Check whether a variable exists under nounset.
Bash Scripts: The plus expansion distinguishes an unset variable without reading its missing value.
set -euo pipefail
unset setting
if [[ ${setting+x} ]]; then printf "Set\n"; else printf "Unset\n"; fi
setting=""
if [[ ${setting+x} ]]; then printf "Set\n"; fiExpected output EXAMPLE
Unset Set
Details
Requires Bash 4 or later. Save as snippet.sh and run bash snippet.sh in an empty working directory. The plus expansion distinguishes an unset variable without reading its missing value.
Tested on Ubuntu 24.04.5 LTS · · GNU bash, version 5.2.21(1)-release (x86_64-pc-linux-gnu) · bash 5.2.21-2ubuntu4; coreutils 9.4-3ubuntu6.3
Test inputs, results and environment
The exact published code ran in a dedicated Linux test container with prepared inputs. Snippet process uid 65534 with a read-only root filesystem and temporary working files. Networking disabled. Fixtures and output records are isolated from the application database. Testing covers this example and its stated checks, not every input, operating system, or runtime version. The Linux container shares its host's kernel; this is not a native Windows test.
- Architecture / host kernel
- x86_64 / 6.18.54.1-microsoft-standard-WSL2
- Snippet identity / locale
- uid=65534, gid=65534 (unprivileged process) / C.UTF-8
- Interpreter invocation
bash -euo pipefail -c <published code>- Container image ID
sha256:ec91410a8f6ec3e985830b5e3a0560306d36f8c1c5ccd91a6699abe707123fa3- Code SHA-256
4bffe0a30be949aa6654f2d5bdb8a20619c013c2244d809862f19200abf785fa- Dependencies
- bash 5.2.21-2ubuntu4; coreutils 9.4-3ubuntu6.3
Test input setup
These are the prepared inputs used by the test harness. See Details above for the prerequisites and how to run this example.
# No input files required.
Observed standard output
Unset Set
Exit code: 0. Standard error: empty. Validation: Exact stdout comparison. System-dependent values above are one recorded run.