Shellcheck warns you about it: $ cat foo.sh #!/usr/bin/env bash set -euxo pipefail declare x="$(err)" $ shellcheck foo.sh In foo.sh line 3: de

Shellcheck warns you about it: $ cat foo.sh #!/usr/bin/env bash set -euxo pipefail declare x="$(err)" $ shellcheck foo.sh In foo.sh line 3: declare x="$(err)" ^-- SC2034 (warning): x appears unused. Verify use (or export if used externally). ^-- SC2155 (warning): Declare and assign separately to avoid masking return values. For more information: https://www.shellcheck.net/wiki/SC2034 -- x appears unused. Verify use (or ... https://www.shellcheck.net/wiki/SC2155 -- Declare and assign separately to ... And the Bash man page mentions it a couple times, particularly around Pipelines, pipefail, Lists, and Compound Commands. You can also check exit status in the PIPESTATUS array. I almost never use set -o pipefail, and often not even set -e. Your scripts end up failing for mysterious reasons and it takes forever to figure out which part of which pipeline failed. Instead, I simply check the result of the end of the pipe (did it return any data? does it look valid?) and that is good enough 99% of the time.