Writing Shell Scripts

Shell Script Development Best Practices

Shebang

• #!/usr/bin/env bash for portability. #!/bin/sh for POSIX-only scripts.
• Executables: no extension or .sh. Libraries: always .sh, not executable.

Error Handling

Always use set -Eeuo pipefail after the shebang:

• -E: ERR trap inherited in functions
• -e: exit on failure
• -u: exit on undefined variables
• -o pipefail: exit if any pipeline command fails

Use || true only when explicitly ignoring failures.

Formatting

• 2-space indent, never tabs. Max 80 characters per line.
• Place ; then and ; do on the same line as if/for/while.
• Split long pipelines across lines with | at the start of continuation lines.

Variables

• Always quote: "$variable", "${prefix}_${suffix}".
• Always $(command) over backticks — nestable and unambiguous.
• readonly or declare -r for constants. UPPER_CASE for constants/environment variables.
• local for function variables. lower_case for local/function-scoped variables.
• Avoid globals; prefer function parameters and return values.

Functions

• Define before use. local for all variables. Return meaningful exit codes.
• lower_case with underscores. Use :: for library namespacing (mylib::helper).
• Document purpose, globals, arguments, outputs, and return values:

  # Downloads and verifies a file
  # Globals: DOWNLOAD_TIMEOUT
  # Args: $1=URL, $2=destination
  # Outputs: writes to stdout on success
  # Returns: 0 on success, 1 on failure
  download_file() {
      local url="${1:?URL required}"
      local dest="${2:?destination required}"
  }
  

• Use a main function in scripts with multiple functions; call it last:

  main() {
      parse_args "$@"
      run
  }
  main "$@"
  

Safe file iteration

while IFS= read -r -d '' file; do
    process "$file"
done < <(find . -name "*.txt" -print0)

Cleanup with trap

cleanup() {
    local exit_code=$?
    # Kill background jobs before cleaning up
    jobs -p | xargs -r kill 2>/dev/null || true
    wait 2>/dev/null || true
    rm -rf "$TEMP_DIR"
    exit "$exit_code"
}
trap cleanup EXIT
trap 'trap - EXIT; cleanup; kill -INT $$' INT
trap 'trap - EXIT; cleanup; kill -TERM $$' TERM

• trap cleanup EXIT runs on normal exit, errors (set -e), and signals.
• Separate INT/TERM traps re-raise the signal so the parent process sees the correct exit status.
• Kill child processes explicitly — by default, backgrounded jobs survive the parent's exit.

Logging

log_info()  { printf '[INFO]  %s\n' "$*"; }
log_warn()  { printf '[WARN]  %s\n' "$*" >&2; }
log_error() { printf '[ERROR] %s\n' "$*" >&2; }
die()       { log_error "$@"; exit 1; }

Input/Output

• printf '%s\n' "$msg" over echo for formatted output.
• Errors to stderr: >&2.
• Use heredocs for multi-line strings. Quote the delimiter (<<'EOF') to prevent variable and command expansion — unquoted heredocs expand $VAR and `cmd`:

  # Unquoted — $VAR and $() expand (use for generating output with runtime values)
  cat <<EOF
  Host: $HOSTNAME  Port: $PORT
  EOF
  
  # Quoted — no expansion (use for writing scripts, SQL, config with literal $ chars)
  cat <<'EOF'
  echo "This $VARIABLE will not be expanded"
  EOF
  

Arithmetic

• Use (( )) or $(( )) for math. Never let, $[ ], or expr.

String Manipulation

Prefer parameter expansion over sed/awk for simple transformations (${var#pattern}, ${var%pattern}, ${var/old/new}).

Defaults and guards:

"${1:-default}"    # use default if $1 is unset or empty
"${1:?error msg}"  # exit with error if $1 is unset or empty
"${var:+alt}"      # use alt if var is set and non-empty, else empty

Working with JSON (jq)

Use jq for JSON processing — never parse JSON with grep/sed/awk.

