Agent Skills: Bash Development

Bash Development

Core patterns and best practices for Bash 5.1+ script development. Provides modern bash idioms, error handling, argument parsing, and pure-bash alternatives to external commands.

Script Foundation

Every script starts with the essential header:

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

set options explained:

• -e - Exit immediately on command failure
• -u - Treat unset variables as errors
• -o pipefail - Pipeline fails if any command fails

Script Metadata Pattern

SCRIPT_NAME=$(basename "${BASH_SOURCE[0]}")
SCRIPT_DIR=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
readonly SCRIPT_VERSION="1.0.0"
readonly SCRIPT_NAME SCRIPT_DIR

Error Handling

Implement trap-based error handling for robust scripts.

Code examples

Argument Parsing

Standard argument parsing template.

Code examples

Variable Best Practices

Always use curly braces and quote variables:

# Correct
"${variable}"
"${array[@]}"

# Incorrect
$variable
${array[*]}  # Use [@] for proper iteration

Use readonly for constants:

readonly CONFIG_FILE="/etc/app/config"
readonly -a VALID_OPTIONS=("opt1" "opt2" "opt3")

Note: Never use readonly in sourced scripts - it causes errors on re-sourcing.

String Operations (Pure Bash)

Prefer native bash parameter expansion over external tools:

# Trim whitespace
trimmed="${string#"${string%%[![:space:]]*}"}"
trimmed="${trimmed%"${trimmed##*[![:space:]]}"}"

# Lowercase/Uppercase (Bash 4+)
lower="${string,,}"
upper="${string^^}"

# Substring extraction
substring="${string:0:10}"      # First 10 chars
suffix="${string: -5}"          # Last 5 chars

# Replace patterns
replaced="${string//old/new}"   # Replace all
replaced="${string/old/new}"    # Replace first

# Strip prefix/suffix
no_prefix="${string#prefix}"    # Shortest match
no_prefix="${string##*/}"       # Longest match (basename)
no_suffix="${string%suffix}"    # Shortest match
no_suffix="${string%%/*}"       # Longest match

Array Operations

# Declaration
declare -a indexed_array=()
declare -A assoc_array=()

# Safe iteration with nullglob
shopt -s nullglob
for file in *.txt; do
    process "${file}"
done
shopt -u nullglob

# Array length
length="${#array[@]}"

# Append element
array+=("new_element")

# Iterate with index
for i in "${!array[@]}"; do
    printf '%d: %s\n' "${i}" "${array[i]}"
done

File Operations

# Read file to string
content="$(<"${file}")"

# Read file to array (Bash 4+)
mapfile -t lines < "${file}"

# Check file conditions
[[ -f "${file}" ]]    # Regular file exists
[[ -d "${dir}" ]]     # Directory exists
[[ -r "${file}" ]]    # Readable
[[ -w "${file}" ]]    # Writable
[[ -x "${file}" ]]    # Executable
[[ -s "${file}" ]]    # Non-empty

# Safe temp file creation
temp_file=$(mktemp)
trap 'rm -f "${temp_file}"' EXIT

Conditional Expressions

Use [[ ]] for conditionals (bash-specific, more powerful):

# String comparisons
[[ "${var}" == "value" ]]       # Equality
[[ "${var}" == pattern* ]]     # Glob matching
[[ "${var}" =~ ^regex$ ]]      # Regex matching

# Numeric comparisons
(( num > 10 ))                  # Arithmetic comparison
[[ "${num}" -gt 10 ]]          # Traditional syntax

# Compound conditions
[[ -f "${file}" && -r "${file}" ]]
[[ "${opt}" == "a" || "${opt}" == "b" ]]

Utility Functions

Code examples

Performance Guidelines

• Use builtins over external commands when possible
• Batch operations instead of loops for large datasets
• Use printf over echo for portability and control
• Avoid unnecessary subshells in tight loops
• Use [[ ]] over [ ] for string comparisons

Additional Resources

Reference Files

For detailed patterns and examples:

• bash_example_file.sh - Complete script template
• bash_example_includes.bash - Reusable utility functions
• bash-agent-notes.markdown - Context-aware review guidance
• pure-bash-bible-strings.md - String manipulation patterns
• pure-bash-bible-arrays.md - Array operations
• pure-bash-bible-files.md - File handling patterns
• pure-bash-bible-variables.md - Parameter expansion reference