SC2337 – ShellCheck Wiki

In pipefail mode, flags like -q, -m, or -L can cause grep to exit early, aborting the pipeline with SIGPIPE. Use a non-pipe input like '< <(cmd)' or '<<<' instead.

Problematic code:

set -o pipefail
find . -exec ls | grep -q myfile

Correct code:

set -o pipefail
grep -q myfile <(find . -exec ls)

Rationale:

When -o pipefail is set and a barrage of input is provided to grep, a race condition makes it possible for the first command in the pipeline to fail to register its SIGPIPE, which leaves grep waiting for input indefinitely. This is dependent on a number of factors, including whether the script is executed interactively, so it can appear inconsistent, which makes the script more difficult to troubleshoot.

In the above example, if there is an issue with the ls command, such as access permissions or non-existent directory, it will either cause the entire pipeline to fail if it produces a non-zero exit code, or hang if it does provide a zero.

Using input redirection avoids a pipe, and therefore does not depend on the setting of -o pipefail in the script, causing consistent behavior.

Exceptions:

If you are not providing a large number of lines through a pipeline to grep, then this warning can be ignored. For instance, it is less likely that a race condition will appear during the output of dig +short as compared to iptables -L.

Original GitHub Issue