Mastering Linux Process Management: From Viewing to Scheduling

Process Delay and Periodic Scheduling

Process Concept

Process: an executing instance of a program that has not yet finished.
Program: a file containing executable code.
A process is created from a program and consumes system resources; it is not the same as the program.
Processes can be interactive, batch, or daemon.
The relationship between processes and programs is many‑to‑one.

Viewing Processes

Command syntax: ps [options]
Options:
-A : show all processes (same as -e)
-a : show all user processes
-f : full format listing
-l : long format
-r : only running processes
-u : user‑oriented format (user, CPU, memory, …)
-x : show processes without a controlling terminal
-p : show process with given PID
-t : show processes attached to a given terminal

Viewing Process Inheritance

pstree displays the process hierarchy as a tree.
Usage: pstree [options] [pid|user]
-a : show full inheritance, marking swapped‑out processes with parentheses.
-c : separate duplicate process names with an asterisk.
Specify a pid or user to limit the view.

Terminating Processes

kill [signal] pid
killall [signal] process_name
Use ps -aux to list processes and pstree to view child processes.

Process Scheduling

Delay Scheduling (at)

Purpose: run a program at a specified time.
Syntax: at [-f file] [-m] time
-f : file containing commands to execute
-m : mail the user when the job finishes
Time formats:
  Absolute: MMDDYY or MM/DD/YYYY, or keywords today/tomorrow.
  Relative: now+N[minutes|hours|days|weeks].
Examples:
  at now+1 minutes
  at -f file 17:30+2 days

Periodic Scheduling (crontab)

Purpose: schedule recurring tasks.
Syntax: crontab -u user {-e | -l | -r}
-e : edit the user’s crontab
-l : list the crontab (default is current user)
-r : remove the crontab
crontab -u user file   # load a file as the crontab
Crontab line format: minute hour day month day‑of‑week command
Special symbols:
  * : any value
  , : list of values
  - : range of values
  / : step values
Examples:
  0 17 * * 1-5          # 17:00 Monday‑Friday
  30 8 * * 1,3,5        # 08:30 on Mon, Wed, Fri
  0 8-18/2 * * *        # every 2 hours from 08 to 18
  0 * */3 * *          # every 3 days
Crontab files are stored under /var/spool/cron/ (or /var/spool/scron/root).