Shell & Bash Scripting

Overview

Shell scripting patterns for automation, system administration, and CLI tools.

Script Fundamentals

Script Structure

#!/usr/bin/env bash

Script: backup.sh

Description: Backup files to remote server

Usage: ./backup.sh [options] <source> <destination>

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 "${BASH_SOURCE[0]}")" readonly LOG_FILE="/var/log/${SCRIPT_NAME%.sh}.log"

Default values

VERBOSE=false DRY_RUN=false COMPRESS=true

Cleanup on exit

cleanup() { local exit_code=$? # Cleanup temporary files rm -f "${TEMP_FILE:-}" exit "$exit_code" } trap cleanup EXIT

Logging functions

log() { local level="$1" shift echo "[$(date '+%Y-%m-%d %H:%M:%S')] [$level] $*" | tee -a "$LOG_FILE" }

info() { log "INFO" "$@"; } warn() { log "WARN" "$@" >&2; } error() { log "ERROR" "$@" >&2; } debug() { [[ "$VERBOSE" == true ]] && log "DEBUG" "$@" || true; }

die() { error "$@" exit 1 }

Usage

usage() { cat <<EOF Usage: $SCRIPT_NAME [options] <source> <destination>

Options: -v, --verbose Enable verbose output -n, --dry-run Show what would be done -h, --help Show this help message

Examples: $SCRIPT_NAME /data /backup $SCRIPT_NAME -v --dry-run /home/user /mnt/backup EOF }

Main function

main() { parse_args "$@" validate_inputs perform_backup }

Run main if script is executed directly

if [[ "${BASH_SOURCE[0]}" == "${0}" ]]; then main "$@" fi

Argument Parsing

Using getopts (POSIX)

parse_args_getopts() { while getopts ":vnh" opt; do case $opt in v) VERBOSE=true ;; n) DRY_RUN=true ;; h) usage; exit 0 ;; ?) die "Invalid option: -$OPTARG" ;; :) die "Option -$OPTARG requires an argument" ;; esac done shift $((OPTIND - 1))

SOURCE="${1:-}"
DESTINATION="${2:-}"

}

Using manual parsing (supports long options)

parse_args() { while [[ $# -gt 0 ]]; do case "$1" in -v|--verbose) VERBOSE=true shift ;; -n|--dry-run) DRY_RUN=true shift ;; -c|--compress) COMPRESS=true shift ;; --no-compress) COMPRESS=false shift ;; -h|--help) usage exit 0 ;; --) shift break ;; -*) die "Unknown option: $1" ;; *) break ;; esac done

# Positional arguments
SOURCE="${1:-}"
DESTINATION="${2:-}"

}

Validation

validate_inputs() { [[ -z "$SOURCE" ]] && die "Source path is required" [[ -z "$DESTINATION" ]] && die "Destination path is required" [[ -e "$SOURCE" ]] || die "Source does not exist: $SOURCE" }

Variables and Data

Variable Operations

Variable assignment

name="John" readonly CONSTANT="immutable"

Default values

name="${name:-default}" # Use default if unset or empty name="${name:=default}" # Assign default if unset or empty name="${name:+alternative}" # Use alternative if set and non-empty name="${name:?error message}" # Error if unset or empty

String manipulation

str="Hello, World!" echo "${str:0:5}" # "Hello" (substring) echo "${str: -6}" # "World!" (last 6 chars) echo "${#str}" # 13 (length) echo "${str/World/Bash}" # "Hello, Bash!" (replace first) echo "${str//o/0}" # "Hell0, W0rld!" (replace all) echo "${str#Hello, }" # "World!" (remove prefix) echo "${str%!}" # "Hello, World" (remove suffix) echo "${str^^}" # "HELLO, WORLD!" (uppercase) echo "${str,,}" # "hello, world!" (lowercase)

