Bash scripts glue systems together. Here’s how to write them without the usual fragility.
Script Header Always start with:
1 2 3 4 5 6 #!/usr/bin/env bash set -euo pipefail # -e: Exit on error # -u: Error on undefined variables # -o pipefail: Fail if any pipe command fails Argument Parsing Simple Positional 1 2 3 4 5 6 7 8 9 #!/usr/bin/env bash set -euo pipefail if [[ $# -lt 1 ]]; then echo "Usage: $0 <filename>" >&2 exit 1 fi FILENAME="$1" With Options (getopts) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 #!/usr/bin/env bash set -euo pipefail usage() { echo "Usage: $0 [-v] [-o output] [-n count] input" echo " -v Verbose mode" echo " -o output Output file" echo " -n count Number of iterations" exit 1 } VERBOSE=false OUTPUT="" COUNT=1 while getopts "vo:n:h" opt; do case $opt in v) VERBOSE=true ;; o) OUTPUT="$OPTARG" ;; n) COUNT="$OPTARG" ;; h) usage ;; *) usage ;; esac done shift $((OPTIND - 1)) if [[ $# -lt 1 ]]; then usage fi INPUT="$1" Long Options 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 #!/usr/bin/env bash set -euo pipefail VERBOSE=false OUTPUT="" while [[ $# -gt 0 ]]; do case $1 in -v|--verbose) VERBOSE=true shift ;; -o|--output) OUTPUT="$2" shift 2 ;; -h|--help) usage ;; -*) echo "Unknown option: $1" >&2 exit 1 ;; *) break ;; esac done Error Handling Trap for Cleanup 1 2 3 4 5 6 7 8 9 10 11 12 13 14 #!/usr/bin/env bash set -euo pipefail TMPDIR="" cleanup() { if [[ -n "$TMPDIR" && -d "$TMPDIR" ]]; then rm -rf "$TMPDIR" fi } trap cleanup EXIT TMPDIR=$(mktemp -d) # Work with $TMPDIR - it's cleaned up on exit, error, or interrupt Custom Error Handler 1 2 3 4 5 6 7 8 9 10 11 12 13 14 #!/usr/bin/env bash set -euo pipefail error() { echo "Error: $1" >&2 exit "${2:-1}" } warn() { echo "Warning: $1" >&2 } # Usage [[ -f "$CONFIG" ]] || error "Config file not found: $CONFIG" Detailed Error Reporting 1 2 3 4 5 6 7 8 #!/usr/bin/env bash set -euo pipefail on_error() { echo "Error on line $1" >&2 exit 1 } trap 'on_error $LINENO' ERR Variables and Defaults 1 2 3 4 5 6 7 8 9 10 11 # Default value NAME="${1:-default}" # Error if unset NAME="${1:?Error: name required}" # Default only if unset (not empty) NAME="${NAME-default}" # Assign default if unset : "${NAME:=default}" String Operations 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 FILE="/path/to/file.txt" # Extract parts echo "${FILE##*/}" # file.txt (basename) echo "${FILE%/*}" # /path/to (dirname) echo "${FILE%.txt}" # /path/to/file (remove extension) echo "${FILE##*.}" # txt (extension only) # Replace echo "${FILE/path/new}" # /new/to/file.txt echo "${FILE//t/T}" # /paTh/To/file.TxT (all occurrences) # Case conversion echo "${FILE^^}" # Uppercase echo "${FILE,,}" # Lowercase # Length echo "${#FILE}" # String length Conditionals 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 # File tests [[ -f "$FILE" ]] # File exists [[ -d "$DIR" ]] # Directory exists [[ -r "$FILE" ]] # Readable [[ -w "$FILE" ]] # Writable [[ -x "$FILE" ]] # Executable [[ -s "$FILE" ]] # Non-empty # String tests [[ -z "$VAR" ]] # Empty [[ -n "$VAR" ]] # Non-empty [[ "$A" == "$B" ]] # Equal [[ "$A" != "$B" ]] # Not equal [[ "$A" =~ regex ]] # Regex match # Numeric tests [[ $A -eq $B ]] # Equal [[ $A -ne $B ]] # Not equal [[ $A -lt $B ]] # Less than [[ $A -le $B ]] # Less or equal [[ $A -gt $B ]] # Greater than [[ $A -ge $B ]] # Greater or equal # Logical [[ $A && $B ]] # And [[ $A || $B ]] # Or [[ ! $A ]] # Not Loops 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 # Over arguments for arg in "$@"; do echo "$arg" done # Over array arr=("one" "two" "three") for item in "${arr[@]}"; do echo "$item" done # C-style for ((i=0; i<10; i++)); do echo "$i" done # Over files for file in *.txt; do [[ -f "$file" ]] || continue echo "$file" done # Read lines from file while IFS= read -r line; do echo "$line" done < "$FILE" # Read lines from command while IFS= read -r line; do echo "$line" done < <(some_command) Functions 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 # Basic function greet() { local name="$1" echo "Hello, $name" } # With return value is_valid() { local input="$1" [[ "$input" =~ ^[0-9]+$ ]] } if is_valid "$value"; then echo "Valid" fi # Return data via stdout get_config() { cat /etc/myapp/config } CONFIG=$(get_config) # Local variables process() { local tmp tmp=$(mktemp) # tmp is local to this function } Arrays 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 # Create arr=("one" "two" "three") arr[3]="four" # Access echo "${arr[0]}" # First element echo "${arr[@]}" # All elements echo "${#arr[@]}" # Length echo "${arr[@]:1:2}" # Slice (start:length) # Add arr+=("five") # Iterate for item in "${arr[@]}"; do echo "$item" done # With index for i in "${!arr[@]}"; do echo "$i: ${arr[$i]}" done Associative Arrays 1 2 3 4 5 6 7 8 9 10 11 12 declare -A config config[host]="localhost" config[port]="8080" echo "${config[host]}" echo "${!config[@]}" # All keys echo "${config[@]}" # All values for key in "${!config[@]}"; do echo "$key: ${config[$key]}" done Process Substitution 1 2 3 4 5 6 7 # Compare two commands diff <(sort file1) <(sort file2) # Feed command output as file while read -r line; do echo "$line" done < <(curl -s "$URL") Subshells 1 2 3 4 5 6 7 8 9 10 11 12 # Run in subshell (changes don't affect parent) ( cd /tmp rm -f *.tmp ) # Still in original directory # Capture output result=$(command) # Capture with error result=$(command 2>&1) Here Documents 1 2 3 4 5 6 7 8 9 10 11 12 13 # Multi-line string cat << EOF This is a multi-line string with $VARIABLE expansion EOF # No variable expansion cat << 'EOF' This preserves $VARIABLE literally EOF # Here string grep "pattern" <<< "$string" Practical Patterns Check Dependencies 1 2 3 4 5 6 7 8 9 check_deps() { local deps=("curl" "jq" "git") for dep in "${deps[@]}"; do if ! command -v "$dep" &> /dev/null; then echo "Missing dependency: $dep" >&2 exit 1 fi done } Logging 1 2 3 4 5 6 7 LOG_FILE="/var/log/myapp.log" log() { echo "[$(date '+%Y-%m-%d %H:%M:%S')] $*" | tee -a "$LOG_FILE" } log "Starting process" Confirmation Prompt 1 2 3 4 5 6 7 8 confirm() { read -rp "$1 [y/N] " response [[ "$response" =~ ^[Yy]$ ]] } if confirm "Delete all files?"; then rm -rf /tmp/data/* fi Retry Logic 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 retry() { local max_attempts="$1" local delay="$2" shift 2 local attempt=1 until "$@"; do if ((attempt >= max_attempts)); then echo "Failed after $attempt attempts" >&2 return 1 fi echo "Attempt $attempt failed, retrying in ${delay}s..." sleep "$delay" ((attempt++)) done } retry 3 5 curl -sf "$URL" Lock File 1 2 3 4 5 6 7 8 9 10 11 12 LOCKFILE="/var/lock/myapp.lock" acquire_lock() { exec 200>"$LOCKFILE" flock -n 200 || { echo "Another instance is running" >&2 exit 1 } } acquire_lock # Script continues only if lock acquired Complete Example 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 #!/usr/bin/env bash set -euo pipefail readonly SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)" readonly SCRIPT_NAME="$(basename "$0")" usage() { cat << EOF Usage: $SCRIPT_NAME [options] <input> Options: -o, --output FILE Output file (default: stdout) -v, --verbose Verbose output -h, --help Show this help Examples: $SCRIPT_NAME data.txt $SCRIPT_NAME -o result.txt -v input.txt EOF exit "${1:-0}" } log() { if [[ "$VERBOSE" == true ]]; then echo "[$(date '+%H:%M:%S')] $*" >&2 fi } error() { echo "Error: $*" >&2 exit 1 } # Defaults VERBOSE=false OUTPUT="/dev/stdout" # Parse arguments while [[ $# -gt 0 ]]; do case $1 in -o|--output) OUTPUT="$2"; shift 2 ;; -v|--verbose) VERBOSE=true; shift ;; -h|--help) usage ;; -*) error "Unknown option: $1" ;; *) break ;; esac done [[ $# -ge 1 ]] || usage 1 INPUT="$1" # Validate [[ -f "$INPUT" ]] || error "File not found: $INPUT" # Main logic log "Processing $INPUT" process_data < "$INPUT" > "$OUTPUT" log "Done" Bash scripts don’t have to be fragile. Apply these patterns and they’ll work reliably for years.
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