Bats Testing Patterns
Comprehensive guidance for writing comprehensive unit tests for shell scripts using Bats (Bash Automated Testing System), including test patterns, fixtures, and best practices for production-grade shell testing.
When to Use This Skill
- Writing unit tests for shell scripts
- Implementing test-driven development (TDD) for scripts
- Setting up automated testing in CI/CD pipelines
- Testing edge cases and error conditions
- Validating behavior across different shell environments
- Building maintainable test suites for scripts
- Creating fixtures for complex test scenarios
- Testing multiple shell dialects (bash, sh, dash)
Bats Fundamentals
What is Bats?
Bats (Bash Automated Testing System) is a TAP (Test Anything Protocol) compliant testing framework for shell scripts that provides:
- Simple, natural test syntax
- TAP output format compatible with CI systems
- Fixtures and setup/teardown support
- Assertion helpers
- Parallel test execution
Installation
bash
# macOS with Homebrew
brew install bats-core
# Ubuntu/Debian
git clone https://github.com/bats-core/bats-core.git
cd bats-core
./install.sh /usr/local
# From npm (Node.js)
npm install --global bats
# Verify installation
bats --version
File Structure
project/
├── bin/
│ ├── script.sh
│ └── helper.sh
├── tests/
│ ├── test_script.bats
│ ├── test_helper.sh
│ ├── fixtures/
│ │ ├── input.txt
│ │ └── expected_output.txt
│ └── helpers/
│ └── mocks.bash
└── README.md
Basic Test Structure
Simple Test File
bash
#!/usr/bin/env bats
# Load test helper if present
load test_helper
# Setup runs before each test
setup() {
export TMPDIR=$(mktemp -d)
}
# Teardown runs after each test
teardown() {
rm -rf "$TMPDIR"
}
# Test: simple assertion
@test "Function returns 0 on success" {
run my_function "input"
[ "$status" -eq 0 ]
}
# Test: output verification
@test "Function outputs correct result" {
run my_function "test"
[ "$output" = "expected output" ]
}
# Test: error handling
@test "Function returns 1 on missing argument" {
run my_function
[ "$status" -eq 1 ]
}
Assertion Patterns
Exit Code Assertions
bash
#!/usr/bin/env bats
@test "Command succeeds" {
run true
[ "$status" -eq 0 ]
}
@test "Command fails as expected" {
run false
[ "$status" -ne 0 ]
}
@test "Command returns specific exit code" {
run my_function --invalid
[ "$status" -eq 127 ]
}
@test "Can capture command result" {
run echo "hello"
[ $status -eq 0 ]
[ "$output" = "hello" ]
}
Output Assertions
bash
#!/usr/bin/env bats
@test "Output matches string" {
result=$(echo "hello world")
[ "$result" = "hello world" ]
}
@test "Output contains substring" {
result=$(echo "hello world")
[[ "$result" == *"world"* ]]
}
@test "Output matches pattern" {
result=$(date +%Y)
[[ "$result" =~ ^[0-9]{4}$ ]]
}
@test "Multi-line output" {
run printf "line1\nline2\nline3"
[ "$output" = "line1
line2
line3" ]
}
@test "Lines variable contains output" {
run printf "line1\nline2\nline3"
[ "${lines[0]}" = "line1" ]
[ "${lines[1]}" = "line2" ]
[ "${lines[2]}" = "line3" ]
}
File Assertions
bash
#!/usr/bin/env bats
@test "File is created" {
[ ! -f "$TMPDIR/output.txt" ]
my_function > "$TMPDIR/output.txt"
[ -f "$TMPDIR/output.txt" ]
}
@test "File contents match expected" {
my_function > "$TMPDIR/output.txt"
[ "$(cat "$TMPDIR/output.txt")" = "expected content" ]
}
@test "File is readable" {
touch "$TMPDIR/test.txt"
[ -r "$TMPDIR/test.txt" ]
}
@test "File has correct permissions" {
touch "$TMPDIR/test.txt"
chmod 644 "$TMPDIR/test.txt"
[ "$(stat -f %OLp "$TMPDIR/test.txt")" = "644" ]
