Master Linux Process Management and Scheduling: From ps to crontab

Process Delay and Periodic Scheduling

Process Concept

Process: an instance of a program that has started execution but 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 are classified as interactive, batch, or daemon.
The relationship between processes and programs is many‑to‑one.

Viewing Processes

Command syntax: ps [options]
Options:
  -A : display information for all processes (same as -e)
  -a : display information for all user processes
  -f : full format, showing all process details
  -l : long format
  -r : show only running processes
  -u : user‑oriented format (includes user name, CPU and memory usage)
  -x : display processes without a controlling terminal
  -p : display information for the process with the given PID
  -t : display processes attached to the specified terminal

Viewing Process Inheritance

pstree command displays the inheritance relationship of system processes as a tree.
If a user is specified, only processes owned by that user are shown.
Syntax: pstree [options] [pid|user]
Options:
  -a : show the complete inheritance chain; swapped‑out processes are shown in parentheses
  -c : separate duplicate process names (default adds an asterisk *)
  pid|user : specify the root process ID or user whose processes should be displayed

Process Termination

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

Process Scheduling

Delay scheduling

Function: schedule a program to run at a specified time.
Syntax: at [-f file] [-m] time
  -f : specify a file containing commands to execute
  -m : mail the user after the job finishes
Time formats:
  Absolute: MMDDYY or MM/DD/YYYY, or keywords "today"/"tomorrow"
  Relative: now+<number><unit> where unit can be minutes, hours, days, weeks
Examples:
  at now+1 minutes          # run in one minute
  at -f file 17:30+2 days   # run at 17:30 two days later

Periodic scheduling

Function: plan recurring tasks.
Syntax: crontab –u user {-e | -l | -r}
  -u : specify the user for which to manage tasks
  -e : edit the task list
  -l : list the task list (default is current user)
  -r : remove the task list
Alternative: crontab –u user file   # file contains the task definitions
Task file format: minute hour day month day-of-week command
Examples:
  0 17 * * 1-5               # at 17:00 Monday‑Friday
  30 8 * * 1,3,5            # at 08:30 on Mon, Wed, Fri
  0 8-18/2 * * *            # every 2 hours between 08:00 and 18:00
  0 * */3 * *               # every three days
Cron directory location: /var/spool/cron/root