Mastering Linux Process Management and Scheduling with ps, pstree, at & crontab

Process Concepts

A process is an executing instance of a program that occupies system resources; it differs from the program file itself. Processes can be interactive, batch, or daemon, and the relationship between processes and programs is many‑to‑one.

Viewing Processes

ps [options]

Common options: -A or -e: show all processes -a: show all user processes -f: full‑format listing -l: long format -r: only running processes -u: user‑oriented format (includes user, CPU, memory) -x: processes without a controlling terminal -p: specify PID -t: specify terminal

Inspecting Process Hierarchy

pstree [options] [pid|user]

pstree

displays processes in a tree structure, rooted at init or a given PID/user. Options: -a: show full command line and swapped‑out processes (in parentheses) -c: separate duplicate process names with an asterisk

Terminating Processes

kill [signal] pid

killall [signal] process_name

Use ps -aux to list processes and pstree to view child processes before terminating.

One‑Time (Delayed) Scheduling with at

at [-f file] [-m] time

Options: -f: specify a file containing commands to execute -m: mail output to the user after the job finishes

Time can be absolute (e.g., MMDDYY or MM/DD/YYYY, today, tomorrow) or relative (e.g., now+5 minutes, now+2 days).

Example:

at now+1 minutes

Periodic Scheduling with crontab

crontab -u user {-e | -l | -r}

-e

: edit the user’s crontab -l: list the crontab -r: remove the crontab

Crontab file format: minute hour day month day-of-week command Special symbols: *: any value ,: list of discrete values -: range of values /: step values

Examples:

Weekdays at 17:00: 0 17 * * 1-5 Mon, Wed, Fri at 08:30: 30 8 * * 1,3,5 Every 2 hours between 08:00 and 18:00: 0 8-18/2 * * * Every 3 days: 0 * */3 * * Crontab files are stored under /var/spool/cron (e.g., /var/spool/cron/root).