9/27    Say I want to kill processes older than x hours. If I make a list of PIDs,
        check the start time, and kill on that basis, there is a small
        possibility that that process will terminate after I've done the check
        but before I do the kill, and that another (possibly important) process
        will take on that PID, and I would thus end up killing this important
        process. Is there any way to avoid this race condition without going down
        to kernel level? Thanks.
        \- sure, lots of ways. check ppid, argv string, start time of
           process, etc. the rightway.easiest way probably depends on
           some details of how you are trying to do this but certainly
           you can with ps | awk/grep | (xargs|) kill. --psb
           \_ well, that mitigates it somewhat further, but does not remove the
              essential race condition.
              \- yes it does, if you do it correctly. or do you think
                 something like PAWS isnt good enough either. --psb
                 \_ no, it doesn't.  it's still a race condition.  pipes are
                    not atomic.  you should know better.  there's still a slim
                    chance that between the ps and the kill, the proc exits and
                    another starts.  -!op
              \_ If you check the start time like the first reply suggests, it
                 should eliminate the problem.
                 \_ yes, but what if time wraps around too? :9
                 \_ No, it's not atomic.  Don't they teach Computer Science
                    anymore?  I haven't done real CS in years yet I still know
                    a race condition when I see one.
        \_ There is a race condition, but it doesn't matter.  Running kill on
           a nonexistant PID doesn't hurt anything, and if you've really got a
           system that can wrap around 100k PIDs in the time it takes to run
           your ps | ... | kill, you've got one whacked machine.