mapfile

Additional Notes

mapfile (also called readarray) is a Bash built-in that reads lines from standard input into an indexed array. Each line of input becomes one element of the array, stripping the trailing newline.

It is useful for reading file contents or command output into an array for further processing in scripts. It is more efficient and cleaner than using a loop with read.

Syntax

mapfile [options] array_name
mapfile -t array_name < file

Parameters

• array_name: The name of the array variable to populate.

Common Options

• -t: Remove trailing newlines from each line (most commonly used).
• -n count: Read at most count lines.
• -O origin: Start filling the array at index origin (default 0).
• -s count: Skip the first count lines.
• -C callback: Evaluate callback every -c lines.
• -c count: Specify the callback frequency (default is 5000).
• -u fd: Read from file descriptor fd instead of stdin.

Examples

mapfile -t lines < /etc/passwd
echo "${lines[0]}"

Read /etc/passwd into an array and print the first line.

mapfile -t -s 1 lines < /etc/fstab

Skip the first line (header comment) and read the rest.

mapfile -t -n 3 lines < /etc/hosts

Read only the first 3 lines.

ls -1 *.txt | mapfile -t files
for file in "${files[@]}"; do
  echo "Processing: $file"
done

Read ls output into an array and iterate.

mapfile -t -C 'echo "Read line"' -c 1 lines < data.txt

Execute a callback after each line read.

Practical Notes

• readarray is a synonym for mapfile; both work identically.
• Always use -t to strip trailing newlines unless you need them.
• mapfile is a Bash 4+ feature. It is not available in POSIX sh or older Bash versions.
• For large files, mapfile is more memory-efficient than reading into a variable with $(cat).