/usr/lib/radare/bin/waiter is in radare-common 1:1.5.2-4.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 | #!/bin/sh
#
# Author: pancake <youterm.com>
#
if [ -z "$RADARE_WAITER_CMD" ]; then
	RADARE_WAITER_CMD="radare -d"
fi
if [ -z "$1" ]; then
	echo "Usage: rsc waiter <pid>|<process-name> <sleep>"
	echo " Waits for a child of pid or process-name created"
	echo "RADARE_WAITER_CMD=\"radare -d\""
	exit
fi
pid="$1"
sleep="$2"
[ -z "$pid" ] && pid=0
[ -z "$sleep" ] && sleep=0
isnum=$(($pid+1))
if [ $isnum = 1 ]; then
	# process name
	while : ; do
		for a in /proc/*/stat ; do
			if [ -f $a ]; then
				target=`cut -d ' ' -f 1 $a`
				name=`cut -d ' ' -f 2 $a | tr '()' '  ' | sed -e 's, ,,g'`
				if [ "$name" = "$pid" ]; then
					echo "Attaching to $target..."
					$RADARE_WAITER_CMD $target
					exit 0
				fi
			fi
		done
		sleep $sleep
	done
else
	# child of pid
	while : ; do
		for a in /proc/*/stat ; do
			if [ -f $a ]; then
				target=`cut -d ' ' -f 1 $a`
				ppid=`cut -d ' ' -f 4 $a`
				if [ "$ppid" = "$pid" ]; then
					echo "Attaching to $target..."
					$RADARE_WAITER_CMD $target
					exit 0
				fi
			fi
		done
		sleep $sleep
	done
fi
 |