Shell Guide

Shell/Bash Guide

Applies to: Bash 4+, POSIX sh, Automation Scripts, CI/CD Pipelines, Makefiles

Core Principles

1. Strict Mode Always: Every script starts with set -euo pipefail to fail fast on errors
2. Quote Everything: All variable expansions must be double-quoted to prevent word splitting and globbing
3. Explicit Over Implicit: Use [[ ]] for conditionals, local for function variables, named constants for magic values
4. Fail Loudly: Never swallow errors silently; use trap for cleanup and meaningful exit codes
5. ShellCheck Clean: All scripts pass shellcheck with zero warnings before commit

Guardrails

Shebang and Strict Mode

• Every script: #!/usr/bin/env bash (or #!/bin/sh for POSIX)
• Immediately follow with set -euo pipefail
• Use set -x only for debugging, never in production scripts
• POSIX scripts must not use bash-specific features ([[ ]], arrays, local)

#!/usr/bin/env bash
set -euo pipefail
[[ "${TRACE:-}" == "1" ]] && set -x

Quoting

• Always double-quote: "$var", "$@", "${arr[@]}", "$(command)"
• Single quotes for literals that must not be interpolated
• Only omit quotes in arithmetic: $(( count + 1 ))

# Correct
grep -r "$pattern" "$directory"
for arg in "$@"; do process "$arg"; done

# Wrong - word splitting and globbing bugs
grep -r $pattern $directory
for arg in $@; do process $arg; done

Error Handling

• Check return codes: if ! command; then handle_error; fi
• Inline: critical_cmd || { echo "Failed" >&2; exit 1; }
• Never use set +e (restructure logic instead)
• Exit codes: 0 = success, 1 = general error, 2 = usage error
• Errors to stderr: echo "Error: message" >&2

Portability

• #!/usr/bin/env bash over #!/bin/bash (varies across systems)
• command -v instead of which for executable checks
• $(command) instead of backticks
• printf over echo for portable output (flags/escapes differ)

Security

• Never use eval (use arrays for dynamic command building)
• Validate all external input (arguments, env vars, file contents)
• mktemp for temp files, never hardcoded /tmp/myapp.tmp
• No secrets in script files; read from environment or secret managers
• umask 077 before creating sensitive files

# Safe temp file
tmpfile="$(mktemp)" || exit 1
trap 'rm -f "$tmpfile"' EXIT

# Never do this
eval "$user_input"        # Command injection
password="hunter2"        # Hardcoded secret

Script Structure

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

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

# ── Configuration (with defaults) ─────────────────────
LOG_LEVEL="${LOG_LEVEL:-info}"
OUTPUT_DIR="${OUTPUT_DIR:-./output}"

# ── Functions ──────────────────────────────────────────
usage() { ... }
cleanup() { ... }
main() { ... }

# ── Traps & Entry ─────────────────────────────────────
trap cleanup EXIT
main "$@"

Key Patterns

Parameter Expansion

db_host="${DB_HOST:-localhost}"              # Default value
api_key="${API_KEY:?Error: API_KEY required}" # Required (fail if unset)

filename="archive.tar.gz"
name="${filename%%.*}"         # "archive"    (longest suffix removal)
ext="${filename#*.}"           # "tar.gz"     (shortest prefix removal)
path="/usr/local/bin/tool"
dir="${path%/*}"               # "/usr/local/bin"
base="${path##*/}"             # "tool"
upper="${var^^}"               # UPPERCASE (Bash 4+)
lower="${var,,}"               # lowercase (Bash 4+)

Trap for Cleanup

cleanup() {
    local exit_code=$?
    rm -f "$tmpfile"
    exit "$exit_code"         # Preserve original exit code
}
trap cleanup EXIT
trap 'echo "Interrupted" >&2; exit 130' INT TERM

Arrays (Avoid eval)

declare -a files=()
files+=("first.txt" "second.txt")
for file in "${files[@]}"; do echo "$file"; done

# Build commands safely with arrays
cmd=(curl --silent --fail)
[[ -n "${TOKEN:-}" ]] && cmd+=(--header "Authorization: Bearer $TOKEN")
"${cmd[@]}" "$url"

Functions

process_file() {
    local file="$1"
    local -r max_lines=1000
    local line_count
    line_count="$(wc -l < "$file")"
    if (( line_count > max_lines )); then
        echo "Warning: $file exceeds $max_lines lines" >&2
    fi
}

All variables inside functions must be declared local.

Here Documents

cat <<EOF                     # Interpolated
Hello, $USER at $(hostname)
EOF

cat <<'EOF'                   # Literal (no expansion)
This $variable stays literal.
EOF

Safe File Iteration

# Handles spaces, newlines, special characters
while IFS= read -r -d '' file; do
    process "$file"
done < <(find "$dir" -type f -name "*.log" -print0)

# Simple globs (Bash 4+)
shopt -s nullglob globstar
for file in "$dir"/**/*.sh; do process "$file"; done

Never use for file in $(find ...) -- it breaks on spaces.

Process Substitution

diff <(sort file1.txt) <(sort file2.txt)

Testing

bats-core

# test/deploy.bats
setup()    { export TMPDIR="$(mktemp -d)"; }
teardown() { rm -rf "$TMPDIR"; }

@test "deploy requires environment argument" {
    run ./deploy.sh
    [ "$status" -ne 0 ]
    [[ "$output" == *"Usage:"* ]]
}

Testing Standards

• Test with bats-core (preferred) or shellspec
• Test files in test/ or spec/ directory
• Test names describe behavior: "deploy requires environment argument"
• Use setup/teardown for temp dirs and fixtures
• Coverage: >80% library functions, >60% scripts
• Each test must be independent

Tooling

ShellCheck

shellcheck script.sh                        # Single file
find . -name "*.sh" -exec shellcheck {} +   # All scripts

# Suppress with justification
# shellcheck disable=SC2034  # Variable used by sourced script
unused_looking_var="value"

shfmt

shfmt -w -i 4 -bn script.sh     # Format in place (4-space indent)
shfmt -d -i 4 script.sh         # Check only (diff output)

Essential Commands

shellcheck *.sh              # Lint
shfmt -d -i 4 *.sh           # Check formatting
bats test/                   # Run tests
bash -n script.sh            # Syntax check (no execution)

References

For detailed patterns and examples, see:

• references/patterns.md -- Script templates, trap patterns, portable scripting examples

External References

• Bash Reference Manual
• POSIX Shell Specification
• ShellCheck Wiki
• Google Shell Style Guide
• bats-core
• shellspec
• shfmt
• Pure Bash Bible