bashuf.sh

#!/usr/bin/env bash
set -o errexit

# A native bash function to mimic the behaviour of shuf(1).
#
# - From the failed experiments department.
# - Unusably slow on long files with thousands of lines.
#
# "Arrays are sparse doubly-linked lists."
#   https://git.savannah.gnu.org/cgit/bash.git/tree/array.c

function bashuf() {
	if [[ ! -v SRANDOM ]]
	then
		echo "This bash function requires SRANDOM support. (Bash 5.1)"
		exit 1
	fi

	mapfile -t lines
	for (( todo=${#lines[@]}; todo > 0; todo-- )) 
	do
		rand=$(( SRANDOM % todo ))
		printf "%s\n" "${lines[${rand}]}"
		# Unless rand gave us the last of the remaining lines,
		# swap the last line to where we just read the
		# random line from:
		if [[ ${rand} -ne ${todo} ]]
		then
			lines[${rand}]="${lines[$((todo-1))]}"
		fi
	done	
}

bashuf << Here
1111
2222
3333
4444
Here