queue

#!/bin/sh
# Queue commands to the existing list
# Useful to copy long list of files, like
# queue cp /tmp/ya /mnt/
# queue cp /tmp/yi /mnt/
# queue cp /mnt/ye /mnt/
set -eu

PIDFILE=~/.queue.pid

#this waits for any PIDs to finish
anywait(){
    for pid in "$@"; do
        while kill -0 "$pid" 2&>1 >/dev/null; do
            sleep 1
        done
    done
}

#read previous instances PID from PIDFILE and write own PID to PIDFILE
updatequeue() {
	OLDPID=$(<$PIDFILE)
	echo $$>$PIDFILE
}

#afterwards: cleanup (if pidfile still contains own PID, truncate it)
clean() {
	if [ $(<$PIDFILE) == $$ ]; then
	truncate -s0 $PIDFILE
	fi
}
#open PIDFILE and aquire lock
exec 9>>$PIDFILE
lockf -t 2 updatequeue || { echo "ERROR: lockf() failed to readpid()." >&2; exit 1; }

#wait for OLDPID
anywait $OLDPID

#do stuff
"$@"

lockf -t 2 clean || { echo "ERROR: flock() failed." >&2; exit 1; }