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Test each script as a defined command before wiring them together: document its inputs, outputs, exit status and side effects; run ShellCheck and shfmt; add repeatable Bats tests for success and failure; then test the combined command through the interface users will call.
Why test in layers before combining scripts?
Several Bash scripts can work perfectly when launched alone and still fail after they share a dispatcher, configuration, working directory or error path. A wrapper may pass arguments differently, hide a non-zero status, overwrite an environment variable, resolve a relative path from another directory or continue after a component has failed.
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Layered testing catches different classes of problems:
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- Formatting checks keep files consistent and make review changes easier to spot.
- Behavior tests execute a script and assert its output, status and filesystem effects.
- Integration tests exercise the dispatcher exactly as a user will invoke it.
Do not treat one layer as a substitute for another. ShellCheck cannot prove that a command produced the intended file, and a passing formatting check says nothing about runtime behavior.
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Start with a contract for every script
Before writing tests, describe each script as a command with observable behavior. The contract is the reference you preserve when scripts are combined.
Record inputs and prerequisites
- Required positional arguments, options and representative valid values.
- Malformed, missing or contradictory arguments that must be rejected.
- Files, directories, standard input and configuration files the script reads.
- Environment variables and external commands it requires.
- The Bash and operating-system versions the finished tool promises to support.
Record outputs and status
- Expected standard output for a successful invocation.
- Expected standard error for warnings and failures. If the exact wording is not part of the interface, assert a stable substring or error class instead of a complete sentence.
- The exit status for success and for each meaningful failure. A wrapper must not accidentally turn a component failure into status 0.
- Files created, changed or deleted, including their contents and permissions when those are part of the command’s behavior.
Write one short contract per script. For example, a converter might accept an input path, print a converted record, create an output file and return 0; a missing input might print an error to standard error, create nothing and return 2. The exact values are yours to define, but tests should make those decisions executable.
Run ShellCheck on every shell file
ShellCheck is a static-analysis tool for Bash and other shell scripts. It reports warnings and suggestions ranging from beginner syntax mistakes to semantic problems and subtle shell pitfalls. Run it on every production script and on test files that use shell syntax.
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shellcheck bin/*.sh lib/*.bash test/*.batsKeep the command in the project’s build or test suite so a new warning fails the same check locally and in continuous integration. Fix the warning, or document a narrowly scoped suppression when the behavior is intentional and understood. Do not silence all diagnostics globally: that removes the signal you are trying to preserve.
Static analysis answers “does this code contain a pattern that is likely wrong?” It does not answer “does this command produce the required output and side effect?” Continue with execution tests even when ShellCheck is clean.
Use shfmt to make formatting a check, not a debate
shfmt formats shell programs. A consistent formatter reduces whitespace-only review noise and makes a combined tool easier to maintain. You can rewrite files locally or ask the formatter to report differences in CI:
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shfmt -w bin/*.sh lib/*.bash
shfmt -d bin/*.sh lib/*.bashChoose one mode for the repository and run the same options everywhere. Formatting should happen before behavior tests so a developer receives style feedback early.
Bats files need a detail that is easy to miss. The alternative Bash-compatible test syntax, such as function_name { # @test, can be linted and formatted with ShellCheck 0.7 or later and shfmt 3.2.0 or later by selecting Bats language mode:
shfmt -ln bats -d testThe standard @test syntax is custom to Bats rather than standard Bash. Confirm the installed ShellCheck and shfmt versions in the environment that runs the checks before depending on this compatibility.
Write repeatable behavior tests with Bats
Bats (Bash Automated Testing System) is a TAP-compliant framework for Bash 3.2 or above. It can run Bash commands and other UNIX programs, give tests names, provide setup and teardown hooks, capture command output and status, and produce output suitable for continuous integration.
A minimal test structure
Put tests in a dedicated directory and test the script through its public command-line interface rather than by calling private functions. Bats supplies a run helper that executes a command and exposes its result for assertions.
