Indexed Arrays

1. Create arrays explicitly

An indexed array stores values under integer indexes. Bash can infer array creation from indexed assignment, but declare -a makes intent explicit.

declare -a services
services[0]="api"
services[1]="worker"
services[2]="cache"

printf 'first=%s\n' "${services[0]}"
printf 'second=%s\n' "${services[1]}"

2. Array literals preserve element boundaries

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

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

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

3. Append with +=

services=("api" "worker")
services+=("cache")
services+=("metrics" "scheduler")

printf '%s\n' "${services[@]}"

Appending multiple elements in one operation is convenient when optional CLI arguments or discovered resources need to extend an existing list.

4. Array indexes can be sparse

declare -a ports
ports[0]=8080
ports[5]=9090

printf 'indexes: %s\n' "${!ports[@]}"
printf 'values: %s\n' "${ports[@]}"
printf 'count: %d\n' "${#ports[@]}"

The element count is not necessarily equal to the highest index plus one. Do not assume dense numbering unless your own code maintains it.

5. Replace elements by index

images=("api:v1" "worker:v1" "cache:v1")
images[1]="worker:v2"

printf '%s\n' "${images[@]}"

6. unset removes elements

services=("api" "worker" "cache")
unset 'services[1]'

printf 'indexes=%s\n' "${!services[@]}"
printf 'values=%s\n' "${services[@]}"

Removing one element can leave a gap. If you need a dense list again, rebuild it or assign from a filtered expansion.

7. Slice arrays without converting them to strings

items=(zero one two three four five)

printf 'from index 2: %s\n' "${items[@]:2}"
printf 'three items: %s\n' "${items[@]:1:3}"

Quoted array slicing preserves one shell word per selected element.

8. Distinguish array length from element string length

items=("api" "worker service" "cache")

printf 'array elements=%d\n' "${#items[@]}"
printf 'length of element 1=%d\n' "${#items[1]}"

9. Arrays are ideal for command construction

curl_args=(
  --fail
  --silent
  --show-error
  --connect-timeout 5
)

[[ $verbose == true ]] && curl_args+=(--verbose)

curl "${curl_args[@]}" "$url"

10. Copy arrays with quoted expansion

original=("api gateway" "worker" "cache")
copy=("${original[@]}")

copy+=("metrics")

printf 'original=%d copy=%d\n' \
  "${#original[@]}" "${#copy[@]}"

11. Hands-on lab: deployment target array

mkdir -p "$HOME/devops-academy/bash/chapter07/lesson01"
cd "$HOME/devops-academy/bash/chapter07/lesson01"

targets=("api gateway" "worker" "cache")
targets+=("metrics")

printf 'target count=%d\n' "${#targets[@]}"

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

unset 'targets[2]'
printf 'remaining indexes: %s\n' "${!targets[@]}"
printf 'remaining values:\n'
printf '  <%s>\n' "${targets[@]}"

12. Knowledge check

13. Summary

Indexed arrays are Bash's safest in-memory representation for small ordered collections. Use them for arguments, paths, service names, and similar values where exact element boundaries matter.

14. Further reading

• GNU Bash Reference Manual — Arrays.
• GNU Bash Reference Manual — Shell Parameter Expansion.
• GNU Bash Reference Manual — Compound Assignments.
• ShellCheck documentation — array expansion and quoting.