--- name: shell-scripting description: Shell scripting best practices and patterns. Use when writing bash/zsh scripts, automating tasks, creating CLI tools, or debugging shell commands. author: Joseph OBrien status: unpublished updated: '2025-12-23' version: 1.0.1 tag: skill type: skill --- # Shell Scripting Comprehensive shell scripting skill covering bash/zsh patterns, automation, error handling, and CLI tool development. ## When to Use This Skill - Writing automation scripts - Creating CLI tools - System administration tasks - Build and deployment scripts - Log processing and analysis - File manipulation and batch operations - Cron jobs and scheduled tasks ## Script Structure ### Template ```bash #!/usr/bin/env bash # Script: name.sh # Description: What this script does # Usage: ./name.sh [options] set -euo pipefail # Exit on error, undefined vars, pipe failures IFS=$'\n\t' # Safer word splitting # Constants readonly SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)" readonly SCRIPT_NAME="$(basename "${BASH_SOURCE[0]}")" # Default values VERBOSE=false DRY_RUN=false # Functions usage() { cat < Options: -h, --help Show this help message -v, --verbose Enable verbose output -n, --dry-run Show what would be done EOF } log() { echo "[$(date +'%Y-%m-%d %H:%M:%S')] $*" >&2 } error() { log "ERROR: $*" exit 1 } # Main logic main() { # Parse arguments while [[ $# -gt 0 ]]; do case "$1" in -h|--help) usage exit 0 ;; -v|--verbose) VERBOSE=true shift ;; -n|--dry-run) DRY_RUN=true shift ;; *) break ;; esac done # Your logic here } main "$@" ``` ## Error Handling ### Set Options ```bash set -e # Exit on any error set -u # Error on undefined variables set -o pipefail # Pipe failure is script failure set -x # Debug: print each command (use sparingly) ``` ### Trap for Cleanup ```bash cleanup() { rm -f "$TEMP_FILE" log "Cleanup complete" } trap cleanup EXIT # Also handle specific signals trap 'error "Script interrupted"' INT TERM ``` ### Error Checking Patterns ```bash # Check command exists command -v jq >/dev/null 2>&1 || error "jq is required but not installed" # Check file exists [[ -f "$FILE" ]] || error "File not found: $FILE" # Check directory exists [[ -d "$DIR" ]] || mkdir -p "$DIR" # Check variable is set [[ -n "${VAR:-}" ]] || error "VAR is not set" # Check exit status explicitly if ! some_command; then error "some_command failed" fi ``` ## Variables & Substitution ### Variable Expansion ```bash # Default values ${VAR:-default} # Use default if VAR is unset or empty ${VAR:=default} # Set VAR to default if unset or empty ${VAR:+value} # Use value if VAR is set ${VAR:?error msg} # Error if VAR is unset or empty # String manipulation ${VAR#pattern} # Remove shortest prefix match ${VAR##pattern} # Remove longest prefix match ${VAR%pattern} # Remove shortest suffix match ${VAR%%pattern} # Remove longest suffix match ${VAR/old/new} # Replace first occurrence ${VAR//old/new} # Replace all occurrences ${#VAR} # Length of VAR ``` ### Arrays ```bash # Declare array declare -a ARRAY=("one" "two" "three") # Access elements echo "${ARRAY[0]}" # First element echo "${ARRAY[@]}" # All elements echo "${#ARRAY[@]}" # Number of elements echo "${!ARRAY[@]}" # All indices # Iterate for item in "${ARRAY[@]}"; do echo "$item" done # Append ARRAY+=("four") ``` ### Associative Arrays ```bash declare -A MAP MAP["key1"]="value1" MAP["key2"]="value2" # Access echo "${MAP[key1]}" # Check key exists [[ -v MAP[key1] ]] && echo "key1 exists" # Iterate for key in "${!MAP[@]}"; do echo "$key: ${MAP[$key]}" done ``` ## Control Flow ### Conditionals ```bash # String comparison [[ "$str" == "value" ]] [[ "$str" != "value" ]] [[ -z "$str" ]] # Empty [[ -n "$str" ]] # Not empty # Numeric comparison [[ "$num" -eq 5 ]] # Equal [[ "$num" -ne 5 ]] # Not equal [[ "$num" -lt 5 ]] # Less than [[ "$num" -gt 5 ]] # Greater than # File tests [[ -f "$file" ]] # File exists [[ -d "$dir" ]] # Directory exists [[ -r "$file" ]] # Readable [[ -w "$file" ]] # Writable [[ -x "$file" ]] # Executable # Logical operators [[ "$a" && "$b" ]] # AND [[ "$a" || "$b" ]] # OR [[ ! "$a" ]] # NOT ``` ### Loops ```bash # For loop for i in {1..10}; do echo "$i" done # While loop while read -r line; do echo "$line" done < "$file" # Process substitution while read -r line; do echo "$line" done < <(command) # C-style for for ((i=0; i<10; i++)); do echo "$i" done ``` ## Input/Output ### Reading Input ```bash # Read from user read -r -p "Enter name: " name # Read password (hidden) read -r -s -p "Password: " password # Read with timeout read -r -t 5 -p "Quick! " answer # Read file line by line while IFS= read -r line; do echo "$line" done < "$file" ``` ### Output & Redirection ```bash # Redirect stdout command > file # Overwrite command >> file # Append # Redirect stderr command 2> file # Redirect both command &> file command > file 2>&1 # Discard output command > /dev/null 2>&1 # Tee (output and save) command | tee file ``` ## Text Processing ### Common Patterns ```bash # Find and process files find . -name "*.log" -exec grep "ERROR" {} + # Process CSV while IFS=, read -r col1 col2 col3; do echo "$col1: $col2" done < file.csv # JSON processing (with jq) jq '.key' file.json jq -r '.items[]' file.json # AWK one-liners awk '{print $1}' file # First column awk -F: '{print $1}' /etc/passwd # Custom delimiter awk 'NR > 1' file # Skip header # SED one-liners sed 's/old/new/g' file # Replace all sed -i 's/old/new/g' file # In-place edit sed -n '10,20p' file # Print lines 10-20 ``` ## Best Practices ### Do - Quote all variable expansions: `"$VAR"` - Use `[[ ]]` over `[ ]` for tests - Use `$(command)` over backticks - Check return values - Use `readonly` for constants - Use `local` in functions - Provide `--help` option - Use meaningful exit codes ### Don't - Parse `ls` output - Use `eval` with untrusted input - Assume paths don't have spaces - Ignore shellcheck warnings - Write one giant script (modularize) ## Reference Files - **`references/one_liners.md`** - Useful one-liner commands ## Integration with Other Skills - **developer-experience** - For tooling automation - **debugging** - For script debugging - **testing** - For script testing patterns

shell-scripting

Install this skill

Comments