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@test 'converts a valid input' {
run "$BATS_TEST_DIRNAME/../bin/convert.sh" "$BATS_TEST_DIRNAME/fixtures/input.txt"
[[ $status -eq 0 ]]
[[ $output == 'converted record' ]]
}
@test 'rejects a missing input' {
run "$BATS_TEST_DIRNAME/../bin/convert.sh" "$BATS_TEST_DIRNAME/fixtures/missing.txt"
[[ $status -ne 0 ]]
[[ $output == *'not found'* ]]
}The first assertion checks the success status and the second checks captured output. The failure test checks that an invalid request is rejected and that a useful diagnostic is present. If exact output is part of your contract, compare the complete value; otherwise assert only the stable part that users or scripts rely on.
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Check standard output, standard error and files deliberately
When the distinction between output streams matters, redirect them to separate files in the test and assert each file independently. A command that prints a warning to standard output may appear to work in a terminal but break a pipeline that expects machine-readable output there.
@test 'writes data to stdout and warnings to stderr' {
stdout_file="$BATS_TEST_TMPDIR/stdout"
stderr_file="$BATS_TEST_TMPDIR/stderr"
run bash -c '"$1" "$2" >"$3" 2>"$4"' _
"$BATS_TEST_DIRNAME/../bin/report.sh"
"$BATS_TEST_DIRNAME/fixtures/input.txt"
"$stdout_file" "$stderr_file"
[[ $status -eq 0 ]]
[[ $(cat "$stdout_file") == 'report' ]]
[[ $(cat "$stderr_file") == *'warning'* ]]
}Use the same principle for side effects: assert that a required file exists and has the expected content, and that a failed invocation did not leave a partial artifact.
Use setup and teardown hooks
Hooks create the data a test needs and remove it afterward. Keep setup small and deterministic; a test should not depend on a developer’s home directory, current shell variables or files left by a previous test.
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setup() {
TEST_ROOT=$(mktemp -d)
INPUT="$TEST_ROOT/input.txt"
OUTPUT="$TEST_ROOT/output.txt"
printf '%s' 'sample' > "$INPUT"
}
teardown() {
rm -rf "$TEST_ROOT"
}
@test 'creates the requested output file' {
run "$BATS_TEST_DIRNAME/../bin/convert.sh" "$INPUT" "$OUTPUT"
[[ $status -eq 0 ]]
[[ -f $OUTPUT ]]
[[ $(cat "$OUTPUT") == 'converted sample' ]]
}The example uses a temporary directory created for the test. Adapt the command and expected text to your script’s contract; the important properties are isolated inputs, explicit assertions and cleanup regardless of whether the test passes.
Isolate working directories, fixtures and environment
Filesystem coupling is a common reason individually passing scripts fail as a group. Run each test with its own temporary root, create fixtures in setup, and pass absolute paths when the behavior under test is not path resolution itself.
- Change to a temporary working directory when the script writes relative paths.
- Set only the environment variables the contract requires; unset variables that could leak from the developer’s shell or CI runner.
- Use fixture files checked into the test tree for stable input, and generate per-test files for data that must be unique.
- Remove temporary directories in teardown, including after a failing assertion.
- Avoid sourcing a production file merely to reach a function unless that is the intended interface. Bats warns that code outside functions can run before setup and interfere with its internals.
Bats documents test-case isolation, but isolation is not magic: tests can still share external services, fixed filenames or mutable global state. Make those dependencies explicit or replace them with fixtures.
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Test failure paths before the dispatcher exists
Success cases are necessary but insufficient. A combined command must define what happens when one component cannot complete. Add a test for each branch that can affect users:
- Missing, unreadable or malformed input.
- Invalid option combinations and missing environment variables.
- An external command that is unavailable or returns failure.
- A destination directory that does not exist or is not writable.
- A downstream script that exits non-zero after an earlier step has already produced output.
For each failure, assert the non-zero status, the diagnostic stream, and the absence or cleanup of partial output. Decide whether the tool should stop immediately, continue with a warning, or roll back work; encode that decision in the test rather than relying on Bash’s default control flow.
Pay particular attention to pipelines and command substitutions. A wrapper can accidentally report the status of the last command in a pipeline instead of the command that failed, or overwrite a useful error with a later successful operation. The behavior test should invoke the script as a user does and inspect the final status.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Exercise the combined command through its public interface
After component tests pass, add integration tests for the dispatcher or wrapper. These tests are not a second copy of every unit case; they verify that invocation, configuration and result propagation remain correct when scripts are composed.
Cover normal composition
- Create a realistic fixture directory and any shared configuration the combined command expects.
- Invoke the top-level executable with the same arguments a user or automation job will pass.
