Safe Array Expansion and Quoting

1. Quoted @ is the default safe expansion

items=("api gateway" "worker" "*.yaml")

for item in "${items[@]}"; do
  printf 'item=<%s>\n' "$item"
done

Each element remains one shell word, regardless of spaces or wildcard characters.

2. Quoted * joins elements

items=("api gateway" "worker" "cache")
IFS=,
joined="${items[*]}"
printf 'joined=%s\n' "$joined"

Quoted "${array[*]}" creates one word by joining elements with the first character of IFS. Use it only when joining is explicitly the goal.

3. Unquoted array expansion reintroduces splitting and globbing

items=("api gateway" "*.log")

# Fragile:
for item in ${items[@]}; do
  printf 'item=<%s>\n' "$item"
done

4. Execute command arrays with quoted @

command=(
  printf
  'service=%s env=%s\n'
  "api gateway"
  "staging blue"
)

"${command[@]}"

The first array element becomes the command name; the remaining elements become exact arguments.

5. Append one array to another safely

base=(--fail --silent)
extra=(--header "X-Mode: staging")

args=("${base[@]}" "${extra[@]}")

printf '<%s>\n' "${args[@]}"

6. Empty arrays should expand to zero arguments

args=()

run() {
  printf 'argc=%d\n' "$#"
}

run "${args[@]}"

Quoted "${args[@]}" on an empty array supplies zero arguments. This is exactly what command construction usually needs.

7. Quote individual element expansions too

files=("release candidate.tar.gz")

file=${files[0]}
printf 'file=<%s>\n' "$file"

cp -- "${files[0]}" destination/

Using -- before path operands protects against filenames that begin with a dash when the target command follows this convention.

8. Expand indexes separately from values

values=([2]="api" [7]="worker")

for index in "${!values[@]}"; do
  printf 'index=%s value=%s\n' "$index" "${values[$index]}"
done

For associative arrays, the same syntax returns keys rather than numeric indexes.

9. Joining for display is different from forwarding

services=("api gateway" "worker" "cache")

printf 'display: %s\n' "$(IFS=,; printf '%s' "${services[*]}")"

# Forwarding remains:
some_command "${services[@]}"

Human-readable serialization and machine-safe argument forwarding are different operations. Do not substitute one for the other.

10. printf can inspect arrays without losing boundaries

items=("api gateway" "worker" "cache*")

printf 'count=%d\n' "${#items[@]}"
printf 'item=%q\n' "${items[@]}"

%q is useful for diagnostics because it shows a shell-reusable representation without changing the underlying array.

11. Pass array data to functions as ordinary arguments

print_items() {
  local item
  for item in "$@"; do
    printf '<%s>\n' "$item"
  done
}

items=("api gateway" worker cache)
print_items "${items[@]}"

For complex shared array mutation, Bash also offers name references via declare -n, but explicit arguments are often easier to understand and test.

12. Hands-on lab: safe kubectl-style argument vector

mkdir -p "$HOME/devops-academy/bash/chapter07/lesson03"
cd "$HOME/devops-academy/bash/chapter07/lesson03"

namespace="team blue"
selector="app=api gateway"

args=(
  get
  pods
  --namespace "$namespace"
  --selector "$selector"
)

printf 'argument count=%d\n' "${#args[@]}"
printf 'arg=%q\n' "${args[@]}"

# Demonstrate exact forwarding without requiring kubectl:
printf 'SIMULATED kubectl'
printf ' %q' "${args[@]}"
printf '\n'

13. Knowledge check

14. Summary

Arrays preserve boundaries only when expansions preserve them. Use quoted "${array[@]}" for iteration and forwarding, quoted "${array[*]}" only for intentional joining, and avoid unquoted expansions in production code.

15. Further reading

• GNU Bash Reference Manual — Arrays.
• GNU Bash Reference Manual — Shell Parameter Expansion.
• GNU Bash Reference Manual — Word Splitting and Filename Expansion.
• ShellCheck documentation — array quoting.