TopolearnArrays are the only safe way to hold lists of file names or arguments that may contain spaces.
An indexed array is created with arr=(a "b c" d) and grows with arr+=(e). Index from 0: ${arr[1]}. The essential expansions:
"${arr[@]}"every element as a separate word (always quote it),${#arr[@]}number of elements,${!arr[@]}the indices,"${arr[@]:1:2}"a slice.
Arrays can be sparse: after unset 'arr[0]' the indices are 1 2 3, so loop over elements or over ${!arr[@]} rather than assuming 0..n-1.
Associative arrays (bash 4+) must be declared: declare -A port=([http]=80 [https]=443). Set with port[ssh]=22, read with ${port[ssh]}, list keys with "${!port[@]}". Key order is not defined, so sort if you need stable output. Test for a key with [[ -v port[ssh] ]].
Arrays shine for building command lines: args=(--verbose); [[ $dry ]] && args+=(--dry-run); rsync "${args[@]}" src/ dst/ passes each option as exactly one argument, which string concatenation can not do safely.
To read lines of a file or command into an array use mapfile -t lines < file (also called readarray). Bash arrays are one-dimensional and cannot be exported to child processes.
Example
cd "$(mktemp -d)"
fruits=(apple "star fruit" kiwi)
fruits+=(mango)
echo "${fruits[1]}" # star fruit
echo "${#fruits[@]}" # 4
for f in "${fruits[@]}"; do printf '<%s>' "$f"; done; echo
# <apple><star fruit><kiwi><mango>
echo "${fruits[@]:1:2}" # star fruit kiwi
unset 'fruits[0]'
echo "${!fruits[@]}" # 1 2 3 (sparse!)
declare -A port=([http]=80 [https]=443)
port[ssh]=22
echo "${port[https]}" # 443
[[ -v port[ftp] ]] || echo "no ftp" # no ftp
for k in "${!port[@]}"; do echo "$k=${port[$k]}"; done # order not guaranteed
printf 'x\ny\n' > two.txt
mapfile -t lines < two.txt
echo "${#lines[@]} lines, first=${lines[0]}" # 2 lines, first=xRemember
- "${arr[@]}" expands each element as one word; always quote it
- declare -A for key/value maps; keys come back in no particular order
- Build command lines in arrays, then run "${cmd[@]}"
Watch out for
- Writing ${arr} or $arr, which gives only the first element.