• Basic Replace Syntax:

  • sed -i 's/KENNY/LENNY/g' filename

  • Substitute all occurrences of "KENNY" with "LENNY"

• Delete Line Containing String:

  • sed -i '/SEINFELD/d' filename

• Delete Empty Lines:

  • sed -i '/^$/d' filename

• Delete First Line:

  • sed '1d' filename

• 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 mode
• I → insert at line start
• ESC → escape to command mode
• R → replace mode
• x → delete character
• dd → delete line
• yy → copy line
• p → paste
• v → visual mode
• o → open new line below and insert
• O → open new line above and insert
• a → append after cursor
• A → append at end of line
• :q! → quit without saving
• :wq or ZZ → save and quit

Crontab Basics

• crontab -e → edit crontab
• crontab -l → list crontab entries
• crontab -r → remove crontab entries
• systemctl 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 01/30/25
• at 4PM + 4 days
• at now + 5 hours
• at 8:00AM Sun
• at 10:00AM next month

Managing AT Jobs

• atq → list the scheduled at jobs
• atrm # → remove a specific at job (use number from atq)
• systemctl status atd → check atd daemon status