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name: shell-dev-practices description: Shell scripting best practices for bash, sh, zsh with focus on error handling, portability, and automation.

Shell development practices

Purpose

Guide for shell scripting best practices covering POSIX compatibility, error handling, and robust automation scripts.

When to use

This skill activates when:

• Writing shell scripts
• Creating build/deploy automation
• Writing bash/sh/zsh code
• Debugging shell issues
• Making scripts portable

Core principles

Always use strict mode

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

# -e: Exit on error
# -u: Error on undefined variables
# -o pipefail: Fail on pipe errors

Quote all variables

Copy# Bad: Word splitting and glob expansion
echo $filename
rm $path/*

# Good: Properly quoted
echo "$filename"
rm "$path"/*

Error handling

Trap for cleanup

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

TEMP_DIR=""

cleanup() {
    if [[ -n "$TEMP_DIR" && -d "$TEMP_DIR" ]]; then
        rm -rf "$TEMP_DIR"
    fi
}

trap cleanup EXIT

TEMP_DIR=$(mktemp -d)
# Use temp dir...

Check command existence

Copy# Check if command exists
if ! command -v git &> /dev/null; then
    echo "Error: git is required but not installed" >&2
    exit 1
fi

Validate inputs

Copyvalidate_args() {
    if [[ $# -lt 1 ]]; then
        echo "Usage: $0 <filename>" >&2
        exit 1
    fi

    if [[ ! -f "$1" ]]; then
        echo "Error: File not found: $1" >&2
        exit 1
    fi
}

validate_args "$@"

Functions

Function definitions

Copy# Good: Local variables, clear return
process_file() {
    local file="$1"
    local output=""

    if [[ ! -f "$file" ]]; then
        return 1
    fi

    output=$(cat "$file")
    echo "$output"
}

Return values

Copy# Use return for status, echo for output
get_value() {
    local key="$1"

    if [[ -z "$key" ]]; then
        return 1
    fi

    echo "value_for_$key"
}

# Capture output
if value=$(get_value "mykey"); then
    echo "Got: $value"
else
    echo "Failed to get value"
fi

Conditionals

Test syntax

Copy# Modern syntax with [[]]
if [[ -f "$file" ]]; then
    echo "File exists"
fi

# String comparison
if [[ "$string" == "value" ]]; then
    echo "Match"
fi

# Numeric comparison
if [[ "$count" -gt 10 ]]; then
    echo "Greater than 10"
fi

Common tests

Copy# File tests
[[ -f "$path" ]]  # Regular file
[[ -d "$path" ]]  # Directory
[[ -e "$path" ]]  # Exists
[[ -r "$path" ]]  # Readable
[[ -w "$path" ]]  # Writable
[[ -x "$path" ]]  # Executable

# String tests
[[ -z "$var" ]]   # Empty
[[ -n "$var" ]]   # Not empty
[[ "$a" == "$b" ]] # Equal
[[ "$a" != "$b" ]] # Not equal

Arrays

Copy# Declare array
declare -a files=()

# Add elements
files+=("file1.txt")
files+=("file2.txt")

# Iterate
for file in "${files[@]}"; do
    echo "Processing: $file"
done

# Array length
echo "Count: ${#files[@]}"

Input handling

Reading input

Copy# Read line
read -r line < "$file"

# Read with prompt
read -rp "Enter name: " name

# Read password
read -rsp "Enter password: " password
echo  # newline after hidden input

Process arguments

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

usage() {
    echo "Usage: $0 [-v] [-o output] input"
    exit 1
}

verbose=false
output=""

while getopts "vo:" opt; do
    case $opt in
        v) verbose=true ;;
        o) output="$OPTARG" ;;
        *) usage ;;
    esac
done

shift $((OPTIND - 1))

if [[ $# -lt 1 ]]; then
    usage
fi

input="$1"

Output

Logging

Copy# Log levels
log_info() { echo "[INFO] $*"; }
log_warn() { echo "[WARN] $*" >&2; }
log_error() { echo "[ERROR] $*" >&2; }

# Usage
log_info "Starting process"
log_error "Failed to open file"

Colors (optional)

Copy# Only use if terminal supports it
if [[ -t 1 ]]; then
    RED='\033[0;31m'
    GREEN='\033[0;32m'
    NC='\033[0m'
else
    RED=''
    GREEN=''
    NC=''
fi

echo -e "${GREEN}Success${NC}"
echo -e "${RED}Error${NC}"

Portability

POSIX compatibility

Copy# More portable alternatives
# Instead of: echo -e
printf '%s\n' "$message"

# Instead of: [[ ]]
[ -f "$file" ]

# Instead of: $()
`command`  # Though $() is preferred when available

Path handling

Copy# Get script directory
SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"

# Resolve symlinks
REAL_PATH="$(realpath "$path")"

ShellCheck

Always validate with ShellCheck:

Copy# Install
brew install shellcheck  # macOS
apt install shellcheck   # Ubuntu

# Run
shellcheck script.sh

Checklist

• Uses set -euo pipefail
• All variables quoted
• Functions use local variables
• Cleanup with trap
• Inputs validated
• ShellCheck passes

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Additional resources:

• Bash manual
• ShellCheck