}
@test "File size is correct" {
echo -n "12345" > "$TMPDIR/test.txt"
[ "$(wc -c < "$TMPDIR/test.txt")" -eq 5 ]
}
Setup and Teardown Patterns
Basic Setup and Teardown
bash
#!/usr/bin/env bats
setup() {
# Create test directory
TEST_DIR=$(mktemp -d)
export TEST_DIR
# Source script under test
source "${BATS_TEST_DIRNAME}/../bin/script.sh"
}
teardown() {
# Clean up temporary directory
rm -rf "$TEST_DIR"
}
@test "Test using TEST_DIR" {
touch "$TEST_DIR/file.txt"
[ -f "$TEST_DIR/file.txt" ]
}
Setup with Resources
bash
#!/usr/bin/env bats
setup() {
# Create directory structure
mkdir -p "$TMPDIR/data/input"
mkdir -p "$TMPDIR/data/output"
# Create test fixtures
echo "line1" > "$TMPDIR/data/input/file1.txt"
echo "line2" > "$TMPDIR/data/input/file2.txt"
# Initialize environment
export DATA_DIR="$TMPDIR/data"
export INPUT_DIR="$DATA_DIR/input"
export OUTPUT_DIR="$DATA_DIR/output"
}
teardown() {
rm -rf "$TMPDIR/data"
}
@test "Processes input files" {
run my_process_script "$INPUT_DIR" "$OUTPUT_DIR"
[ "$status" -eq 0 ]
[ -f "$OUTPUT_DIR/file1.txt" ]
}
Global Setup/Teardown
bash
#!/usr/bin/env bats
# Load shared setup from test_helper.sh
load test_helper
# setup_file runs once before all tests
setup_file() {
export SHARED_RESOURCE=$(mktemp -d)
echo "Expensive setup" > "$SHARED_RESOURCE/data.txt"
}
# teardown_file runs once after all tests
teardown_file() {
rm -rf "$SHARED_RESOURCE"
}
@test "First test uses shared resource" {
[ -f "$SHARED_RESOURCE/data.txt" ]
}
@test "Second test uses shared resource" {
[ -d "$SHARED_RESOURCE" ]
}
Mocking and Stubbing Patterns
Function Mocking
bash
#!/usr/bin/env bats
# Mock external command
my_external_tool() {
echo "mocked output"
return 0
}
@test "Function uses mocked tool" {
export -f my_external_tool
run my_function
[[ "$output" == *"mocked output"* ]]
}
Command Stubbing
bash
#!/usr/bin/env bats
setup() {
# Create stub directory
STUBS_DIR="$TMPDIR/stubs"
mkdir -p "$STUBS_DIR"
# Add to PATH
export PATH="$STUBS_DIR:$PATH"
}
create_stub() {
local cmd="$1"
local output="$2"
local code="${3:-0}"
cat > "$STUBS_DIR/$cmd" <<EOF
#!/bin/bash
echo "$output"
exit $code
EOF
chmod +x "$STUBS_DIR/$cmd"
}
@test "Function works with stubbed curl" {
create_stub curl "{ \"status\": \"ok\" }" 0
run my_api_function
[ "$status" -eq 0 ]
}
Variable Stubbing
bash
#!/usr/bin/env bats
@test "Function handles environment override" {
export MY_SETTING="override_value"
run my_function
[ "$status" -eq 0 ]
[[ "$output" == *"override_value"* ]]
}
@test "Function uses default when var unset" {
unset MY_SETTING
run my_function
[ "$status" -eq 0 ]
[[ "$output" == *"default"* ]]
}
Fixture Management
Using Fixture Files
bash
#!/usr/bin/env bats
# Fixture directory: tests/fixtures/
setup() {
FIXTURES_DIR="${BATS_TEST_DIRNAME}/fixtures"
WORK_DIR=$(mktemp -d)
export WORK_DIR
}
teardown() {
rm -rf "$WORK_DIR"
}
@test "Process fixture file" {
# Copy fixture to work directory
cp "$FIXTURES_DIR/input.txt" "$WORK_DIR/input.txt"
# Run function
run my_process_function "$WORK_DIR/input.txt"
# Compare output
diff "$WORK_DIR/output.txt" "$FIXTURES_DIR/expected_output.txt"
}
Dynamic Fixture Generation
bash
#!/usr/bin/env bats
generate_fixture() {
local lines="$1"
local file="$2"
for i in $(seq 1 "$lines"); do
echo "Line $i content" >> "$file"
done
}
@test "Handle large input file" {
generate_fixture 1000 "$TMPDIR/large.txt"
run my_function "$TMPDIR/large.txt"
[ "$status" -eq 0 ]
[ "$(wc -l < "$TMPDIR/large.txt")" -eq 1000 ]
}
Advanced Patterns
Testing Error Conditions
