Master Linux Process Management: From Programs to Commands and Scheduling

What Is a Program

A program is a set of instructions that a computer can recognize and execute to perform a specific task or solve a problem, typically composed of code, data, and resource files and compiled into a binary.

What Is a Process

A process is an independent execution instance of a program that operates on a specific data set; it is the basic unit for resource allocation and scheduling in an operating system.

Relationship Between Program and Process

The following diagram illustrates how a program becomes a process when it is loaded and executed.

Process States

The diagram below shows the lifecycle of a process.

Basic states:

Created: The process requests a blank PCB (process control block) and allocates resources; if allocation fails, it cannot be scheduled.

Ready: All required resources are allocated and the process can run as soon as it gets CPU time.

Running: The process is executing after being scheduled from the ready state.

Blocked: The process is temporarily unable to run due to I/O requests or other events; it returns to ready when the event is resolved.

Terminated: The process ends normally, crashes, or is killed by the system.

Additional states:

Running (running)

Ready (ready)

Sleep (interruptible / uninterruptible)

Stopped (stopped, can be resumed manually)

Zombie (zombie, remains until the parent process reaps it)

How to Remove a Zombie Process

Example of creating a zombie process and then using top or ps to locate it (state Z) and its PID.

Terminate a zombie by sending a signal to its parent process: kill -18 6904 Alternatively, killing the zombie’s parent process works but is not recommended.

Process Types

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

Foreground: Processes started from a terminal and associated with it.

Process Priority

Static priority range: 100‑139.

Dynamic priority range: -20 to 19.

Process Management Commands

ps

Displays static process statistics. Process information is stored in the /proc directory. Without options, ps shows limited data; the common usage is ps aux.

Key fields:

USER : Process owner.

PID : Process ID.

%CPU : CPU usage percentage.

%MEM : Memory usage percentage.

VSZ : Virtual memory size (KB).

RSS : Resident set size, physical memory used (KB).

STAT : Process state.

START : Start time.

TIME+ : Cumulative CPU time.

COMMAND : Command name.

top

Shows real‑time process statistics.

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

Field explanations (us, sy, ni, id, wa, hi, si, st) are provided in the article.

pgrep

Searches for processes matching criteria.

Common options: -u: Specify user. -l: Show process name. -a: Show full command line.

# pgrep -u asdjkl
9828
9834
...

pstree

Displays processes in a tree structure.

Useful options: -a: Show full command line. -p: Show PIDs. -u: Show user IDs. -H pid: Highlight a specific PID and its ancestors.

systemd─┬─ModemManager───{ModemManager}
   └─NetworkManager───{NetworkManager}

free

Shows memory usage details.

Mem: 1867048 total 1247944 used 119912 free 13656 shared 499192 buff/cache 358276 available

iostat

Provides detailed 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

Monitors real‑time I/O usage, including network traffic.

Manual Process Control

Run a command in the background with command &. Use Ctrl+Z to suspend a running job and then bg to resume it in the background. List background jobs with jobs, bring a job to the foreground with fg, and run multiple commands in parallel by separating them with &.

Ending Processes

kill

The kill command sends a signal to a process; the signal type is user‑specified. Use trap -l to list available signals.

1) SIGHUP   2) SIGINT   3) SIGQUIT   4) SIGILL   5) SIGTRAP
6) SIGABRT  7) SIGBUS   8) SIGFPE    9) SIGKILL 10) SIGUSR1
...

Scheduled Tasks

One‑time tasks (at)

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

Recurring tasks (crontab)

Common options: -u user: Operate on a specific user’s crontab. -e: Edit the crontab. -l: List the crontab. -r: Remove the crontab. -i: Prompt before removal.

Crontab time fields: minute hour day month weekday.