Arrays

Indexed arrays (bash and zsh)

files=("a.txt" "b.txt" "c.txt")
echo "${files[0]}"        # a.txt (bash) or empty (zsh — see below)
echo "${files[@]}"        # all elements
echo "${#files[@]}"       # count: 3
files+=("d.txt")          # append
unset 'files[1]'           # remove the b.txt slot

bash arrays start at 0, zsh at 1 by default. Add setopt KSH_ARRAYS in zsh to get bash semantics. The difference catches everyone the first time.

Iterating

for f in "${files[@]}"; do
  echo "$f"
done

# With index
for i in "${!files[@]}"; do
  echo "$i: ${files[i]}"
done

Associative arrays (bash 4+, zsh)

declare -A user
user[name]="Alice"
user[age]=24
for k in "${!user[@]}"; do
  echo "$k = ${user[$k]}"
done

macOS bash 3.2 doesn't have associative arrays. Either install bash 5 via brew, or use zsh, or fall back to parallel indexed arrays.

Reading a command into an array

readarray -t lines < ~/.zshrc        # bash 4+
lines=("${(@f)$(< ~/.zshrc)}")             # zsh
files=(*.txt)                              # glob

Slicing

echo "${files[@]:1:2}"   # 2 elements starting at index 1
echo "${files[@]: -1}"   # last element (note the space)

An array is not a space-joined string

Arrays preserve each filename or user value as a separate item. Concatenating items="$a $b" loses whether spaces belonged to data. Store values separately and pass "${items[@]}".

Crossing a shell boundary requires serialization

You cannot export a Bash array intact as an environment variable. Pass multiple arguments or serialize with an explicit line, NUL, or JSON contract. Similar array syntax does not imply the same data protocol.