Master Linux Process Management: ps, Signals, Priorities & Job Control

1. The ps Command

Processes are programs running on your system, each identified by a unique Process ID (PID) assigned sequentially by the kernel. Running ps displays a snapshot of active processes:

ubuntu@ubuntu:~$ ps
  PID TTY          TIME CMD
 3309 pts/1    00:00:00 bash
 3794 pts/1    00:00:00 ps

PID – Process ID

TTY – Controlling terminal

TIME – Cumulative CPU time

CMD – Executable name

The manual page lists many options; a common usage is ps aux, which shows detailed columns such as USER, %CPU, %MEM, VSZ, RSS, STAT, START, TIME, and COMMAND.

2. Process Details

A process is an instance of a program that the kernel loads into memory, allocates CPU and I/O resources, and monitors. For example, running ps aux | grep cat reveals two separate cat processes even though they invoke the same binary.

3. Process Creation

New processes are created with the fork() system call, which clones the calling process and assigns a new PID. The child can continue executing the same program or replace its image with execve(). The original process becomes the parent, identified by PPID.

Using ps l shows a long format that includes the PPID column.

4. Process Termination

Processes exit via the _exit system call, returning a status code (0 for success). The parent must call wait() to reap the child’s exit status. Unreaped children become zombie processes , while children whose parents die first become orphan processes and are adopted by the init process (PID 1).

5. Signals

Signals are asynchronous notifications sent to a process to indicate events such as user interrupts, hardware faults, or inter‑process communication. Common signals include:

SIGHUP (1) – Hangup

SIGINT (2) – Interrupt (Ctrl‑C)

SIGKILL (9) – Forceful kill (cannot be caught)

SIGTERM (15) – Graceful termination

SIGSTOP – Stop execution

Processes can ignore, catch, block, or terminate in response to signals.

6. The kill Command

Use kill PID to send the default SIGTERM to a process, or specify a signal explicitly, e.g., kill -9 12345 to send SIGKILL.

7. Process Priority

CPU time is allocated in time slices. The scheduler can be nudged using nice (to start a process with a lower priority) and renice (to change the priority of an existing process):

nice -n 5 apt upgrade
renice 10 -p 3245

Lower nice values mean higher priority; negative values increase priority further.

8. Process States

The STAT column of ps aux shows process states:

R – Running or runnable

S – Interruptible sleep

D – Uninterruptible sleep (usually I/O)

Z – Zombie (awaiting parent reap)

T – Stopped (e.g., by SIGSTOP)

9. The /proc Filesystem

Linux represents processes as directories under /proc, each named by its PID. Inside, files such as status, cmdline, and fd provide detailed information beyond what ps shows.

10. Job Control

Appending & to a command runs it in the background. Use jobs to list background jobs, fg %1 to bring a job to the foreground, and bg %1 to resume a stopped job in the background.