Bash Scripts

Bash Scripts Practice

CRITICAL: Bash Script Safety (ABSOLUTE)

You MUST ALWAYS start scripts with:

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

• set -e - Exit immediately if any command fails
• set -u - Exit if undefined variable is used
• set -o pipefail - Catch failures in pipes

EXCEPTION: Only use set +e temporarily for commands you EXPECT to fail, then re-enable.

CRITICAL: Test Every Bash Function (ABSOLUTE)

You MUST NEVER add bash functions without bats tests.

Every public function needs a @test annotation in bats:

# test/validator.bats
@test "validate_config accepts valid JSON" {
  run validate_config valid.json
  [ "$status" -eq 0 ]
}

@test "validate_config rejects invalid JSON" {
  run validate_config invalid.json
  [ "$status" -eq 1 ]
  [[ "$output" =~ "Invalid JSON" ]]
}

CRITICAL: Parameter Validation (ABSOLUTE)

Validate in every script:

• Required parameters exist
• Parameter count is correct
• File paths exist (if expected)
• Enums match expected values

if [ $# -lt 1 ]; then
  echo "Error: Missing required parameter" >&2
  echo "Usage: $0 <config_file>" >&2
  exit 1
fi

CRITICAL: Idempotency (ABSOLUTE)

Scripts MUST be safe to run multiple times.

# WRONG:
mkdir output/      # Fails on second run
rm config.old      # Fails if file doesn't exist

# CORRECT:
mkdir -p output/
rm -f config.old

Script Structure Pattern

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

readonly SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
readonly SCRIPT_NAME="$(basename "${BASH_SOURCE[0]}")"

show_help() { ... }
log_info() { echo -e "\033[0;32m✓\033[0m $*"; }
log_error() { echo -e "\033[0;31m✗\033[0m $*" >&2; }

main() {
  # Parse arguments, validate, execute
}

main "$@"

Bats Testing Patterns

Basic assertions:

[ "$status" -eq 0 ]                    # Exit code
[[ "$output" =~ "expected pattern" ]]  # Regex match
[[ "$lines[0]" == "first line" ]]      # Specific line
[ -f "$file" ]                         # File exists

Setup/teardown:

setup() {
  export TEST_TEMP_DIR="$(mktemp -d)"
}
teardown() {
  rm -rf "$TEST_TEMP_DIR"
}

Verification Loops

Loop 1 (TDD): bats test/{script_name}.bats Loop 2 (Full Suite): bats test/**/*.bats Loop 3 (Manual): Run script with test data, verify output/side effects

Code Quality Checklist

• Script starts with set -euo pipefail
• Every function has bats test
• Required parameters validated
• Script is idempotent
• ShellCheck passes: shellcheck {script_name}.sh
• All bats tests pass
• No debug statements (set -x)
• Functions small and focused (< 20 lines)
• Error messages descriptive and go to stderr
• Help text clear with examples

Anti-patterns

You MUST NEVER:

• Omit set -euo pipefail
• Skip parameter validation
• Write bash functions without tests
• Use set +e globally
• Ignore exit codes
• Write non-idempotent scripts
• Parse ls output (for file in $(ls))
• Use eval
• Put secrets in scripts
• Forget to quote variables ($var -> "$var")

Prefer:

• Small, focused functions
• Bats tests for every function
• Descriptive error messages to stderr
• Help text with examples
• ShellCheck linting
• Docker-aware scripts (check /.dockerenv)