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" &#x3C;&#x3C;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 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

#!/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

#!/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

#!/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

#!/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

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

.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

Bats GitHub: https://github.com/bats-core/bats-core
Bats Documentation: https://bats-core.readthedocs.io/
TAP Protocol: https://testanything.org/
Test-Driven Development: https://en.wikipedia.org/wiki/Test-driven_development

bats-testing-patterns

Safety Notice

Copy this and send it to your AI assistant to learn

macOS with Homebrew

Ubuntu/Debian

From npm (Node.js)

Verify installation

Load test helper if present

Setup runs before each test

Teardown runs after each test

Test: simple assertion

Test: output verification

Test: error handling

Load shared setup from test_helper.sh

setup_file runs once before all tests

teardown_file runs once after all tests

Mock external command

Fixture directory: tests/fixtures/

Source script under test

Common test utilities

Create temporary test directory

Source Transparency

Related Skills

premium-saas-design

design-principles

debugging-strategies