Process · 2 steps

A process is stuck

Read the process state, find what it is blocked on, and know which states cannot be killed and why.

Every step

Read the process state

The state letter tells you immediately whether the process can be signalled at all. Sending kill -9 to a D-state process does nothing, and knowing that saves an hour of confusion.

$ ps -o pid,stat,wchan:30,comm,etime -p <PID>

$ cat /proc/<PID>/status | grep -E "State|SigPnd|SigBlk"

Look for: The STAT letter: R running, S interruptible sleep, D uninterruptible sleep, Z zombie, T stopped. WCHAN names the kernel function it is parked in.

Find what it is waiting for

A process that is alive but idle is nearly always waiting on something outside itself: a lock, a socket that will never answer, or a child that never exits.

$ strace -p <PID> -f -tt # ctrl-c after a few seconds

$ cat /proc/<PID>/stack # kernel stack, needs root

$ ls -l /proc/<PID>/fd | tail -20

$ cat /proc/<PID>/wchan; echo

Look for: The syscall it is parked in. `read` on a socket fd, `futex`, `flock`, and `wait4` each point somewhere completely different.