advanced-array-patterns

## CRITICAL GUIDELINES ### Windows File Path Requirements **MANDATORY: Always Use Backslashes on Windows for File Paths** When using Edit or Write tools on Windows, you MUST use backslashes (`\`) in file paths, NOT forward slashes (`/`). --- # Advanced Bash Array Patterns (2025) ## Overview Comprehensive guide to bash arrays including indexed arrays, associative arrays, mapfile/readarray, and advanced manipulation patterns following 2025 best practices. ## Indexed Arrays ### Declaration and Initialization ```bash #!/usr/bin/env bash set -euo pipefail # Method 1: Direct assignment files=("file1.txt" "file2.txt" "file with spaces.txt") # Method 2: Compound assignment declare -a numbers=(1 2 3 4 5) # Method 3: Individual assignment fruits[0]="apple" fruits[1]="banana" fruits[2]="cherry" # Method 4: From command output (CAREFUL with word splitting) # ✗ DANGEROUS - splits on spaces files_bad=$(ls) # ✓ SAFE - preserves filenames with spaces mapfile -t files_good < <(find . -name "*.txt") # Method 5: Brace expansion numbers=({1..100}) letters=({a..z}) ``` ### Array Operations ```bash #!/usr/bin/env bash set -euo pipefail arr=("first" "second" "third" "fourth" "fifth") # Length echo "Length: ${#arr[@]}" # 5 # Access elements echo "First: ${arr[0]}" echo "Last: ${arr[-1]}" # Bash 4.3+ echo "Second to last: ${arr[-2]}" # All elements (properly quoted for spaces) for item in "${arr[@]}"; do echo "Item: $item" done # All indices for idx in "${!arr[@]}"; do echo "Index $idx: ${arr[$idx]}" done # Slice (offset:length) echo "${arr[@]:1:3}" # second third fourth # Slice from offset to end echo "${arr[@]:2}" # third fourth fifth # Append element arr+=("sixth") # Insert at position (complex) arr=("${arr[@]:0:2}" "inserted" "${arr[@]:2}") # Remove element by index unset 'arr[2]' # Remove by value (all occurrences) arr_new=() for item in "${arr[@]}"; do [[ "$item" != "second" ]] && arr_new+=("$item") done arr=("${arr_new[@]}") # Check if empty if [[ ${#arr[@]} -eq 0 ]]; then echo "Array is empty" fi # Check if element exists contains() { local needle="$1" shift local item for item in "$@"; do [[ "$item" == "$needle" ]] && return 0 done return 1 } if contains "third" "${arr[@]}"; then echo "Found 'third'" fi ``` ### Array Transformation ```bash #!/usr/bin/env bash set -euo pipefail arr=("apple" "banana" "cherry" "date") # Map (transform each element) upper_arr=() for item in "${arr[@]}"; do upper_arr+=("${item^^}") # Uppercase done # Filter filtered=() for item in "${arr[@]}"; do [[ ${#item} -gt 5 ]] && filtered+=("$item") done # Join array to string IFS=',' joined="${arr[*]}" unset IFS echo "$joined" # apple,banana,cherry,date # Split string to array IFS=',' read -ra split_arr <<< "one,two,three" # Unique values declare -A seen unique=() for item in "${arr[@]}"; do if [[ -z "${seen[$item]:-}" ]]; then seen[$item]=1 unique+=("$item") fi done # Sort array readarray -t sorted < <(printf '%s\n' "${arr[@]}" | sort) # Reverse array reversed=() for ((i=${#arr[@]}-1; i>=0; i--)); do reversed+=("${arr[$i]}") done # Or using tac readarray -t reversed < <(printf '%s\n' "${arr[@]}" | tac) ``` ## Associative Arrays (Bash 4+) ### Declaration and Usage ```bash #!/usr/bin/env bash set -euo pipefail # MUST declare with -A declare -A config # Assignment config["host"]="localhost" config["port"]="8080" config["debug"]="true" # Or compound assignment declare -A user=( [name]="John Doe" [email]="john@example.com" [role]="admin" ) # Access echo "Host: ${config[host]}" echo "User: ${user[name]}" # Default value if key missing echo "${config[missing]:-default}" # Check if key exists if [[ -v config[host] ]]; then echo "Host is set" fi # Alternative check if [[ -n "${config[host]+x}" ]]; then echo "Host key exists (even if empty)" fi # All keys echo "Keys: ${!config[@]}" # All values echo "Values: ${config[@]}" # Length (number of keys) echo "Size: ${#config[@]}" # Iterate for key in "${!config[@]}"; do echo "$key = ${config[$key]}" done # Delete key unset 'config[debug]' # Clear entire array config=() ``` ### Real-World Associative Array Patterns ```bash #!