Master Linux Process Management: From Basics to Advanced Commands

What is a program

A set of instructions that a computer can recognize and execute, guiding it to perform specific tasks or solve problems. Programs typically consist of code, data, and resource files, involving syntax, algorithms, and data structures, and are compiled into binary files.

What is a process

A process is an independent execution instance of a program that operates on a data set. It is the basic unit of resource allocation and scheduling in an operating system, forming the foundation of OS structure.

Relationship between program and process

Process states

Basic process states

Created: the process requests a blank PCB (process control block) and fills control information; if resources are insufficient, it cannot be scheduled.

Ready: the process has all required resources and can run as soon as the CPU is allocated.

Running: the process moves to execution after being scheduled from the ready state.

Blocked: the process temporarily cannot run due to I/O requests or resource waits; it returns to ready when the condition is satisfied.

Terminated: the process ends due to completion, error, or system termination and cannot run further.

Additional states

Running (running)

Ready (ready)

Sleep: interruptible and uninterruptible

Stopped: paused in memory, not scheduled unless manually started

Zombie: terminated process whose parent hasn't waited; can be cleared by killing the parent.

How to eliminate zombie state

Creating a zombie process

Use top or ps to view zombie (Z) PID

Send a kill signal to wake the zombie's parent process and recycle the zombie

# kill -18 6904

Alternatively, kill the zombie's parent process, which is not recommended.

Process types

Daemon: a background service started during system boot, unrelated to any terminal.

Foreground: a process associated with a terminal, started from the terminal.

Process priority

Static priority: 100‑139

Dynamic priority: -20 to 19

Process management commands

ps

View static process statistics; information is stored in the /proc directory. Without options, only limited information is shown.

# ps
PID TTY TIME CMD
6950 pts/2 00:00:00 bash
7441 pts/2 00:00:00 ps

Common usage: ps aux Options for ps aux:

a – show all processes

u – display in user‑oriented format

x – show processes without a controlling terminal

k / --sort – sort by attribute (prefix - for descending)

ps output fields

USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND

top

Real‑time view of process statistics.

# top
top - 17:01:45 up 4:08, 4 users, load average: 0.00, 0.01, 0.05
Tasks: 209 total, 1 running, 208 sleeping, 0 stopped, 0 zombie
%Cpu(s): 0.0 us, 0.1 sy, 0.0 ni, 99.9 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
KiB Mem : 1867048 total, 76652 free, 796612 used, 993784 buff/cache
KiB Swap: 2097148 total, 2097004 free, 144 used. 810116 avail Mem
...

Field meanings:

PID – process ID

USER – owning user

PR – priority (lower number = higher priority)

NI – nice value (lower number = higher priority)

VIRT – virtual memory size (KB)

RES – resident (physical) memory size (KB)

SHR – shared memory size (KB)

S – process state

%CPU – CPU usage percentage

%MEM – memory usage percentage

TIME+ – total CPU time used

COMMAND – command name

Commands in top

q – quit

s – change refresh interval

k – kill a specified process

w – write configuration to file

pgrep

Find processes by name or user.

# pgrep -u asdjkl
9828
9834
9841
...

pstree

Display processes in a tree structure.

# pstree -a
systemd --switched-root --system --deserialize 21
  ├─ModemManager
  │   └─[{ModemManager}]
  ├─NetworkManager --no-daemon
  │   └─[{NetworkManager}]
...

free

Show detailed memory usage.

# free
              total   used   free  shared buff/cache available
Mem:        1867048 1247944 119912   13656   499192   358276
Swap:       2097148   6572 2090576

iostat

Provide I/O performance statistics.

Total DISK READ: 0.00 B/s  Total DISK WRITE: 0.00 B/s
Actual DISK READ: 0.00 B/s  Actual DISK WRITE: 0.00 B/s
...

iotop

Monitor network traffic.

Process management

Manual start

Use command & to run a command in the background.

Running jobs: Ctrl+Z sends a job to the background but stops it.

Unstarted jobs: command & List background tasks: jobs Bring a background task to the foreground: fg Continue a stopped task: bg Parallel execution: separate commands with &.

Terminate process

kill

The kill command sends a signal to a process; the signal type is user‑specified.

# trap -l
1) SIGHUP 2) SIGINT 3) SIGQUIT 4) SIGILL 5) SIGTRAP
6) SIGABRT 7) SIGBUS 8) SIGFPE 9) SIGKILL 10) SIGUSR1
11) SIGSEGV 12) SIGUSR2 13) SIGPIPE 14) SIGALRM 15) SIGTERM
16) SIGSTKFLT 17) SIGCHLD 18) SIGCONT 19) SIGSTOP 20) SIGTSTP
21) SIGTTIN 22) SIGTTOU 23) SIGURG 24) SIGXCPU 25) SIGXFSZ
26) SIGVTALRM 27) SIGPROF 28) SIGWINCH 29) SIGIO 30) SIGPWR
31) SIGSYS 34) SIGRTMIN 35) SIGRTMIN+1 ... 64) SIGRTMAX

Scheduled tasks

One‑time tasks

# at 13:50
at> mkdir kk
at> ^D
job 1 at Thu Apr 18 13:50:00 2024

Periodic tasks

Use crontab to schedule recurring jobs.

-u user – set crontab for a specific user

-e – edit crontab

-l – list crontab

-r – remove crontab

-i – confirm before removal

* * * * *
# minute hour day month week