Bash Scripting Patterns and Practical Recipes

#!/usr/bin/env bash

# Use env to locate Bash on PATH.

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

# Resolve the directory of the current script.

while (($#)); do
  case "$1" in
    -v|--verbose) verbose=1 ;;
    -o|--output) output=$2; shift ;;
    *) echo "unknown arg: $1" >&2; exit 1 ;;
  esac
  shift
done

# Implement manual flag parsing in Bash.

while getopts ':f:o:v' opt; do
  case $opt in
    f) file=$OPTARG ;;
    o) output=$OPTARG ;;
    v) verbose=1 ;;
    \?) echo "invalid option: -$OPTARG" ;;
  esac
done

# Parse short options using the getopts builtin.

shift $((OPTIND - 1))

# Drop parsed options after getopts completes.

tmp=$(mktemp)

# Create a secure temporary file.

tmpdir=$(mktemp -d)

# Create a secure temporary directory.

cmp -s new.conf current.conf || cp new.conf current.conf

# Avoid replacing files unnecessarily.

if ! command -v jq >/dev/null 2>&1; then echo 'jq required'; exit 1; fi

# Require a dependency before running script logic.

dir=$PWD
while [[ $dir != / && ! -d $dir/.git ]]; do dir=$(dirname "$dir"); done
echo "$dir"

# Walk upward until a matching parent directory is found.

log() { printf '[%s] %s
' "$(date +%H:%M:%S)" "$*"; }

# Print timestamped log lines.

warn() { printf 'WARN: %s
' "$*" >&2; }
die() { printf 'ERROR: %s
' "$*" >&2; exit 1; }

# Use reusable stderr helpers for diagnostics.

./deploy.sh 2>&1 | tee deploy.log

# Write output to terminal and log file.

for attempt in {1..5}; do
  curl -sf http://localhost:8080/health && break
  sleep 2
done

# Retry a command several times before giving up.

timeout 30s bash -lc 'long_running_job'

# Stop a command if it runs too long.

job1 &
pid1=$!
job2 &
pid2=$!
wait "$pid1" "$pid2"

# Start jobs in background and wait for both.

user=$(jq -r '.user.name' payload.json)
echo "$user"

# Combine Bash with jq for reliable JSON parsing.

cat > app.env <<EOF
APP_ENV=${APP_ENV:-dev}
PORT=${PORT:-3000}
EOF

# Create small config files from shell variables.