bats-testing-patterns

Bats Testing Patterns

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Install skill "bats-testing-patterns" with this command: npx skills add wshobson/agents/wshobson-agents-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" &#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

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