awk-based mktemp(1) implementation (no guarantees!)

mktemp.awk

#!/usr/bin/awk -f

function count_x(tmpl) {
	match(tmpl, "X+$")
	return RLENGTH
}

function replace_x(tmpl, tok) {
	# ATTENTION: this function destroys the template if the tok contains more
	# characters than the tmpl contains Xs
	return substr(tmpl, 1, length(tmpl) - length(tok)) tok
}

function random_tok(len,    _cmd, _ln) {
	_cmd = "LC_ALL=C tr -cd 'A-Za-z0-9' </dev/urandom | dd bs=1 count=" int(len) " 2>/dev/null"
	_cmd | getline _ln
	close(_cmd)
	return _ln
}

function creat_excl(path) {
	gsub(/'/, "'\\''", path)
	return !system("exec >/dev/null 2>&1 && umask 077 && set -C && :>'" path "'")
}

BEGIN {
	tmpl = ARGV[1]
	num_x = count_x(tmpl)

	while (1) {
		tok = random_tok(num_x)
		if (length(tok) != num_x) continue  # assert length is okay
		fname = replace_x(tmpl, tok)
		if (creat_excl(fname)) break
	}
	print fname
}