Zombie process

A zombie process refers to a child process that has completed its execution but still has an entry in the system’s process table. This happens because the parent process has not performed the necessary system call (wait()) to retrieve the child's exit status. Zombies don't use system resources beyond their process table entry, but accumulating a large number of them can exhaust the system’s available process IDs. Essentially, zombie processes indicate that a program isn’t correctly managing its child processes, typically due to poor programming practices or unexpected termination of parent processes.

Zombie processes cannot be killed using traditional signals (kill, kill -9) because they're already terminated. To remove them, the parent process must execute a wait() system call to read their exit status, thereby fully cleaning up the child’s entry in the process table. If the parent process itself has stopped functioning, killing or restarting the parent will allow the Linux init or systemd process (PID 1) to adopt the zombies and automatically clear them. Alternatively, rebooting the Linux system completely clears all zombie processes, though this method is less practical on production systems. Properly designing applications to handle child processes correctly using wait() is the most effective long-term solution.