Skip to main content
Every running program on a Linux system is a process — an instance of an executable with its own memory space, file descriptors, and system resources. The kernel assigns each process a unique Process ID (PID) and tracks its state, priority, CPU usage, and memory consumption. Knowing how to list, inspect, and control processes lets you diagnose runaway applications, gracefully restart services, and tune system performance without rebooting.

​
Viewing Processes

Use ps to take a snapshot of running processes at a given moment. 
ps aux                        # all processes with details
ps -ef                        # full-format listing
ps aux | grep nginx           # filter by name
pgrep -l nginx                # list matching processes

USER       PID  %CPU  %MEM    VSZ   RSS  STAT  COMMAND
root         1   0.0   0.1  22560  4096  Ss    /sbin/init
alice     1234   1.2   0.8 512000 32000  Sl    /usr/bin/python3 app.py

​
Interactive Monitoring

For a live, updating view of system activity, use top or the more feature-rich htop. 
top                           # real-time process monitor
htop                          # enhanced interactive monitor (if installed)
While inside top, use these keyboard shortcuts to interact with it:
  • q — quit
  • k — kill a process (prompts for PID and signal)
  • r — renice a process (change its priority)
  • 1 — toggle a per-CPU breakdown at the top
  • M — sort by memory usage
  • P — sort by CPU usage

​
Sending Signals

Linux communicates with processes through signals. Use kill, pkill, or killall to send them. 
kill 1234                     # SIGTERM (graceful stop)
kill -9 1234                  # SIGKILL (force stop)
kill -HUP 1234                # SIGHUP (reload config)
pkill nginx                   # kill by name
killall nginx                 # kill all matching
SignalNumberMeaning
SIGTERM15Graceful termination (default for kill)
SIGKILL9Immediate, unblockable termination
SIGHUP1Reload configuration (many daemons support this)
SIGINT2Interrupt (same as pressing Ctrl+C)
SIGSTOP19Pause/suspend a process
SIGCONT18Resume a paused process
kill -9 (SIGKILL) does not allow the process to clean up, flush buffers, or release locks. Only use it as a last resort after SIGTERM has failed.

​
Background and Foreground Jobs

Run tasks in the background or move them between the foreground and background without opening a new terminal session. 
command &                     # run in background
jobs                          # list background jobs
fg %1                         # bring job 1 to foreground
bg %1                         # resume job 1 in background
nohup command &               # persist after logout
1

Start a job in the background

Append & to any command to run it without blocking your shell prompt.
2

Check running jobs

Run jobs to list all background and suspended jobs in the current shell session, along with their job numbers.
3

Move jobs between foreground and background

Press Ctrl+Z to suspend the current foreground process, then use bg %N to resume it in the background or fg %N to bring it back to the foreground.
4

Persist jobs after logout

Prefix your command with nohup (and append &) so it keeps running even after you close the terminal or disconnect an SSH session.

​
Process Priority

Linux schedules processes using a priority value called “niceness,” ranging from -20 (highest priority) to 19 (lowest priority). The default niceness is 0. 
nice -n 10 command            # start with lower priority
renice 5 -p 1234              # change running process priority
For processes managed by systemd (web servers, databases, cron jobs, etc.), use systemctl commands instead of kill and nice. Commands like sudo systemctl restart nginx and sudo systemctl status postgresql give you structured control and logging through the journal.