20 Essential Unix Command‑Line Tricks Every Sysadmin Should Know

1. Delete a large file safely

When a huge file (e.g., 200 GB) makes rm or ls hang, truncate it first, then remove it.

: > /path/to/file.log
rm /path/to/file.log

2. Record terminal output

Use the script utility to capture everything displayed in the terminal.

script my.session
# …run commands…
exit   # or Ctrl‑D to stop recording

Inspect the session file with more, less or cat.

3. Re‑create a missing /tmp directory

If /tmp is removed, recreate it with the correct permissions and ownership.

mkdir /tmp
chmod 1777 /tmp
chown root:root /tmp
ls -ld /tmp

4. Lock a directory from ordinary users

Set the directory mode to 0000 so non‑root users cannot list or enter it. Root can still traverse. chmod 0000 /downloads Restore normal access with:

chmod 0755 /downloads

5. Password‑protect a file in vim

Open a file with encryption enabled: vim +X filename Or, after editing, type :X inside vim to set a password.

6. Clear garbled characters on the screen

Run the reset command to restore a clean terminal state.

reset

7. Produce human‑readable sizes

Pass -h (or -H) to GNU/BSD utilities such as ls, df, du, free, stat, sort, lscpu, tree to display sizes in KiB, MiB, GiB, etc.

ls -lh
df -h
free -h
stat -c %A /boot
lscpu -e
tree -h /boot

8. Show known user information

On Linux use lslogins; on BSD use logins to list users, UID, login times and other fields.

lslogins
logins

9. Remove accidentally extracted files

If a tarball was unpacked in the wrong directory, delete the extracted files by feeding the tarball’s file list to rm:

cd /var/www/html/
/bin/rm -f "$(tar ztf /path/to/file.tar.gz)"

10. Replace top with htop

htop

provides an interactive, colour‑rich view of processes.

sudo htop

11. Re‑run the previous command

Enter !! to execute the last command again. Prefix with sudo to run it as root.

!!
sudo !!

To repeat the most recent command that starts with a specific word, use !foo. Example: sudo !service runs the last service command as root.

12. Use the last argument of the previous command

The !$ expansion inserts the last argument of the preceding command.

sudo vi /etc/nginx/nginx.conf
/sbin/nginx -t -c /etc/nginx/nginx.conf
sudo vi !$

13. Schedule a leave‑the‑terminal reminder

Use the leave command with a time in hhmm (12‑ or 24‑hour format).

leave +0930

14. Return to the previous directory

Use cd - to go back to the directory you were in before the last cd.

cd -

15. Jump directly to your home directory

Simply type cd without arguments. cd Define CDPATH to shorten navigation, e.g. export CDPATH=/var/www:/nas10 lets you reach /var/www/html with cd html.

16. Create a directory tree in one command

Use mkdir -p with brace expansion to create multiple nested directories.

mkdir -p /jail/{dev,bin,sbin,etc,usr,lib,lib64}
ls -l /jail/

17. Copy a file to many directories at once

Pipe a list of target directories to xargs instead of issuing several cp commands.

echo /usr/dir1 /var/dir2 /nas/dir3 | xargs -n 1 cp -v /path/to/file

18. Quickly find differences between two directories

Use diff on the directory paths to see line‑by‑line differences.

diff /tmp/r/ /tmp/s/

19. Reformat text files

The fmt command wraps paragraphs to a uniform width. Use -s to split long lines without re‑flowing short ones.

fmt file.txt
fmt -s file.txt

20. View output while writing it to a file

Pipe a command through tee to display its output on the screen and simultaneously save it.

mycoolapp arg1 arg2 input.file | tee my.log

Original source: http://www.cyberciti.biz/open-source/command-line-hacks/20-unix-command-line-tricks-part-i/