Learning UNIX is a seemingly daunting task, there are thousands of commands out there, each with hundreds of options. But in reality you only need to know a few of them.

I use unix quite a bit, usually either on one of our Linux servers, or on my Powerbook with OS X. And these are the 15 commands that I use most. If you can memorize these 15 commands, you can do quite a bit on a unix operating system, and add unix as a skill on your resume.

The 15 Most Important UNIX commands

1. man - show manual for a command, example: man ls hit q to exit the man page.
2. cd - change directory, example: cd /etc/
3. ls - list directory, similar to dir on windows. example: ls /etc, use ls -l /etc to see more detail
4. cp - copy a file or directory, example: cp source dest if you want to copy a directory use the -R option for recursive: cp -R /source /dest
5. mv - move a file, example: mv source dest
6. rm - remove a file, example: rm somefile to remove a directory you may need the -R option, you can also use the -f option which tells it not to confirm each file: rm -Rf /dir
7. cat - concatenate, or output a file cat /var/log/messages
8. more - outputs one page of a file and pauses. example: more /var/log/messages press q to exit before getting to the bottom. You can also pipe to more | more from other commands, for example ls -l /etc | more
9. scp - secure copy, copies a file over SSH to another server. example: scp /local/file user@host.com:/path/to/save/file
10. tar - tape archiver, tar takes a bunch of files, and munges them into one .tar file, the files are often compressed with the gzip algorithm, and use the .tar.gz extension. to create a tar tar -cf archive.tar /directory, then to extract the archive to the current directory run tar -xf archive.tar to use gzip, just add a z to the options, to create a tar.gz: tar -czf archive.tar.gz /dir to extract it tar -xzf archive.tar.gz
11. grep - pattern matcher, grep takes a regular expression, or to match a simple string you can use fast grep, fgrep failure /var/log/messages, I'm usually just looking for a simple pattern so I tend to use fgrep more than regular grep.
12. find - lists files and directories recursively on a single line, I usually pipe grep into the mix when I use find, eg: find / | fgrep log
13. tail - prints the last few lines of a file, this is handy for checking log files tail /var/log/messages if you need see more lines, use the -n option, tail -n 50 /var/log/messages you can also use the -f option, which will continuously show you the end of the file as things are added to it (very handy for watching logs) tail -f /var/log/messages
14. head - same as tail, but shows the first few lines the file
15. vi - text editor, there are several text editors such as emacs, and nano, but vi is usually installed on any server so its a good one to learn. To edit a file type vi file to edit a line press Esc i then to save changes and exit use Esc wq, or to quit without saving use Esc q!. There are a million other commands, but that will enable you to edit files at a basic level.

Once you learn these commands, and are comfortable with them, you shouldn't stop there, there are lots of other commands that can make your life easier.

Did I miss any commands that you think are essential to using a UNIX based operating system?