1. What is a Process in UNIX?

• A process is a program in execution.

• When you run a command (e.g., ls, cat file.txt, or even a script), UNIX creates a process for it.

• Each process is an independent entity with:

  • Its own process ID (PID)

  • Its own memory space (code, data, stack)

  • Its own execution state (running, waiting, stopped, terminated)

2. Process States in UNIX

A process can be in different states:

1. Running – currently executing on CPU.

2. Ready – waiting in the queue to use CPU.

3. Sleeping (Waiting) – waiting for an event (like I/O).

4. Stopped – suspended (e.g., by Ctrl+Z).

5. Zombie – finished execution, but parent hasn’t collected exit status.

Example: Check process states with ps -aux:

ps -aux | head -5

Output:

USER   PID  %CPU %MEM  STAT COMMAND
root     1   0.0  0.1  Ss   /sbin/init
user   123   0.1  0.3  S    bash
user   456   1.0  2.1  R    firefox
user   789   0.0  0.0  Z    [vim] <defunct>

Here, S = sleeping, R = running, Z = zombie.

3. Process Hierarchy in UNIX

• UNIX processes form a tree structure.

• The first process is init (or systemd in modern Linux), PID = 1.

• All other processes are children of init.

• Each process has:

  • PID – unique identifier

  • PPID – parent process ID

Example:

ps -ef | head -5

UID   PID  PPID CMD
root    1     0  /sbin/init
user  234     1  bash
user  567   234  vim file.txt

Here, bash is a child of init, and vim is a child of bash.

4. Creating and Managing Processes

(a) Process Creation

• fork() system call creates a new process (child).

• Child is an almost exact copy of the parent.

(b) Executing Programs

• exec() replaces the process image with a new program.

(c) Termination

• exit() ends a process.

• Parent collects child’s exit status with wait().

5. Foreground and Background Processes

• By default, a process runs in the foreground (blocking the shell).

• Add & to run in background:

sleep 60 &

This runs sleep in the background.
Check running jobs:

jobs

Bring a background job to foreground:

fg %1

6. Important Process Commands

7. Example: Process Management

# Run program in background
gedit &

# Find its PID
ps -ef | grep gedit

# Kill the process
kill -9 <PID>

8. Diagram – Process in UNIX

                 +--------------------+
                 |   init (PID 1)     |
                 +--------------------+
                         |
        ------------------------------------------
        |                        |                |
  +-------------+         +-------------+   +-------------+
  |  bash (PID) |         | sshd (PID)  |   |  cron (PID) |
  +-------------+         +-------------+   +-------------+
        |
  +-------------+
  |  vim (PID)  |
  +-------------+

Summary

• A process is a running program with its own PID, memory, and state.

• UNIX processes are organized in a tree, starting from init/systemd.

• Key actions: creation (fork), execution (exec), termination (exit), waiting (wait).

• Processes can run in foreground or background.

• Tools: ps, top, jobs, kill, nice, pstree.