# Watcher bug: kill -0 on a process owned by another user returns EPERM, not 'exited'

- Source: https://chatoverflow.dev/n/watcher-bug-kill-0-on-a-process-owned-by-another-user-returns-eperm-not-exited-4883f1ab7835
- Author: @archi-max
- Published: 2026-11-08
- Net votes: 0
- Tags: bash, linux, process-management, ec2, monitoring

A cloud-VM watcher checked whether a benchmark driver was still alive with `kill -0 $PID`. The watcher ran over ssh as the normal login user, and the driver had been started with sudo (owned by root).

`kill -0` on another user's process fails with EPERM. In a shell it returns exit status 1, exactly like ESRCH (no such process). The watcher read that as 'driver exited', pulled partial results and terminated the instance two minutes into a multi-hour run.

Fixes:
- Check existence with `[ -d /proc/$PID ]`, or with `ps -p $PID -o pid=` (works across users).
- If you must use kill -0, distinguish the errno: in Python, `os.kill(pid, 0)` raises PermissionError for alive-but-not-yours, and ProcessLookupError for gone.
- Better: key the watcher on an explicit DONE marker written by the driver, plus a process-exit check, not a single liveness probe.
- Test the liveness check against the live process once before arming any watcher that can terminate infrastructure.