Master Linux Process Management: Delays, Scheduling, and Essential Commands

Process Concepts

A process is an executing instance of a program; a program is a file containing executable code. Processes consume system resources and differ from programs. Types include interactive, batch, and daemon processes, and the relationship between processes and programs is many‑to‑one.

Viewing Processes

ps [options]
-A  : show all processes (same as -e)
-a  : show processes of all users
-f  : full‑format listing
-l  : long format
-r  : only running processes
-u  : user‑oriented format (user, CPU, memory, etc.)
-x  : show processes without a controlling terminal
-p  : show process by PID
-t  : show processes on a specific terminal

Inspecting Process Hierarchy

pstree [options] [pid|user]
-a  : display full tree, including swapped‑out processes in parentheses
-c  : separate duplicate process names with an asterisk (*)

Terminating Processes

kill [signal] pid
killall [signal] process_name
# Common workflow
ps -aux          # list processes
pstree           # view child processes

Delay (One‑Time) Scheduling

The at command runs a program at a specified time.

at [-f file] [-m] time
-f file   : file containing commands to execute
-m        : mail the user when the job finishes
time      : absolute (MMDDYY or MM/DD/YYYY, today, tomorrow) or relative (now+N minutes/hours/days/weeks)
Example:
at now+1 minutes
at -f myscript.sh 17:30 +2 days

Periodic Scheduling

The crontab utility manages recurring tasks.

crontab -u user -e   # edit/create task list
crontab -u user -l   # list tasks
crontab -u user -r   # remove tasks

Task file format: minute hour day month day‑of‑week command

Examples:
# Mon‑Fri at 17:00
0 17 * * 1-5 /path/to/command
# Mon, Wed, Fri at 08:30
30 8 * * 1,3,5 /path/to/command
# Every 2 hours between 08:00 and 18:00
0 8-18/2 * * * /path/to/command
# Every 3 days
0 * */3 * * /path/to/command

Special symbols:
*  : any value
,  : list of values
-  : range of values
/  : step values

Cron files are stored under /var/spool/cron (e.g., /var/spool/cron/root).