- Assert the final standard output and exit status.
- Inspect every expected filesystem effect, including files generated by more than one stage.
Cover boundaries between scripts
- Pass an empty result from one stage to the next and verify the documented behavior.
- Use paths containing spaces and paths relative to a different working directory.
- Omit or alter shared configuration to ensure the wrapper reports a useful error.
- Force a downstream failure and verify that the top-level status is non-zero and that no misleading success message is printed.
- Run the command more than once against the same fixture if idempotence is promised; assert whether the second run replaces, skips or rejects existing output.
Keep integration fixtures small. A focused test that proves argument forwarding and status propagation is easier to diagnose than a giant scenario that fails for several unrelated reasons.
Make the suite useful in continuous integration
Run ShellCheck, shfmt and Bats in the same CI job or in clearly named jobs. Bats exits with status 0 when all tests pass and 1 when any test fails, so a normal CI command can reject a change on failure. Bats can emit TAP and JUnit-compatible reports; use the format your CI system can display and archive.
Keep the human-readable output for local debugging and save the machine-readable report as a build artifact when supported. A failed job should identify the test name, command status and relevant output without requiring a developer to reproduce the entire environment.
Test the Bash and operating-system versions that the finished tool actually supports. Bats’ Bash 3.2-or-later baseline describes the framework’s compatibility, not the complete matrix for your scripts, external utilities or target distributions.
Add parallel execution only after serial tests are reliable
Bats supports parallel execution through GNU parallel or a compatible replacement. Parallel runs can shorten a large suite, but order is not guaranteed. Shared files, fixed ports, global environment changes and dependencies between tests can then produce nondeterministic failures.
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- Search for shared mutable paths, caches and environment variables.
- Give each test a unique temporary directory and avoid assuming another test has run first.
- Enable parallelism and repeat the suite enough times to expose hidden coupling.
- Keep a serial diagnostic command for investigating failures.
Parallelism is an optimization, not a correctness feature. A faster suite that occasionally passes for the wrong reason is less valuable than a slower deterministic one.
Choose the lightest approach that still proves behavior
| Approach | What it checks | Assertions and isolation | CI use | Best fit |
|---|---|---|---|---|
| ShellCheck | Static warnings and suggestions about shell syntax and semantics | Does not execute the command or verify output and side effects | Can run in a build or test suite | Every shell file, before runtime testing |
| shfmt | Consistent shell formatting | Reports or applies formatting changes; it does not test behavior | Diff mode can fail a formatting check | Repositories that want a consistent style and reviewable diffs |
| Bats | Runtime behavior of Bash scripts and other UNIX programs | Named tests, run status/output capture, setup and teardown, and fixture isolation |
TAP and JUnit-compatible output; status 0 for all passing tests and 1 when any test fails | Scripts needing repeatable assertions, reusable fixtures and CI reports |
| Direct shell invocations | Runtime behavior with no dedicated framework | Whatever status, output and filesystem assertions you write; setup and reporting are your responsibility | Depends on the surrounding script and CI conventions | A very small utility where a few explicit checks are sufficient |
Even a small project should state what it checks. A handful of direct invocations is reasonable when the script is simple, but do not leave exit status, output or side effects to visual inspection.
A pre-combination checklist
- Each script has documented valid and invalid inputs.
- Expected standard output, standard error and exit statuses are explicit.
- Required files, environment variables and external commands are listed.
- ShellCheck runs cleanly or has narrowly justified suppressions.
- shfmt has been applied, and the same formatting check runs in CI.
- Bats tests cover representative success and failure behavior.
- Fixtures and temporary directories are isolated and cleaned up.
- Tests do not depend on execution order or shared mutable files.
- The dispatcher is tested through its public command-line interface.
- Integration tests verify argument forwarding, path handling, shared configuration, downstream failures, final output, final status and filesystem effects.
- The suite runs against the Bash and operating-system versions the tool promises to support.
- CI stores a useful TAP or JUnit-compatible report when a test fails.
What “ready to combine” means
A script is ready to become part of a larger tool when its contract is explicit, static checks are repeatable, behavior tests prove both success and failure, and its side effects are isolated enough to run reliably. The combined command then needs its own tests: component confidence does not prove that the wrapper forwards arguments, preserves statuses or handles shared paths correctly.
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