php+bash script

hello,
can anyone give me any hints about running proccesses in linux from php leve
l.
i'd like php script run bash-script witch search some data throught google.c
om
by opening links session. i want to allow only one proccess with bash script
running. if one user already started the bash-script another have to wait un
til
proccess is done.

greetings,
plx