Shell Script Quality

Shell Script Quality

When to Use

• Creating new shell scripts in scripts/
• Modifying existing shell scripts
• Scripts with complex logic (conditionals, loops, error handling)
• Scripts called by CI/CD pipelines

Process

1. Write Script
   - Follow ShellCheck-compliant patterns from start
   - Use set -euo pipefail for strict mode

2. Run ShellCheck
   shellcheck scripts/my-script.sh
   Fix all warnings before committing

3. Add BATS Tests (if complex)
   - Create test file in tests/bats/
   - Test happy path, error cases, edge cases

4. Verify
   - Script runs correctly locally
   - ShellCheck passes with no warnings
   - BATS tests pass (if applicable)

Strict Mode Template

#!/usr/bin/env bash
set -euo pipefail

# Script description
# Usage: scripts/my-script.sh [options] <arg>

# Constants (use readonly)
readonly SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
readonly PROJECT_ROOT="$(cd "${SCRIPT_DIR}/.." && pwd)"

# Functions before main logic
log_info() {
    echo "[INFO] $*"
}

log_error() {
    echo "[ERROR] $*" >&2
}

# Exit handlers
cleanup() {
    # Cleanup logic here
    :
}
trap cleanup EXIT

# Main logic
main() {
    local arg="${1:-default_value}"

    # Validate inputs
    if [[ ! -f "$arg" ]]; then
        log_error "File not found: $arg"
        exit 1
    fi

    log_info "Processing $arg"
    # ... main logic
}

main "$@"

Common ShellCheck Warnings

ShellCheck Inline Directives

# shellcheck disable=SC2086  # Explanation of why
some_command $unquoted_args  # Intentional word splitting

# shellcheck source=scripts/common.sh
source "${SCRIPT_DIR}/common.sh"

BATS Test Template

#!/usr/bin/env bats

# tests/bats/my-script.bats

setup() {
    # Create temp directory for test files
    TEST_DIR="$(mktemp -d)"
    export TEST_DIR
}

teardown() {
    rm -rf "$TEST_DIR"
}

@test "happy path: process valid input" {
    echo "test content" > "${TEST_DIR}/input.txt"
    run scripts/my-script.sh "${TEST_DIR}/input.txt"

    [ "$status" -eq 0 ]
    [[ "$output" == *"Processing"* ]]
}

@test "error case: missing file" {
    run scripts/my-script.sh "${TEST_DIR}/nonexistent.txt"

    [ "$status" -eq 1 ]
    [[ "$output" == *"not found"* ]]
}

@test "edge case: empty input" {
    touch "${TEST_DIR}/empty.txt"
    run scripts/my-script.sh "${TEST_DIR}/empty.txt"

    [ "$status" -eq 0 ]
}

BATS Installation

# Ubuntu/Debian
sudo apt-get install bats

# macOS
brew install bats-core

# Run tests
bats tests/bats/

CI Integration

# In .github/workflows/ci.yml
shellcheck:
  runs-on: ubuntu-latest
  steps:
    - uses: actions/checkout@v4
    - name: Install ShellCheck
      run: sudo apt-get install shellcheck
    - name: Run ShellCheck
      run: shellcheck scripts/*.sh

bats-tests:
  runs-on: ubuntu-latest
  steps:
    - uses: actions/checkout@v4
    - name: Install BATS
      run: sudo apt-get install bats
    - name: Run BATS tests
      run: bats tests/bats/

Quality Checklist

• Script starts with #!/usr/bin/env bash
• Uses set -euo pipefail for strict mode
• All variables are quoted ("$var")
• Uses [[ ]] for conditionals (not [ ])
• Uses $((..)) for arithmetic
• Uses local for function-scoped variables
• ShellCheck passes with no warnings
• BATS tests cover happy path and errors (if complex)
• CI pipeline runs ShellCheck automatically