Shell by Example: Arrays

Shell by Example: Arrays Bash

Bash Arrays in Bash let you store multiple values in a single variable. Note: Arrays are a Bash feature and are not available in POSIX sh. This example creates an array with four elements.	Edit `#!/bin/bash fruits=("apple" "banana" "cherry" "date")`

Bash Access elements using index (0-based).	Edit `#!/bin/bash fruits=("apple" "banana" "cherry" "date") echo "First fruit: ${fruits[0]}" echo "Second fruit: ${fruits[1]}"`
	Output: First fruit: apple Second fruit: banana

Bash Get all elements with `[@]` or `[*]`.	Edit `#!/bin/bash fruits=("apple" "banana" "cherry" "date") echo "All fruits: ${fruits[*]}"`
	Output: All fruits: apple banana cherry date

Bash Get the number of elements.	Edit `#!/bin/bash fruits=("apple" "banana" "cherry" "date") echo "Number of fruits: ${#fruits[@]}"`
	Output: Number of fruits: 4

Bash Add an element to the end.	Edit `#!/bin/bash fruits=("apple" "banana" "cherry" "date") fruits+=("elderberry") echo "After adding: ${fruits[*]}"`
	Output: After adding: apple banana cherry date elderberry

Bash Set a specific index.	Edit `#!/bin/bash fruits=("apple" "banana" "cherry" "date") fruits[1]="blueberry" echo "After replacing: ${fruits[*]}"`
	Output: After replacing: apple blueberry cherry date

Bash Loop over array elements.	Edit `#!/bin/bash fruits=("apple" "banana" "cherry" "date") for fruit in "${fruits[@]}"; do echo " - $fruit" done`
	Output: - apple - banana - cherry - date

Bash Get array indices with `${!array[@]}`, and loop with indices.	Edit `#!/bin/bash fruits=(apple banana cherry) echo "Indices: ${!fruits[*]}" echo "With indices:" for i in "${!fruits[@]}"; do echo " [$i] = ${fruits[$i]}" done`
	Output: Indices: 0 1 2 With indices: [0] = apple [1] = banana [2] = cherry

Bash Array slicing: `${array[@]:start:length}`	Edit `#!/bin/bash fruits=("apple" "banana" "cherry" "date") echo "First three: ${fruits[]:0:3}" echo "From index 2: ${fruits[]:2}"`
	Output: First three: apple banana cherry From index 2: cherry date

Bash Check if array is empty.	Edit `#!/bin/bash empty_array=() if [ ${#empty_array[@]} -eq 0 ]; then echo "Array is empty" fi`
	Output: Array is empty

Bash Create array from command output.	Edit `#!/bin/bash touch file1.txt file2.txt file3.txt # create array from command output using mapfile mapfile -t files < <(find . -maxdepth 1 -type f -print) echo "Shell files: ${files[*]}"`
	Output: Shell files: ./file3.txt ./file2.txt ./file1.txt

Bash Delete an element (leaves a gap in indices).	Edit `#!/bin/bash fruits=("apple" "banana" "cherry" "date") unset 'fruits[2]' echo "After unset [2]: ${fruits[]}" echo "Indices now: ${!fruits[]}"`
	Output: After unset [2]: apple banana date Indices now: 0 1 3

Bash Bash provides associative arrays (hash maps) using `declare -A`.	Edit `#!/bin/bash declare -A user user[name]="Alice" user[email]="alice@example.com" user[age]="30" echo "Name: ${user[name]}" echo "Email: ${user[email]}"`
	Output: Name: Alice Email: alice@example.com

Bash Iterate over associative array keys:	Edit `#!/bin/bash declare -A user user[name]="Alice" user[email]="alice@example.com" user[age]="30" for key in "${!user[@]}"; do echo "$key: ${user[$key]}" done`
	Output: email: alice@example.com age: 30 name: Alice

Bash Check if a key exists:	Edit `#!/bin/bash if [[ -v user[name] ]]; then echo "name key exists" fi`

Bash Bash provides `mapfile` (or `readarray`) to read lines from a file or command into an array:	Edit `#!/bin/bash mapfile -t lines </etc/passwd for line in "${lines[@]}"; do echo "$line" done`
	Output: root:x:0:0:root:/root:/bin/sh bin:x:1:1:bin:/bin:/sbin/nologin daemon:x:2:2:daemon:/sbin:/sbin/nologin lp:x:4:7:lp:/var/spool/lpd:/sbin/nologin sync:x:5:0:sync:/sbin:/bin/sync shutdown:x:6:0:shutdown:/sbin:/sbin/shutdown halt:x:7:0:halt:/sbin:/sbin/halt mail:x:8:12:mail:/var/mail:/sbin/nologin news:x:9:13:news:/usr/lib/news:/sbin/nologin uucp:x:10:14:uucp:/var/spool/uucppublic:/sbin/nologin cron:x:16:16:cron:/var/spool/cron:/sbin/nologin ftp:x:21:21::/var/lib/ftp:/sbin/nologin sshd:x:22:22:sshd:/dev/null:/sbin/nologin games:x:35:35:games:/usr/games:/sbin/nologin ntp:x:123:123:NTP:/var/empty:/sbin/nologin guest:x:405:100:guest:/dev/null:/sbin/nologin nobody:x:65534:65534:nobody:/:/sbin/nologin

Bash Read only first 5 lines:	Edit `#!/bin/bash mapfile -t -n 5 lines </etc/passwd for line in "${lines[@]}"; do echo "$line" done`
	Output: root:x:0:0:root:/root:/bin/sh bin:x:1:1:bin:/bin:/sbin/nologin daemon:x:2:2:daemon:/sbin:/sbin/nologin lp:x:4:7:lp:/var/spool/lpd:/sbin/nologin sync:x:5:0:sync:/sbin:/bin/sync

Bash Read from a command using process substitution:	Edit `#!/bin/bash mapfile -t users < <(cut -d: -f1 /etc/passwd \| head -5) echo "Users: ${users[*]}"`
	Output: Users: root bin daemon lp sync

Bash Read with a specific delimiter:	Edit `#!/bin/bash mapfile -t -d ':' fields <<<"a:b:c:d" echo "Fields: ${fields[*]}"`
	Output: Fields: a b c d

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