bash
#!/usr/bin/env bats
@test "Function fails with missing file" {
run my_function "/nonexistent/file.txt"
[ "$status" -ne 0 ]
[[ "$output" == *"not found"* ]]
}
@test "Function fails with invalid input" {
run my_function ""
[ "$status" -ne 0 ]
}
@test "Function fails with permission denied" {
touch "$TMPDIR/readonly.txt"
chmod 000 "$TMPDIR/readonly.txt"
run my_function "$TMPDIR/readonly.txt"
[ "$status" -ne 0 ]
chmod 644 "$TMPDIR/readonly.txt" # Cleanup
}
@test "Function provides helpful error message" {
run my_function --invalid-option
[ "$status" -ne 0 ]
[[ "$output" == *"Usage:"* ]]
}
Testing with Dependencies
bash
#!/usr/bin/env bats
setup() {
# Check for required tools
if ! command -v jq &>/dev/null; then
skip "jq is not installed"
fi
export SCRIPT="${BATS_TEST_DIRNAME}/../bin/script.sh"
}
@test "JSON parsing works" {
skip_if ! command -v jq &>/dev/null
run my_json_parser '{"key": "value"}'
[ "$status" -eq 0 ]
}
Testing Shell Compatibility
bash
#!/usr/bin/env bats
@test "Script works in bash" {
bash "${BATS_TEST_DIRNAME}/../bin/script.sh" arg1
}
@test "Script works in sh (POSIX)" {
sh "${BATS_TEST_DIRNAME}/../bin/script.sh" arg1
}
@test "Script works in dash" {
if command -v dash &>/dev/null; then
dash "${BATS_TEST_DIRNAME}/../bin/script.sh" arg1
else
skip "dash not installed"
fi
}
Parallel Execution
bash
#!/usr/bin/env bats
@test "Multiple independent operations" {
run bash -c 'for i in {1..10}; do
my_operation "$i" &
done
wait'
[ "$status" -eq 0 ]
}
@test "Concurrent file operations" {
for i in {1..5}; do
my_function "$TMPDIR/file$i" &
done
wait
[ -f "$TMPDIR/file1" ]
[ -f "$TMPDIR/file5" ]
}
Test Helper Pattern
test_helper.sh
bash
#!/usr/bin/env bash
# Source script under test
export SCRIPT_DIR="${BATS_TEST_DIRNAME%/*}/bin"
# Common test utilities
assert_file_exists() {
if [ ! -f "$1" ]; then
echo "Expected file to exist: $1"
return 1
fi
}
assert_file_equals() {
local file="$1"
local expected="$2"
if [ ! -f "$file" ]; then
echo "File does not exist: $file"
return 1
fi
local actual=$(cat "$file")
if [ "$actual" != "$expected" ]; then
echo "File contents do not match"
echo "Expected: $expected"
echo "Actual: $actual"
return 1
fi
}
# Create temporary test directory
setup_test_dir() {
export TEST_DIR=$(mktemp -d)
}
cleanup_test_dir() {
rm -rf "$TEST_DIR"
}
Integration with CI/CD
GitHub Actions Workflow
yaml
name: Tests
on: [push, pull_request]
jobs:
test:
runs-on: ubuntu-latest
steps:
- uses: actions/checkout@v3
- name: Install Bats
run: |
npm install --global bats
- name: Run Tests
run: |
bats tests/*.bats
- name: Run Tests with Tap Reporter
run: |
bats tests/*.bats --tap | tee test_output.tap
Makefile Integration
makefile
.PHONY: test test-verbose test-tap
test:
bats tests/*.bats
test-verbose:
bats tests/*.bats --verbose
test-tap:
bats tests/*.bats --tap
test-parallel:
bats tests/*.bats --parallel 4
coverage: test
# Optional: Generate coverage reports
Best Practices
- Test one thing per test - Single responsibility principle
- Use descriptive test names - Clearly states what is being tested
- Clean up after tests - Always remove temporary files in teardown
- Test both success and failure paths - Don't just test happy path
- Mock external dependencies - Isolate unit under test
- Use fixtures for complex data - Makes tests more readable
- Run tests in CI/CD - Catch regressions early
- Test across shell dialects - Ensure portability
- Keep tests fast - Run in parallel when possible
- Document complex test setup - Explain unusual patterns
Resources