Array Operations

Learn advanced array operations including searching, sorting, filtering, and transforming arrays in bash.

Joining Array Elements

#!/bin/bash
FRUITS=("apple" "banana" "cherry")

# Join with delimiter
join_by() {
    local IFS="$1"
    shift
    echo "$*"
}

echo $(join_by ',' "${FRUITS[@]}")   # apple,banana,cherry
echo $(join_by ' | ' "${FRUITS[@]}") # apple | banana | cherry

# Simple join with printf
printf '%s,' "${FRUITS[@]}"          # apple,banana,cherry,

Splitting String into Array

#!/bin/bash
STR="apple,banana,cherry"

# Using IFS
IFS=',' read -ra FRUITS <<< "$STR"
echo "${FRUITS[0]}"    # apple
echo "${FRUITS[1]}"    # banana

# Using parameter expansion
STR2="one:two:three"
PARTS=(${STR2//:/ })   # Replace : with space, then split
echo "${PARTS[@]}"

Exercise: Split and Join

Convert between string and array:

Searching Arrays

#!/bin/bash
FRUITS=("apple" "banana" "cherry" "date")

# Check if contains (linear search)
contains() {
    local needle="$1"
    shift
    for item in "$@"; do
        [[ "$item" == "$needle" ]] && return 0
    done
    return 1
}

if contains "banana" "${FRUITS[@]}"; then
    echo "Found!"
fi

# Find index of element
index_of() {
    local needle="$1"
    shift
    local i=0
    for item in "$@"; do
        [[ "$item" == "$needle" ]] && echo $i && return
        ((i++))
    done
    echo -1
}

echo "banana is at index: $(index_of 'banana' "${FRUITS[@]}")"

Filtering Arrays

#!/bin/bash
NUMBERS=(1 2 3 4 5 6 7 8 9 10)

# Filter: keep even numbers
EVENS=()
for n in "${NUMBERS[@]}"; do
    ((n % 2 == 0)) && EVENS+=("$n")
done
echo "${EVENS[@]}"    # 2 4 6 8 10

# Filter files by extension
FILES=(*.txt *.sh)
SCRIPTS=()
for f in "${FILES[@]}"; do
    [[ "$f" == *.sh ]] && SCRIPTS+=("$f")
done

Mapping Arrays

Transform each element:

#!/bin/bash
NAMES=("alice" "bob" "charlie")

# Map: uppercase each
UPPER=()
for name in "${NAMES[@]}"; do
    UPPER+=("${name^^}")
done
echo "${UPPER[@]}"    # ALICE BOB CHARLIE

# Map: add prefix
PREFIXED=()
for name in "${NAMES[@]}"; do
    PREFIXED+=("user_$name")
done
echo "${PREFIXED[@]}"  # user_alice user_bob user_charlie

Exercise: Filter Array

Keep only matching elements:

Sorting Arrays

#!/bin/bash
FRUITS=("cherry" "apple" "banana" "date")

# Sort alphabetically
readarray -t SORTED < <(printf '%s\n' "${FRUITS[@]}" | sort)
echo "${SORTED[@]}"    # apple banana cherry date

# Sort numerically
NUMS=(10 2 33 4 15)
readarray -t SORTED < <(printf '%s\n' "${NUMS[@]}" | sort -n)
echo "${SORTED[@]}"    # 2 4 10 15 33

# Reverse sort
readarray -t SORTED < <(printf '%s\n' "${FRUITS[@]}" | sort -r)
echo "${SORTED[@]}"    # date cherry banana apple

Removing Duplicates

#!/bin/bash
ITEMS=("apple" "banana" "apple" "cherry" "banana")

# Remove duplicates (preserves order)
declare -A seen
UNIQUE=()
for item in "${ITEMS[@]}"; do
    if [[ ! -v seen["$item"] ]]; then
        seen["$item"]=1
        UNIQUE+=("$item")
    fi
done
echo "${UNIQUE[@]}"    # apple banana cherry

# Using sort -u (doesn't preserve order)
readarray -t UNIQUE < <(printf '%s\n' "${ITEMS[@]}" | sort -u)

Array Comparison

#!/bin/bash
A=(1 2 3)
B=(1 2 3)
C=(1 2 4)

# Compare arrays
arrays_equal() {
    [[ "${1[*]}" == "${2[*]}" ]]
}

if [[ "${A[*]}" == "${B[*]}" ]]; then
    echo "A and B are equal"
fi

if [[ "${A[*]}" != "${C[*]}" ]]; then
    echo "A and C are different"
fi

Reversing Arrays

#!/bin/bash
ARR=(1 2 3 4 5)

# Reverse into new array
REVERSED=()
for ((i=${#ARR[@]}-1; i>=0; i--)); do
    REVERSED+=("${ARR[$i]}")
done
echo "${REVERSED[@]}"    # 5 4 3 2 1

# In-place reverse
reverse_array() {
    local -n arr=$1
    local temp
    local len=${#arr[@]}
    for ((i=0; i<len/2; i++)); do
        temp="${arr[$i]}"
        arr[$i]="${arr[$len-1-i]}"
        arr[$len-1-i]="$temp"
    done
}

Practical Example: Process Arguments

#!/bin/bash

POSITIONAL=()
VERBOSE=false
OUTPUT=""

while [[ $# -gt 0 ]]; do
    case $1 in
        -v|--verbose)
            VERBOSE=true
            shift
            ;;
        -o|--output)
            OUTPUT="$2"
            shift 2
            ;;
        *)
            POSITIONAL+=("$1")
            shift
            ;;
    esac
done

echo "Positional args: ${POSITIONAL[@]}"
echo "Verbose: $VERBOSE"
echo "Output: $OUTPUT"

Key Takeaways

• Join with IFS or custom function
• Split strings with IFS=',' read -ra
• Filter by looping and conditionally adding
• Sort with readarray -t SORTED < <(... | sort)
• Remove duplicates with associative array or sort -u
• Compare arrays with [[ "${A[*]}" == "${B[*]}" ]]
• Use readarray -t to capture command output into array
• Bash 4+ features: readarray, -v test for set variables