Process Management

What is a Process?

A process is a running instance of a program. Each process has a unique process ID (PID) and occupies certain system resources (CPU, memory, etc.).

Process Related Concepts

Process States

State	Symbol	Description
Running	R	Running or in run queue
Sleeping	S	Interruptible sleep, waiting for events
Uninterruptible Sleep	D	Uninterruptible sleep, usually waiting for I/O
Stopped	T	Stopped by signal
Zombie	Z	Terminated but not yet collected by parent process

Process Types

• Foreground process: Occupies terminal until it completes
• Background process: Runs in background without occupying terminal
• Daemon process: System service that runs in background for long time

Viewing Processes

ps - Process Snapshot

# Current terminal processes
$ ps
  PID TTY          TIME CMD
 1234 pts/0    00:00:00 bash
  5678 pts/0    00:00:00 ps

# All processes (standard syntax)
$ ps -ef
UID        PID  PPID  C STIME TTY          TIME CMD
root         1     0  0 Jan09 ?        00:00:01 /sbin/init
root         2     0  0 Jan09 ?        00:00:00 [kthreadd]
maxwell   1234  1233  0 10:00 pts/0    00:00:00 bash

# All processes (BSD syntax)
$ ps aux
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root         1  0.0  0.1 169512 11284 ?        Ss   Jan09   0:01 /sbin/init
maxwell   1234  0.0  0.0  21388  5364 pts/0    Ss   10:00   0:00 bash

# Display process tree
$ ps -ef --forest
$ ps auxf

# Specify display columns
$ ps -eo pid,ppid,user,%cpu,%mem,stat,cmd

# Sort by CPU usage
$ ps aux --sort=-%cpu | head

# Sort by memory usage
$ ps aux --sort=-%mem | head

# Find specific process
$ ps aux | grep nginx
$ ps -ef | grep nginx

ps Output Explanation

Column	Description
PID	Process ID
PPID	Parent process ID
USER	Process owner
%CPU	CPU usage
%MEM	Memory usage
VSZ	Virtual memory size
RSS	Actual memory size
TTY	Terminal
STAT	Process state
START	Start time
TIME	CPU time
COMMAND	Command

top - Dynamic Process Monitoring

$ top
top - 10:30:00 up 5 days,  3:00,  2 users, load average: 0.15, 0.10, 0.05
Tasks: 200 total,   1 running, 199 sleeping,   0 stopped,   0 zombie
%Cpu(s):  2.0 us,  1.0 sy,  0.0 ni, 96.5 id,  0.5 wa,  0.0 hi,  0.0 si
MiB Mem :  16000.0 total,   8000.0 free,   4000.0 used,   4000.0 buff/cache
MiB Swap:   2000.0 total,   2000.0 free,      0.0 used. 11000.0 avail Mem

  PID USER      PR  NI    VIRT    RES    SHR S  %CPU  %MEM     TIME+ COMMAND
 1234 maxwell   20   0  500000  50000  30000 S   5.0   0.3   0:10.50 firefox
  5678 root      20   0  100000  10000   5000 S   2.0   0.1   0:05.00 systemd

top Interactive Commands

Key	Function
q	Quit
h	Help
P	Sort by CPU
M	Sort by memory
N	Sort by PID
T	Sort by runtime
k	Kill process
r	Change priority
u	Show specific user
1	Show each CPU
c	Show full command
s	Modify refresh interval

htop - Enhanced top

# Install
$ sudo apt install htop

# Run
$ htop

Features:

• Color display
• Mouse support
• Vertical and horizontal scrolling
• Tree view
• Direct search and filtering

pgrep - Find Process by Name

# Find PID by name
$ pgrep nginx
1234
5678

# Show process name
$ pgrep -l nginx
1234 nginx
5678 nginx

# Full command match
$ pgrep -f "python script.py"

# Find by user
$ pgrep -u maxwell

# Newest process
$ pgrep -n nginx

# Oldest process
$ pgrep -o nginx

pidof - Get PID

$ pidof nginx
1234 5678

Controlling Processes

kill - Send Signals

# Default send SIGTERM (15)
$ kill PID
$ kill 1234

# Force terminate SIGKILL (9)
$ kill -9 PID
$ kill -KILL PID

# Reload configuration SIGHUP (1)
$ kill -1 PID
$ kill -HUP PID

# Send to multiple processes
$ kill 1234 5678 9012

Common Signals

Signal	Number	Description
SIGHUP	1	Hang up, commonly used to reload configuration
SIGINT	2	Interrupt (Ctrl+C)
SIGQUIT	3	Quit (Ctrl+)
SIGKILL	9	Force terminate, cannot be caught
SIGTERM	15	Terminate, default signal
SIGSTOP	19	Stop process
SIGCONT	18	Continue running

