Master Essential Linux Commands: Poweroff, User Switching, File Management, and Text Processing

1. Login and Power Management

halt

, poweroff, init 0 – shut down the system. reboot, init 6 – restart. shutdown – schedule shutdown; examples: shutdown -r (reboot), shutdown -h (halt), shutdown -c (cancel), shutdown +0 (immediate), shutdown +1 (in one minute), shutdown -X (in X minutes), shutdown 20:00 (at 20:00). whoami – display current username. who am i – show username, terminal, login time. who – list all logged‑in users. w – detailed info about logged‑in users and their activities.

2. Switch User and Working Directory

su username

– switch to another user and go to that user’s home directory. su username (without login) – switch user without loading login environment. cd [path] – change directory; cd -P follows physical directory when encountering symlinks. .. – parent directory; cd or cd ~ – home directory; cd - – previous directory. pwd – print current working directory.

3. Basic Common Commands

date

– show system time; date -s "2018-1-20 13:50:11" – set system time. hwclock or clock – read hardware clock; -s writes hardware time to system, -w writes system time to hardware. cal – display calendar. history – command history. hostname – show host name.

Interrupt commands: Ctrl+C (force), Ctrl+D (normal).

4. Help and Documentation

type

– determine command type. enable / enable -n – manage internal commands. help – internal command help. command –help – external command help. whereis command – locate binary and manual. which command – locate executable. man command – view manual page.

5. File Listing

ls

– list directory contents; options: -a, -A, -l, -R, -ld, -1. stat filename – detailed file attributes. file filepath – determine file type; -b – brief output; -f filelist – test each file. cat filename – view file. vim filename – edit file.

6. Create, Move, Copy, Delete Files

touch newfile

– create empty file; touch -a, touch -m, touch -t, touch -c – modify timestamps. mkdir -p – create parent directories; -v – verbose; -m mode – set permissions. rmdir – remove empty directory. cp src dst – copy files; options: -i, -r, --preserve=mode,owner,timestamp, -p, -d, -v, -a, -f. mv src dst – move/rename; -i, -f. rm filename – delete; -i, -f, -r; use ls | xargs rm -rf for many files; --no-preserve-root to override safety.

7. Text Processing Commands

tr [set1] [set2]

– translate characters; -d delete, -c complement, -s squeeze repeats. cut -d delim -f list filename – extract fields; --output-delimiter=delim sets output separator. sort – sort lines; -t delim, -k, -n, -r, -u, -f. uniq – remove duplicate lines; -c, -d, -u.

8. Regular Expressions with grep and sed

grep

options: --color=auto, -v, -n, -c, -i, -q, -A n, -B n, -C n, -e, -w, -E, -F. sed options: -i (in‑place), -i.bak (backup), -n (suppress output), -e (script), -f file (script file), -r (extended regex); commands: p, d, a, i, c, w, r, =, !.