# Extract a field
name=$(jq -r '.user.name' response.json)

# Iterate over array elements
jq -r '.items[] | .id' data.json | while IFS= read -r id; do
    process "$id"
done

# Build JSON safely with --arg (handles escaping)
jq -n --arg name "$USER" --arg host "$HOSTNAME" \
    '{"name": $name, "host": $host}'

# Modify JSON in-place
jq '.config.timeout = 30 | .config.retries = 3' config.json > tmp.json \
    && mv tmp.json config.json

# Select and filter
jq '[.[] | select(.status == "active")]' users.json

• Always use -r (raw output) when piping jq output to other commands — without it, strings include quotes.
• Use --arg and --argjson to pass shell variables into jq expressions safely.
• Use --exit-status (or -e) to make jq return non-zero when the filter produces false or null.

Arrays

# Indexed arrays
files=("one.txt" "two.txt" "three.txt")
files+=("four.txt")             # append
echo "${files[0]}"              # first element
echo "${files[@]}"              # all elements (preserves quoting)
echo "${#files[@]}"             # length

# Iterate safely (handles spaces in elements)
for f in "${files[@]}"; do
    process "$f"
done

# Associative arrays (bash 4+)
declare -A config
config[host]="localhost"
config[port]="5432"
for key in "${!config[@]}"; do
    echo "$key=${config[$key]}"
done

Security

• Validate all user inputs. Use -- to separate options from arguments.
• Avoid eval. Secure permissions (chmod 600) for sensitive files.
• SUID/SGID forbidden on shell scripts — use sudo instead.
• Use mktemp for temp files — never hardcode /tmp/myfile:

  TEMP_DIR="$(mktemp -d)" || die "failed to create temp dir"
  

ShellCheck

Run shellcheck on every script. Fix all warnings — don't suppress without justification.

# Suppress a specific rule with an explanation
# shellcheck disable=SC2086  # word splitting intended for $flags
docker run $flags "$image"

# Suppress for an entire file (top of file, after shebang)
# shellcheck disable=SC2034  # variables sourced by another script

# Source directive for files shellcheck can't find
# shellcheck source=lib/common.sh
source "${SCRIPT_DIR}/lib/common.sh"

Rule ranges: SC1xxx = syntax/parsing, SC2xxx = best practices, SC3xxx = POSIX compatibility.

Portability

• Check required tools: command -v git >/dev/null || die "git required".
• Avoid GNU-specific options when possible (see portability-matrix.md).
• POSIX sh vs bash: POSIX sh has no arrays, no [[ ]], no <() process substitution, no {a,b} brace expansion, no += append. Use #!/bin/sh only when you avoid all bash-isms.
• macOS ships bash 3.2 (2007, GPLv2). Associative arrays (declare -A) require bash 4+. If targeting macOS, either use #!/bin/sh (POSIX) or require users to install modern bash via Homebrew.

New script workflow

- [ ] Start with shebang and set -Eeuo pipefail
- [ ] Add argument parsing with usage/help
- [ ] Add prerequisite checks (required tools)
- [ ] Implement main logic with cleanup trap
- [ ] Add logging and error handling
- [ ] Run validation loop (below)

Validation loop

1. shellcheck script.sh — fix all warnings (SC1xxx=syntax, SC2xxx=best practices, SC3xxx=POSIX)
2. bash -n script.sh — fix syntax errors
3. Test with valid input — verify expected behavior
4. Test with invalid input — verify error messages and exit codes
5. Test on macOS if targeting both platforms
6. Repeat until shellcheck is clean and all cases pass

Deep-dive references

CLI tool template: See patterns/cli-tool-template.md for a production-ready script template Deployment scripts: See patterns/deployment-scripts.md for deploy, rollback, health check patterns Testing: See patterns/testing-patterns.md for bats framework, mocking, CI integration GNU vs BSD: See portability-matrix.md for cross-platform command differences

Official references

• Google Shell Style Guide — naming, formatting, features, best practices
• ShellCheck Wiki — rule-by-rule documentation (SC1xxx syntax, SC2xxx quality, SC3xxx POSIX)