# List all signals
$ kill -l

killall - Kill by Name

# Terminate all processes with same name
$ killall nginx

# Force terminate
$ killall -9 nginx

# Interactive confirmation
$ killall -i nginx

# Kill by user
$ killall -u maxwell

pkill - Kill by Pattern

# Kill by name
$ pkill nginx

# Kill by full command
$ pkill -f "python script.py"

# Kill by user
$ pkill -u maxwell

# Send specific signal
$ pkill -HUP nginx

Foreground/Background Control

Running in Background

# Run command in background
$ command &
$ sleep 100 &
[1] 1234

# View background jobs
$ jobs
[1]+  Running                 sleep 100 &

# Put command into background running
$ command
Ctrl+Z        # Pause
$ bg          # Continue in background

Running in Foreground

# Bring background job to foreground
$ fg
$ fg %1       # Specify job number

# View jobs
$ jobs
[1]-  Running                 sleep 100 &
[2]+  Stopped                 vim file.txt

nohup - No-hangup Running

# Run command, continue running even if terminal closes
$ nohup command &
$ nohup ./script.sh &

# Output redirection
$ nohup command > output.log 2>&1 &

# View output
$ cat nohup.out

disown - Disassociate from Terminal

# Run in background
$ command &

# Disassociate
$ disown

# Or directly
$ disown %1

Process Priority

nice - Set Priority at Startup

# nice value range: -20 (highest priority) to 19 (lowest priority)
# Default value is 0

# Run with lower priority
$ nice -n 10 command

# Run with higher priority (requires root)
$ sudo nice -n -10 command

# Lowest priority
$ nice -n 19 command

renice - Modify Running Process Priority

# Modify process priority
$ renice 10 -p 1234

# Modify all user's processes
$ renice 5 -u maxwell

# Modify group's processes
$ renice 5 -g groupname

# Raise priority (requires root)
$ sudo renice -10 -p 1234

Monitoring System Resources

free - Memory Usage

$ free -h
              total        used        free      shared  buff/cache   available
Mem:           16Gi       4.0Gi       8.0Gi       500Mi       4.0Gi        11Gi
Swap:         2.0Gi          0B       2.0Gi          0B. 11000.0Gi avail Mem

# Continuous monitoring
$ watch free -h

uptime - System Load

$ uptime
 10:30:00 up 5 days,  3:00,  2 users, load average: 0.15, 0.10, 0.05

vmstat - Virtual Memory Statistics

$ vmstat
procs -----------memory---------- ---swap-- -----io---- -system-- ------cpu-----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa st
 1  0      0 8000000 200000 4000000  0    0     5    10   50 100  2  1 97  0  0  0  0

# Refresh every 2 seconds
$ vmstat 2

iostat - I/O Statistics

# Install sysstat
$ sudo apt install sysstat

$ iostat
Linux 5.15.0-generic    01/09/2025  _x86_64_    (4 CPU)

avg-cpu:  %user   %nice %system %iowait  %steal   %idle
            2.00    0.00    1.00    0.50    0.00   96.50

Device             tps    kB_read/s    kB_wrtn/s    kB_read    kB_wrtn
sda              10.00        50.00       100.00    1000000    2000000

/proc Filesystem

# View process information
$ cat /proc/1234/status
Name:   bash
State:  S (sleeping)
Pid:    1234
PPid:   1233
...

# View command line
$ cat /proc/1234/cmdline

# View environment variables
$ cat /proc/1234/environ

# View open files
$ ls -l /proc/1234/fd

# View memory mapping
$ cat /proc/1234/maps

# System information
$ cat /proc/cpuinfo
$ cat /proc/meminfo
$ cat /proc/version

Practical Tips

Find Resource-Hogging Processes

# Top 10 CPU-using processes
$ ps aux --sort=-%cpu | head -11

# Top 10 memory-using processes
$ ps aux --sort=-%mem | head -11

Find Zombie Processes

$ ps aux | grep Z
$ ps aux | awk '$8=="Z"'

Monitor Specific Process

# Use watch
$ watch -n 1 "ps -p 1234 -o pid,ppid,%cpu,%mem,cmd"

# Use top
$ top -p 1234

View Process Open Files

$ lsof -p 1234
$ ls -l /proc/1234/fd

View Port Usage

$ lsof -i :80
$ ss -tlnp | grep :80
$ netstat -tlnp | grep :80

Summary

This chapter introduced Linux process management:

• Viewing processes: ps, top, htop, pgrep
• Controlling processes: kill, killall, pkill
• Foreground/background control: &, fg, bg, jobs, nohup
• Priority: nice, renice
• System monitoring: free, vmstat, iostat

Mastering process management is a foundational skill for system administration and troubleshooting.

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