-Builtins

More discussions in here: https://unix.stackexchange.com/questions/330660/prevent-grep-from-exiting-in-case-of-nomatch

Things about CircleCI

You might want to ask: "But, I pipe the output to another command and the result of echo $? is 0."

According to https://circleci.com/docs/2.0/configuration-reference/#default-shell-options CircleCI uses /bin/bash -eo pipefail for linux job. It will check the exit code of every command between pipes, not just the $? after the whole line of commands exectued.

-e
Exit immediately if a pipeline (which may consist of a single simple command), a subshell command enclosed in parentheses, or one of the commands executed as part of a command list enclosed by braces exits with a non-zero status.

-o pipefail
If pipefail is enabled, the pipeline’s return status is the value of the last (rightmost) command to exit with a non-zero status, or zero if all commands exit successfully. The shell waits for all commands in the pipeline to terminate before returning a value.

Once your grep doesn't find any matching, You will get this error on CircleCI:

Exited with code exit status 1
CircleCI received exit code 1