Linux Basic — Looping Array Elements

Looping Array Elements

Arrays + for is the bread and butter of every Bash script. Done right, it handles any value cleanly. Done wrong, it breaks on whitespace and metacharacters.

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The Three Loop Forms

1. By value

fruits=(apple banana cherry)

for f in "${fruits[@]}"; do
    echo "[$f]"
done
# [apple]
# [banana]
# [cherry]

Always quote, always use @. The quotes prevent word-splitting; @ (not *) keeps each element separate.

2. By index

for i in "${!fruits[@]}"; do
    echo "$i → ${fruits[$i]}"
done
# 0 → apple
# 1 → banana
# 2 → cherry

Use this when you need the position, or when the array might be sparse (and for f in "${arr[@]}" would silently skip holes by value):

arr=(a b c d)
unset 'arr[1]'

for f in "${arr[@]}"; do echo "$f"; done
# a
# c
# d                  ← no clue index 1 was missing

for i in "${!arr[@]}"; do echo "$i $f"; done
# 0 a
# 2 c
# 3 d                ← positions visible

3. C-style

for ((i=0; i<${#fruits[@]}; i++)); do
    echo "$i → ${fruits[$i]}"
done

Use C-style when you need arithmetic on the index — steps of 2, counting down, etc.:

for ((i=${#fruits[@]}-1; i>=0; i--)); do
    echo "${fruits[$i]}"        # reversed
done

---

Loop Over an Associative Array

declare -A cfg=([host]=example.com [port]=8080 [debug]=true)

for k in "${!cfg[@]}"; do
    printf '%-8s = %s\n' "$k" "${cfg[$k]}"
done

No order guarantee — sort the keys if you need predictable output:

for k in $(printf '%s\n' "${!cfg[@]}" | sort); do
    printf '%-8s = %s\n' "$k" "${cfg[$k]}"
done

---

Parallel Iteration — Two Related Arrays

When two arrays share indexes (e.g. names + ages):

names=(alice  bob  carol)
ages=(   30    25    40)

for i in "${!names[@]}"; do
    printf '%-6s is %d\n' "${names[$i]}" "${ages[$i]}"
done
# alice  is 30
# bob    is 25
# carol  is 40

Bash has no native zip. The index loop is the idiomatic form.

For a hash-table style join across two unrelated arrays, build an associative array:

names=(alice bob carol)
ages=(30 25 40)

declare -A age_of
for i in "${!names[@]}"; do
    age_of[${names[$i]}]=${ages[$i]}
done

echo "${age_of[bob]}"           # 25

---

break and continue

for f in "${files[@]}"; do
    [[ "$f" == .* ]] && continue        # skip hidden
    [[ "$f" == STOP ]] && break          # stop on marker
    process "$f"
done

break N / continue N break or continue out of N levels of nested loops:

for outer in 1 2 3; do
    for inner in a b c; do
        if [[ "$inner" == b ]]; then
            continue 2                  # back to outer loop
        fi
        echo "$outer $inner"
    done
done
# 1 a
# 2 a
# 3 a

---

for x in {1..N} — Range Loops

for i in {1..5}; do
    echo "$i"
done
# 1 2 3 4 5

for i in {10..1..2}; do            # 10..1 step 2 (descending)
    echo "$i"
done
# 10 8 6 4 2

for c in {a..f}; do
    echo "$c"
done
# a b c d e f

Brace expansion happens before variable expansion. So this does not work:

n=10
for i in {1..$n}; do echo $i; done
# Output: {1..10}            ← literal, not expanded

Use C-style for variable bounds:

for ((i=1; i<=n; i++)); do echo "$i"; done

Or seq (Linux) / jot (BSD):

for i in $(seq 1 "$n"); do echo "$i"; done

---

A Real Example — Process Files in Batches

files=( *.csv )

# Process in batches of 5
batch_size=5
for ((i=0; i<${#files[@]}; i+=batch_size)); do
    batch=( "${files[@]:i:batch_size}" )
    echo "Batch starting at $i:"
    printf '  %s\n' "${batch[@]}"
    # process_batch "${batch[@]}"
done

Slicing inside an arithmetic-driven loop — the C-style for shines here.

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Common Mistakes

❌ Wrong	✅ Right	Issue
for x in ${arr[@]} (unquoted)	for x in "${arr[@]}"	unquoted splits on IFS
for x in ${arr[*]}	for x in "${arr[@]}"	* joins; @ separates
for ((i=0; i<${#arr}; i++))	for ((i=0; i<${#arr[@]}; i++))	${#arr} is byte-length of element 0
for i in {1..$n}	for ((i=1; i<=n; i++))	brace expansion can’t see variables
Iterating sparse array by value	use for i in "${!arr[@]}"	by-value loop hides the holes

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