top-20-ubuntu-linux-commands-with-examples.md

I use Ubuntu servers all the time, and there are some commands that everyone that uses them should know. I have compiled a list of what I think are the most useful commands a Ubuntu user can know. They most likely work on most other Linux systems but I haven't tested them.

Before we get started

There are some useful things to know about the command line before you get started with the below. These aren't exactly commands but are used within commands.

• / is the root directory of your system. Its used in directory paths.
• ~ is your home directory.
• \ will escape a character for you. For example whitespace, if you have a space in a directory name you will need to use this before the space. E.g. Action\ Movies

1. ls

Lets start with some basics, the ls command simply lists the current working directory structure. You will probably also want to know some of the ls option keys. The most useful I find are -l, '-a' and -h which do the following:

• a - will show hidden files as well
• l - will show files in a list format
• h - will show the size of the files in human readable format.

You can also view the structure of a different directory by stating the directory after your options. For example

ls -lah downloads/movies/

will list the files in the downloads/movies/ directory.

2. cd

No good doing anything on command line unless you can move about within the directory structure. cd will change directory.

cd downloads/movies/

will move you into the movies directory.

3. mkdir

Sticking with the directory theme, mkdir will make a directory.

mkdir movies

will make a directory called movies in your current working directory. You can specify to make a directory in a different location by concatenating the location in front of the new one.

mkdir downloads/movies

will make a directory called movies in the downloads directory.

4. rm

rm will remove your unwanted files or directories

rm index.html

will remove the index.html file in your current location. You may also need some of the flags for rm, namely -f and -R.

• '-f' - will attempt to force the removal regardless of permissions and without prompting for confirmation
• '-R' - will remove recursively, meaning it will remove a directory and its contents including other directories.

rm -Rf /downloads/movies

will recursively remove the movies directory and all of its contents without prompting for confirmation. Now you will understand the jokes and/or scams trying to get you to run rm -Rf / on your system (dont run it). This will remove your entire directory structure given that / is your root directory.

5. mv

This is another useful directory/file based command. mv will move something for you.

mv index.html var/www/

will move the index.html file into your var/www directory. Another use of the mv command though is to also rename a file.

mv index.html index_old.html

will rename index.html to index_old.html.

6. cp

If you dont want to move something, but rather copy it then cp is the command you want.

cp index.html var/www

will copy your inde.html file into the var/www directory.

7. cat

cat will print out the contents of a file to your screen.

cat ~/.ssh/.id_rsa.pub

will print out your public SSH key to the screen.

8. restart

Moving on from directories now we have restart which I personally normally use in conjunction with service. The restart command will restart your system. Used with service it will just restart a service. Taking Nginx for example

service nginx restart

will restart the nginx service. You will need that one a lot when you are configuring Nginx

9. reload

However, sometimes you dont want to restart a service. Maybe you just want to reload the config.

service nginx reload

will do just that. The difference being that reload will continue the service running, a restart will stop the service, then start the service again. Giving a momentary downtime.

10. df

You probably wont need this one if you are just running a basic website or something on your server. However it can become quite useful if you have multiple sites or a small disk. df will tell you how much space is being used and how much is free on your disk. Similar to the ls command above, you will likely want the -h flag for humanly readable.

df -h

11. top

top will bring up a screen with your currently running process, how much memory and CPU they are using among other things. This can be useful if you are having issues and need to diagnose some problems like memory leaks.

12. wget

This is used to get the contents of a url. It has many uses, i mainly use it for 2 specific reasons. Either I simply want to download the contents or I want to run the script that is at that location.

wget http://example.com/file.iso

this will download the iso from that location.

wget http://example.com/sendNotification.php

this will run the sendNotification.php script. Which could be run on say a cron job.

13. apt-get

apt-get is a essential tool on a Ubuntu server. There are three parts to it, install, remove and update. This is what you will use to install all your packages like Nginx or MySQL.

apt-get update

this will update the list of repositories in the apt get lists so you are install the most up-to-date stable releases.

apt-get install nginx

this will install the nginx package.

apt-get remove nginx

this will remove the nginx package.

14. sudo

When it comes to installing things on your system, its likely you will need to be a root user. Or at least have root level permissions for the system. sudo will ensure you are running your commands with those privileges. This will in most cases then ask for your password.

sudo apt-get install nginx

15. chmod

Talking of permissions, there may come a time when you need to change these. chmod will change the permissions levels for the given file. Its too in depth to explain Linux permissions here, however I will give a brief overview.

Permissions on a Ubuntu (Linux) system are based on a numbering system. With 3 classes of user. The owner, the group and the world. The owner is the user than owns the file/directory. The group is the group of users that the directory belongs to and the world users are everyone else.

There are then 3 levels of permissions, read (r), write (w) and execute (x). There are fairly self explainatory. Each of these permissions is given a point level and those points added together for each user and then concatenated give the file/directory its permission level.

• 4 points for r
• 2 points for w
• 1 point for x

So take index.php for example. If I want r, w and x for the owner, r and w for the group and just r for everyone else my persmissions would be as follows:

4 + 2 + 1 = 7 # owner
4 + 2 = 6 # group
4 = 4 # world

Concatenate those numbers together you get 764 so running

chmod 764 index.php

will give those permissions for those users for the index.php file.

Another useful note here is the -R command is used for recursion through directories.

16. chown

This will chnage the owner of the file / directory.

chown john app/

This will make john the owner of the app directory.

17. chgrp

This will change the group of the file / directory.

chgrp www-data app/

This will change the app directory group to www-data

18. adduser

Not much good chnaging permissions for users if you dont have any. The adduser command will do just that. Some people like to user useradd which technically does the same thing, but adduser will guide you through adding the user with passwords, names etc and create the home directory for you. useradd wont, you will need to do that manually.

adduser john

19. usermod

You will need this one to add an existing user to a group.

user mod www-data john

this will add the user john to the group www-data.

20. deluser

This unsurprisingly will remove a user.

deluser john

Will remove the user john from the system.