Parameter expansion

filename="/path/to/file.tar.gz" echo "${filename##/}" # "file.tar.gz" (basename) echo "${filename%/}" # "/path/to" (dirname) echo "${filename%%.*}" # "/path/to/file" (remove all extensions) echo "${filename%.gz}" # "/path/to/file.tar" (remove last extension)

Arrays

Indexed arrays

declare -a fruits=("apple" "banana" "cherry") fruits+=("date") # Append echo "${fruits[0]}" # "apple" (first element) echo "${fruits[-1]}" # "date" (last element) echo "${fruits[@]}" # All elements echo "${#fruits[@]}" # 4 (length) echo "${!fruits[@]}" # 0 1 2 3 (indices)

Iterate

for fruit in "${fruits[@]}"; do echo "$fruit" done

With indices

for i in "${!fruits[@]}"; do echo "$i: ${fruits[i]}" done

Associative arrays (bash 4+)

declare -A user=( [name]="John" [email]="john@example.com" [age]=30 )

echo "${user[name]}" # "John" echo "${!user[@]}" # Keys: name email age echo "${user[@]}" # Values

Iterate key-value

for key in "${!user[@]}"; do echo "$key: ${user[$key]}" done

Array slicing

echo "${fruits[@]:1:2}" # "banana" "cherry" (from index 1, 2 elements)

Array filtering (bash 4+)

evens=() for n in "${numbers[@]}"; do (( n % 2 == 0 )) && evens+=("$n") done

Control Flow

Conditionals

Test operators

Strings

[[ -z "$str" ]] # Empty [[ -n "$str" ]] # Not empty [[ "$a" == "$b" ]] # Equal [[ "$a" != "$b" ]] # Not equal [[ "$a" < "$b" ]] # Less than (lexicographic) [[ "$a" =~ ^[0-9]+$ ]] # Regex match

Numbers

[[ "$a" -eq "$b" ]] # Equal [[ "$a" -ne "$b" ]] # Not equal [[ "$a" -lt "$b" ]] # Less than [[ "$a" -le "$b" ]] # Less than or equal [[ "$a" -gt "$b" ]] # Greater than [[ "$a" -ge "$b" ]] # Greater than or equal

Files

[[ -e "$file" ]] # Exists [[ -f "$file" ]] # Regular file [[ -d "$dir" ]] # Directory [[ -r "$file" ]] # Readable [[ -w "$file" ]] # Writable [[ -x "$file" ]] # Executable [[ -s "$file" ]] # Size > 0 [[ "$a" -nt "$b" ]] # Newer than [[ "$a" -ot "$b" ]] # Older than

If-elif-else

if [[ "$status" == "success" ]]; then echo "Success!" elif [[ "$status" == "pending" ]]; then echo "Still pending..." else echo "Failed" fi

Case statement

case "$command" in start|begin) start_service ;; stop|end) stop_service ;; restart) stop_service start_service ;; *) echo "Unknown command: $command" exit 1 ;; esac

Short-circuit

[[ -f "$file" ]] && process_file "$file" [[ -d "$dir" ]] || mkdir -p "$dir"

Loops

For loop

for item in item1 item2 item3; do echo "$item" done

C-style for

for ((i = 0; i < 10; i++)); do echo "$i" done

While loop

counter=0 while [[ $counter -lt 5 ]]; do echo "$counter" ((counter++)) done

Read file line by line

while IFS= read -r line; do echo "$line" done < "$file"

Process command output

while IFS= read -r file; do echo "Processing: $file" done < <(find . -name "*.txt")

Until loop

until [[ -f "$lockfile" ]]; do sleep 1 done

Break and continue

for i in {1..10}; do [[ $i -eq 5 ]] && continue [[ $i -eq 8 ]] && break echo "$i" done

Functions

Basic function

greet() { local name="$1" echo "Hello, $name!" }

Function with return value

is_valid_email() { local email="$1" [[ "$email" =~ ^[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+.[A-Za-z]{2,}$ ]] }

Check return value

if is_valid_email "test@example.com"; then echo "Valid email" fi

Function with output capture

get_user_count() { wc -l < /etc/passwd } count=$(get_user_count)

