Bash syntax used under a POSIX sh shell

May 23, 2026 · View on GitHub

Playbook ID: shell-dialect-mismatch Category: build Severity: medium Tags: source, shell, ci, bash, sh

What this failure means

A CI workflow or script selects sh but uses Bash-only syntax such as [[ ... ]], source, heredoc strings, functions, or set -o pipefail.

Common log signals

(This playbook uses source-code pattern matching rather than log signals.)

Diagnosis

The command is likely to run under POSIX sh while using syntax that requires Bash. This causes CI-only failures on runners where /bin/sh is not Bash.

Fix steps

  1. Change the workflow step to shell: bash or update the script shebang to Bash.
  2. If Bash is not required, rewrite the command using POSIX-compatible syntax.
  3. Keep the selected shell and script syntax aligned in CI and local runs.

Validation

  • Run faultline inspect . from the repository root and confirm this source finding is absent or intentionally mitigated.
  • Run the script with the configured shell and confirm it exits successfully.

Likely files to inspect

  • .github/workflows/*.yml
  • .github/workflows/*.yaml
  • .gitlab-ci.yml
  • scripts/*.sh

Run Faultline

faultline analyze build.log
faultline explain shell-dialect-mismatch
faultline workflow build.log --json --mode agent

Search phrases this page answers

  • Bash syntax used under a POSIX sh shell
  • Build: bash syntax used under a posix sh shell
  • GitHub Actions bash syntax used under a posix sh shell
  • faultline explain shell-dialect-mismatch

Generated from playbooks/bundled/source/shell-dialect-mismatch.yaml. Do not edit directly — run make docs-generate.