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

#!/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

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

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

Error handling

Trap for cleanup

#!/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

# 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

validate_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

# 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

# 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

# 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

# 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

# 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

# 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

#!/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

# 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)

# 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

# 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

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

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

ShellCheck

Always validate with ShellCheck:

# 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

Additional resources:

• Bash manual
• ShellCheck