Function with array parameter

process_files() { local -a files=("$@") for file in "${files[@]}"; do echo "Processing: $file" done } process_files file1.txt file2.txt file3.txt

Function with named reference (bash 4.3+)

modify_array() { local -n arr=$1 arr+=("new_element") }

my_array=("a" "b" "c") modify_array my_array echo "${my_array[@]}" # "a b c new_element"

Error handling in functions

safe_divide() { local dividend="$1" local divisor="$2"

if [[ "$divisor" -eq 0 ]]; then
    echo "Error: Division by zero" >&2
    return 1
fi

echo $((dividend / divisor))

}

result=$(safe_divide 10 2) && echo "Result: $result"

Text Processing

grep - search patterns

grep "error" logfile.txt # Find lines with "error" grep -i "error" logfile.txt # Case-insensitive grep -E "error|warning" logfile.txt # Extended regex grep -v "debug" logfile.txt # Invert match grep -c "error" logfile.txt # Count matches grep -l "error" *.log # List files with matches grep -r "TODO" src/ # Recursive search

sed - stream editor

sed 's/old/new/' file.txt # Replace first occurrence sed 's/old/new/g' file.txt # Replace all sed -i.bak 's/old/new/g' file.txt # In-place with backup sed '/pattern/d' file.txt # Delete matching lines sed -n '10,20p' file.txt # Print lines 10-20 sed 's/^/prefix: /' file.txt # Add prefix

awk - field processing

awk '{print $1}' file.txt # First field awk -F: '{print $1}' /etc/passwd # Custom delimiter awk '{sum += $1} END {print sum}' data.txt # Sum first column awk 'NR > 1' file.txt # Skip header awk '$3 > 100 {print $1, $3}' data.txt # Conditional print awk '{print NR": "$0}' file.txt # Add line numbers

cut - extract fields

cut -d: -f1 /etc/passwd # First field cut -c1-10 file.txt # Characters 1-10 cut -d, -f1,3 data.csv # Fields 1 and 3

sort and uniq

sort file.txt # Sort lines sort -n numbers.txt # Numeric sort sort -r file.txt # Reverse sort sort -t: -k3 -n /etc/passwd # Sort by field uniq file.txt # Remove adjacent duplicates sort file.txt | uniq -c # Count occurrences sort file.txt | uniq -d # Show duplicates only

tr - translate characters

echo "hello" | tr 'a-z' 'A-Z' # Uppercase tr -d '\r' < file.txt # Remove carriage returns tr -s ' ' < file.txt # Squeeze spaces

xargs - build commands

find . -name ".txt" | xargs wc -l find . -name ".log" -print0 | xargs -0 rm echo "a b c" | xargs -n1 echo cat urls.txt | xargs -P4 -I{} curl {} # Parallel execution

Process Management

Background processes

long_running_command & pid=$! # Get PID of last background job wait $pid # Wait for specific process

Run multiple in background and wait

for file in *.txt; do process_file "$file" & done wait # Wait for all

Parallel processing with xargs

find . -name "*.jpg" -print0 | xargs -0 -P4 -I{} convert {} {}.png

Job control

jobs # List jobs fg %1 # Bring job 1 to foreground bg %1 # Resume job 1 in background kill %1 # Kill job 1

Process substitution

diff <(sort file1.txt) <(sort file2.txt) while read -r line; do echo "$line" done < <(command_that_outputs)

Command groups

{ cmd1; cmd2; cmd3; } > output.txt # Group and redirect ( cd /tmp && cmd1; cmd2 ) # Subshell (doesn't affect current shell)

Coprocesses

coproc my_coproc { while read -r line; do echo "Got: $line"; done; } echo "Hello" >&"${my_coproc[1]}" read -r response <&"${my_coproc[0]}"

Related Skills

• [[automation-scripts]] - Build automation

• [[devops-cicd]] - CI/CD pipelines

• [[development-environment]] - Environment setup