Using Array Variables in Shell Scripting (Modern Bash)

Indexed arrays (0-based)

Declare and initialize:

# Compact initialization
flights=(1424 1425 492 1720)

# Explicit indices (0-based by default)
flights[0]=1424
flights[1]=1425
flights[2]=492
flights[3]=1720

Access elements and length:

echo "First flight: ${flights[0]}"
echo "Count: ${#flights[@]}"     # number of elements

Append and insert:

flights+=("1880")     # append to the end
flights[10]=9001      # creates a sparse array (indices 4..9 empty)

Delete elements:

unset 'flights[1]'    # remove element at index 1
echo "${flights[@]}"  # index 1 is now empty

Array slicing:

# Get 2 elements starting from index 1
subset=("${flights[@]:1:2}")
echo "${subset[@]}"

Iterate (indices and values):

# Values (preserves spaces/newlines in elements)
for f in "${flights[@]}"; do
  echo "Flight: $f"
done

# Indices + values
for i in "${!flights[@]}"; do
  echo "flights[$i] = ${flights[$i]}"
done

All elements vs single string:

# "${arr[@]}"  → expands to N separate words (safe for spaces)
# "${arr[*]}"  → expands to one word (joined by IFS)
printf '%s\n' "${flights[@]}"   # preferred for robust loops

Associative arrays (key → value)

Great for lookups (Bash 4+):

declare -A airport_city=(
  [SFO]="San Francisco"
  [JFK]="New York"
  [DFW]="Dallas–Fort Worth"
)

echo "${airport_city[JFK]}"     # → New York

for k in "${!airport_city[@]}"; do
  echo "$k → ${airport_city[$k]}"
done

Reading data into arrays

# From a newline-separated file
mapfile -t flights < flights.txt   # each line becomes one element (no trailing newlines)

# From a command safely (no word-splitting surprises)
mapfile -t users < <(getent passwd | cut -d: -f1)

Passing arrays to functions

Use name references (Bash 4.3+):

print_list() {
  local -n ref="$1"   # name reference to an array variable
  for x in "${ref[@]}"; do printf '%s\n' "$x"; done
}

print_list flights

Common pitfalls & best practices

• Quoting: Use "${arr[@]}" to preserve spaces/newlines in elements.
• Index base: Bash arrays are 0-based; do not assume 1-based indices.
• Sparse arrays: Assigning to a high index creates gaps; iterate using "${!arr[@]}".
• set -u (nounset): Check existence with [[ ${arr[index]+x} ]] before access.
• Portability: If you must target pure POSIX sh, emulate with delimited strings or files, or switch to Bash.

Array (indexed)
flights[0] = 1424
flights[1] = 1425
flights[2] = 492
flights[3] = 1720

Scalars (separate variables)
FLIGHT1 = 1424
FLIGHT2 = 1425
FLIGHT3 = 492
FLIGHT4 = 1720

Print all elements quickly

echo "${flights[@]}"     # prints each element as a separate word
printf '%s\n' "${flights[@]}"   # one per line (safer for logs)

Summary

Arrays make Bash scripts clearer and safer whenever you manage lists or key–value pairs. Prefer arrays over ad-hoc variable naming, iterate with "${!arr[@]}", and always quote expansions. For strict POSIX sh, plan alternatives or use Bash.