set -euo pipefail Reference for CI Scripts

set -euo pipefail is the strict-mode preamble that makes a bash script abort on the first real error.

Strict mode is the single most valuable line in a CI script. It turns silent failures into loud ones, but each flag has edge cases worth knowing before you rely on it.

Common flags/usage

• -e / errexit: exit immediately when a command returns non-zero
• -u / nounset: error when an unset variable is referenced
• -o pipefail: a pipeline fails if any stage fails, not just the last
• -x / xtrace: print each command before running it (debugging)
• set +e: temporarily disable errexit around a command you handle

Example

#!/usr/bin/env bash
set -euo pipefail

: "${DEPLOY_ENV:?DEPLOY_ENV must be set}"
curl -fsS "https://api/${DEPLOY_ENV}/health" | jq -e '.ok'

In CI

set -e does NOT fire for a command in an if/while condition, on the left of && or ||, or in a function whose result is tested, so a failing command there is swallowed. pipefail is what makes curl ... | jq fail when curl fails. With -u, reference optional vars as "${VAR:-}" to avoid an unbound-variable abort.

Key takeaways

• set -euo pipefail aborts on the first error, unset variable, or failed pipe stage.
• -e is suppressed in conditionals and before && / ||, so failures there are swallowed.
• Use "${VAR:-}" for optional variables so -u does not abort on them.

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