Script exits with error when var=$(… | grep “value”) is empty, but works when grep has results

I have the following bash code (running on Red Hat) that is exiting when I enable set -o errexit and the variable in the code is empty, BUT works fine when the variable is set; the code is designed to test if a screen session matching .monitor_* exists, and if so do something.

I have the following turned on:

set -o errexit
set -x xtrace; PS4='$LINENO: '

If there is a session matching the above pattern it works; however, if nothing matches it just exits with no information other than the following output from xtrace

someuser:~/scripts/tests> ./if_test.sh
+ ./if_test.sh
+ PS4='$LINENO: '
4: set -o errexit
5: set -o pipefail
6: set -o nounset
88: /usr/bin/ls -lR /var/run/uscreens/S-storage-rsync
88: grep '.monitor_*'
88: awk '{ print $9 }'
88: /usr/bin/grep -Ev 'total|uscreens'
8: ms=

I tested the command I am using to set the ms var and it agrees with the xtrace output, it’s not set.

someuser:~/scripts/tests> test -n "${mn}"
+ test -n ''

I have tried using a select statement and got the same results… I can’t figure it out, anyone able to help? Thanks.

I read through all the possible solution recommendations, nothing seems to address my issue.

The code:

#!/usr/bin/env bash

set -o xtrace; PS4='$LINENO: '
set -o errexit
set -o pipefail
set -o nounset

ms="$(/usr/bin/ls -lR /var/run/uscreens/S-"${USER}" | /usr/bin/grep -Ev "total|uscreens" | grep ".monitor_*" | awk '{ print $9 }')"

if [[ -z "${ms}" ]]; then
    echo "Handling empty result"
elif [[ -n "${ms}" ]]; then
    echo "Handling non-empty result"
fi

The following answer was proposed: Test if a variable is set in bash when using “set -o nounset”; however, it doesn’t address the issue at all. In my case the variable being tested is set and as stated in my detail, it’s set to "", or nothing. Thank you; however, it doesn’t help.

It really seems to be the variable declaration that it isn’t liking.

ms="$(/usr/bin/ls -lR /var/run/uscreens/S-"${USER}" | /usr/bin/grep -Ev "total|uscreens" | grep ".monitor_*" | awk '{ print $9 }')"

Answer

• You’re running set -o pipefail, so if any component in a pipeline has a nonzero exit status, the entire pipeline is treated as having a nonzero exit status.
• Your pipeline runs grep. grep has a nonzero status whenever no matches are found.
• You’re running set -o errexit (aka set -e). With errexit enabled, the script terminates whenever any command fails (subject to a long and complicated set of exceptions; some of these are presented in the exercises section of BashFAQ #105, and others touched on in this excellent reference).

Thus, when you have no matches in your grep command, your script terminates on the command substitution running the pipeline in question.

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If you want to exempt a specific command from set -e‘s behavior, the easiest way to do it is to simply append ||: (shorthand for || true), which marks the command as “checked”.