Basic System Info Commands
System Monitoring
systemctl → interact with systemd system managerps → view current processestop → dynamic view of process usage
Systemctl Usage
systemctl start|stop|status servicename.service → manage service statesystemctl enable servicename.service → start service at bootsystemctl restart|reload servicename → restart or reload servicesystemctl list-units --all → list all units
To add a service:
Create a unit file in /etc/systemd/system/servicename.service
Process Management
Killing and Prioritizing
Jobs Monitoring
jobs → list background/foreground job states
Essential Commands:
useradd, groupadd, userdel, groupdel, usermodModify 3 files: /etc/passwd, /etc/group, /etc/shadow
Create User with Custom Options
useradd -G mygroup -s /bin/bash \
-c "User Description" -m \
-d "/home/myuser" myuserOther Useful User Commands
useradd myuser → create userid myuser → check if user existsgroupadd → create groupcat /etc/group → check groupuserdel -r myuser → delete user and home directorygroupdel mygroup → delete groupusermod -G mygroup myuser → change user's groupchgrp -R mygroup myuser → change group ownership recursivelypasswd myuser → set or update user's password
Password Aging with chage
Edit Password Policies
File: /etc/login.defs
PASS_MAX_DAYS 9999PASS_MIN_DAYS 0PASS_MIN_LEN 5PASS_WARN_AGE 7
Switch User Privileges
User Info and Monitoring Commands
who → see logged in userslast → list last login recordsw → who is logged in with more detailsfinger → show user detailsid → show UID, GID and groupsid myuser → check user info
Send Messages to Users
Basic Replace Syntax:
Delete Line Containing String:
sed -i '/SEINFELD/d' filename
Delete Empty Lines:
Delete First Line:
sed '1,2d' filename → delete the first two lines
sed 's/\t/ /g' filename → replace tabs with spaces
sed -n '12,18p' filename → print only lines 12 to 18
sed '12,18d' filename → delete lines 12 to 18
sed G filename → insert an empty line after every line
sed '8!s/seinfeld/S1/' filename → replace "seinfeld" with "S1" on every line except line 8
Vi Editor Commands
i → insert modeI → insert at line startESC → escape to command modeR → replace modex → delete characterdd → delete lineyy → copy linep → pastev → visual modeo → open new line below and insertO → open new line above and inserta → append after cursorA → append at end of line:q! → quit without saving:wq or ZZ → save and quit
Crontab Basics
crontab -e → edit crontabcrontab -l → list crontab entriescrontab -r → remove crontab entriessystemctl status crond → check crond service status
AT: One-time Scheduled Tasks
Used to schedule jobs only once.
Scheduling Examples:
at HH:MM PM → schedule a job interactively
at 11:23AM 12/24/25
Enter job (e.g., echo "test") and press Ctrl + D to finish
at 2:45PM 01/30/25at 4PM + 4 daysat now + 5 hoursat 8:00AM Sunat 10:00AM next month
Managing AT Jobs
atq → list the scheduled at jobsatrm # → remove a specific at job (use number from atq)systemctl status atd → check atd daemon status
/boot -> Grub.cfg
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