Zombie process

In Terminal, run ps -axo pid,ppid,stat,comm | awk '$3 ~ /^Z/'. Each line is a zombie, and the PPID column shows the parent process responsible for it.

You cannot kill a zombie, because it has already exited. The fix is the parent. Quit or restart the parent app, and the zombie is handed to launchd, which cleans it up.

Do not quit launchd or other system processes to clear zombies. If the parent is a system process, a restart of the Mac clears them.

In the full command column of ps, a zombie is shown as <defunct>, because its program has already exited. The parent is the process to deal with.

1. Run ps -axo pid,ppid,stat,comm | awk '$3 ~ /^Z/' to list zombies. The Z in the STAT column marks a zombie.
2. Note the number in the PPID column.
3. Run ps -p <PPID> -o pid,user,command to see which program is the parent.
4. In Activity Monitor, choose View > All Processes, Hierarchically to see the same parent and child tree.
5. Deal with the parent. If it is an app you opened, quit and reopen it. If it is a shell or a script in Terminal, close that window or stop the script. If it is a system process, restart the Mac.

A parent program collects a child's exit status with the wait family of system calls. A program that starts child processes but never waits for them leaves zombies behind. That is a bug in the parent, and only its developer can fix it for good.

Seeing a zombie for a moment is normal. It exists between the time a process exits and the time its parent reads its exit status, which is usually very short.

It is worth a look when the same parent keeps many zombies around, or when their number keeps growing over hours. To count them, run ps -axo stat | grep -c '^Z'. To see the process limits on your Mac, run sysctl kern.maxproc kern.maxprocperuid.

These three terms are often mixed up, and each one needs a different fix.

• A zombie has finished running. Only its entry in the process table is left.
• An orphan is still running, but its parent has exited. macOS hands it to launchd, PID 1, which becomes its new parent.
• A hung process is still running but stuck, often shown as Not Responding in Activity Monitor. Unlike a zombie, it can be quit or force quit.

Zombies are harmless in small numbers, so there is no reason to take risks to clear them.

• Do not run kill -9 on the zombie's PID. It has already exited, so the signal has no effect.
• Do not quit launchd or other system processes to clear zombies. A restart does the same thing safely.
• Do not quit the parent without checking what it is. If it is an app with open documents, save your work first.