Processes

Quick check before you start: Do you know the difference between a program and a process? If not, read on. If you can explain what a PID is, skip to Job Control.

Practice this topic: Processes skill drill

After this lesson, you will be able to:

• Explain the difference between a program (file on disk) and a process (running instance)
• Use ps and ps aux to view running processes
• Manage background jobs with &, fg, bg, and jobs
• Send signals with Ctrl+C, Ctrl+Z, and kill
• Check exit status with echo $?

Programs vs Processes

A program is a file on disk — a recipe sitting in a cookbook. A process is that recipe being executed — ingredients measured, oven on, timer running.

which ls
# /usr/bin/ls — this is the program (a file)

ls
# running ls creates a process — it gets a PID, uses memory, runs, and exits

Every process has:

You can run the same program multiple times simultaneously. Each run creates a separate process with its own PID.

Viewing Processes

ps — Process Snapshot

ps shows your processes:

ps
#   PID TTY          TIME CMD
#  1234 pts/0    00:00:00 bash
#  1267 pts/0    00:00:00 ps

ps aux shows every process on the system:

ps aux | head -5
# USER       PID %CPU %MEM    VSZ   RSS TTY  STAT START   TIME COMMAND
# root         1  0.0  0.1 169436 11568 ?    Ss   09:00   0:01 /sbin/init
# root         2  0.0  0.0      0     0 ?    S    09:00   0:00 [kthreadd]
# student   1234  0.0  0.0  21476  5192 pts/0 Ss  09:05   0:00 -bash

Key columns: PID (process ID), STAT (state), COMMAND (what is running). Pipe through grep to find specific processes:

ps aux | grep firefox

Job Control

Background and Foreground

Append & to run a command in the background:

sleep 60 &
# [1] 5678

The shell prints the job number [1] and PID 5678, then gives you your prompt back.

Ctrl+C and Ctrl+Z

These two key combinations are fundamentally different:

A suspended process sits in memory doing nothing. Resume it with fg (foreground) or bg (background):

sleep 300          # starts in foreground
# (press Ctrl+Z)
# [1]+  Stopped     sleep 300
fg                 # bring it back to foreground

Signals and kill

The kill command sends a signal to a process by PID:

kill 5678          # sends SIGTERM (polite request to exit)
kill -9 5678       # sends SIGKILL (forced termination, no cleanup)

Common signals:

Always try kill PID first. Use kill -9 only when a process refuses to terminate.

Exit Status

Every process returns an exit status when it finishes — an integer between 0 and 255:

ls /etc/passwd
echo $?
# 0 — success

ls /nonexistent
echo $?
# 2 — failure (file not found)

The convention: 0 means success, anything else means failure. The specific non-zero value sometimes indicates what went wrong. You will use exit status extensively when writing shell scripts and C programs — return 0; in main() follows this same convention.

What Comes Next

You now understand how Unix runs and manages programs. Next, you will write your first C program and learn how gcc compiles source code into an executable.