/usr/bin/env bash set -euo pipefail # Configuration parser parse_config() { local config_file="$1" declare -gA CONFIG # Global associative array while IFS='=' read -r key value; do # Skip comments and empty lines [[ "$key" =~ ^[[:space:]]*# ]] && continue [[ -z "$key" ]] && continue # Trim whitespace key="${key//[[:space:]]/}" value="${value#"${value%%[![:space:]]*}"}" # Left trim value="${value%"${value##*[![:space:]]}"}" # Right trim CONFIG["$key"]="$value" done < "$config_file" } # Usage parse_config "/etc/myapp.conf" echo "Database: ${CONFIG[database]:-not set}" # Counter/frequency map count_words() { local file="$1" declare -A word_count while read -ra words; do for word in "${words[@]}"; do # Normalize: lowercase, remove punctuation word="${word,,}" word="${word//[^a-z]/}" [[ -n "$word" ]] && ((word_count[$word]++)) done done < "$file" # Print sorted by count for word in "${!word_count[@]}"; do echo "${word_count[$word]} $word" done | sort -rn | head -10 } # Caching pattern declare -A CACHE cached_expensive_operation() { local key="$1" # Check cache if [[ -n "${CACHE[$key]+x}" ]]; then echo "${CACHE[$key]}" return 0 fi # Compute and cache local result result=$(expensive_computation "$key") CACHE["$key"]="$result" echo "$result" } # JSON-like nested data (using delimited keys) declare -A data data["user.name"]="John" data["user.email"]="john@example.com" data["user.address.city"]="NYC" data["user.address.zip"]="10001" # Access nested echo "City: ${data[user.address.city]}" ``` ## mapfile / readarray (Bash 4+) ### Basic Usage ```bash #!/usr/bin/env bash set -euo pipefail # Read file into array (each line = element) mapfile -t lines < file.txt # Or equivalently: readarray -t lines < file.txt # -t removes trailing newlines # Without -t, each element includes \n # Process each line for line in "${lines[@]}"; do echo "Line: $line" done # Read from command output mapfile -t files < <(find . -name "*.sh") # Read from here-doc mapfile -t data <<'EOF' line1 line2 line3 EOF ``` ### Advanced mapfile Options ```bash #!/usr/bin/env bash set -euo pipefail # -n COUNT: Read at most COUNT lines mapfile -t -n 10 first_10 < large_file.txt # -s COUNT: Skip first COUNT lines mapfile -t -s 1 skip_header < data.csv # Skip header row # -O INDEX: Start at INDEX instead of 0 existing_array=("a" "b") mapfile -t -O "${#existing_array[@]}" existing_array < more_data.txt # -d DELIM: Use DELIM instead of newline (Bash 4.4+) # Read NUL-delimited data (safe for filenames with newlines) mapfile -t -d '' files < <(find . -name "*.txt" -print0) # -C CALLBACK: Execute callback every QUANTUM lines # -c QUANTUM: Number of lines between callbacks (default 5000) process_chunk() { local index=$1 echo "Processing lines around index $index" >&2 } export -f process_chunk mapfile -t -c 1000 -C process_chunk lines < huge_file.txt ``` ### CSV Processing with mapfile ```bash #!/usr/bin/env bash set -euo pipefail # Parse CSV file parse_csv() { local csv_file="$1" local -n result_array="$2" # nameref (Bash 4.3+) while IFS=',' read -ra row; do result_array+=("${row[*]}") # Store as delimited string done < "$csv_file" } # Better: Store as 2D array simulation declare -A csv_data row_num=0 while IFS=',' read -ra fields; do for col_num in "${!fields[@]}"; do csv_data["$row_num,$col_num"]="${fields[$col_num]}" done ((row_num++)) done < data.csv # Access cell echo "Row 2, Col 3: ${csv_data[2,3]}" ``` ## Performance Patterns ### Efficient Array Building ```bash #!/usr/bin/env bash set -euo pipefail # ✗ SLOW - Command substitution in loop slow_build() { local arr=() for i in {1..1000}; do arr+=("$(echo "$i")") # Subshell for each! done } # ✓ FAST - Direct assignment fast_build() { local arr=() for i in {1..1000}; do