bats-testing-patterns

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

# 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

#!/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

#!/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

#!/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

#!/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

#!/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

#!/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

#!/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

#!/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

#!/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

#!/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

#!/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

#!/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

#!/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