Command Line

You may be asking yourself about the virtue using the Commandline. In many way the Graphic User Interface is friendlier. For many users it feels familiar and to other Operating Systems. The reason for learning the commandline is primarily a way to introduce you to what administering a server that could be in a remote location where you have no physical access. Your Ubuntu virtual machine is a good place to get practice. This segment is driven towards some repetitive exercises with a goal to make you comfortable on your Ubuntu virtual machine.

Navigation

Location

We will start with the pwd which stands for Present Working Directory. Many of the commands in UNIX are often an abbreviation of a word of words describing what they do.

username@hostname:~$ pwd
/home/username
username@hostname:~$

The command ls which is short for list will produce the following on your termimal

username@hostname:~$ ls
Desktop Documents Downloads Music Pictures Public Templates Videos

Before we proceed much futher it is important to remember that all UNIX systems are designed with a built-in manual to explain what the commands do. The way to find out what it does is to prepend the command with man In the examples above it would be man pwd and man ls. Go ahead and type those and read what the manual says about them. As you may have see the ls command can take arguments while the pwd does not. Below is an outline of the use of ls.

ls [options][location]

The square brackets above are optional. Here is an example with the echo of the results shown.

username@hostname:~$ ls
Desktop Documents Downloads Music Pictures Public Templates Videos
username@hostname:~$ ls -l
total 48
drwxr-xr-x  3 fkayiwa fkayiwa 4096 Mar  7 14:09 Desktop
drwxr-xr-x  2 fkayiwa fkayiwa 4096 Dec  9 17:32 Documents
drwxr-xr-x  2 fkayiwa fkayiwa 4096 Dec  9 17:32 Downloads
-rw-r--r--  1 fkayiwa fkayiwa 8445 Dec  9 17:21 examples.desktop
drwxr-xr-x  2 fkayiwa fkayiwa 4096 Dec  9 17:32 Music
drwxr-xr-x  2 fkayiwa fkayiwa 4096 Dec  9 17:32 Pictures
drwxr-xr-x  2 fkayiwa fkayiwa 4096 Dec  9 17:32 Public
drwxr-xr-x  2 fkayiwa fkayiwa 4096 Dec  9 17:32 Templates
drwxr-xr-x  2 fkayiwa fkayiwa 4096 Dec  9 17:32 Videos

So what did we just do?

• The ls will list the contents of our current directory.

• The ls -l adds the optional switch ( -l) to do the long listing. A long listing has the following:

  • First character indicates whether it is a normal file ( - ) or directory ( d )
  • Next 9 characters are permissions for the file or directory (we’ll learn more about them in a later segment)
  • The next file is the number of blocks.
  • The next field is the owner of the file or directory (fkayiwa/root in the examples above).
  • The next field is the group the file or directory belongs to (fkayiwa).
  • Following this is the file size.
  • Next up is the file modification time.
  • The actual name of the file or directory.

Here is another example.

username@hostname:~$ls -l /var
total 52
drwxrws---  5 archivematica archivematica 4096 Mar  6 11:38 archivematica
drwxr-xr-x  2 root          root          4096 Apr  1 07:35 backups
drwxr-xr-x 19 root          root          4096 Mar  6 11:10 cache
drwxrwsrwt  2 root          whoopsie      4096 Dec 26 07:35 crash
drwxr-xr-x  2 root          root          4096 Aug 20  2013 games
drwxr-xr-x 68 root          root          4096 Mar  6 11:23 lib
drwxrwsr-x  2 root          staff         4096 Dec 11 15:04 local
lrwxrwxrwx  1 root          root             9 Mar 10 13:40 lock -> /run/lock
drwxr-xr-x 24 root          root          4096 Apr  3 07:55 log
drwxrwsr-x  2 root          mail          4096 Mar 31 07:58 mail
drwxr-xr-x  2 root          root          4096 Aug 20  2013 opt
lrwxrwxrwx  1 root          root             4 Mar 10 13:40 run -> /run
drwxr-xr-x 10 root          root          4096 Mar  6 11:23 spool
drwxrwxrwt  2 root          root          4096 Apr 19  2012 tmp
drwxr-xr-x  2 root          root          4096 Dec 11 11:59 www

• We ran ls with a command line argument ( /var ). When we do this it tells ls not to list our current directory but instead to list that directories contents.

Spend a little time perusing the ls manual by doing the following. man ls

Paths

The previous section touched on something about navigating your Ubuntu machine, namely the paths on the operating system. Understanding paths is essential in using your new computer. Whenever we refer to a file or directory on the command line we are in fact referring to the actual path to it.

Referring to paths can be done using the absolute path or a relative path.

At the top of the unix system is what is called the root directory. It is denoted by a single dash (/). It has sub-directories which have other sub-directories contained within them. Files can reside in any of these directories.

Absolute paths refer to the location relative to the root directory. Relative paths specify location based on your current working directory. The example below will illustrate what we mean.

username@hostname:~$ pwd
/home/fkayiwa
username@hostname:~$
username@hostname: ls Desktop
test.mrc
username@hostname:~$ ls /home/fkayiwa/Desktop
test.mrc
username@hostname:~$

So what just happened?

• We ran pwd to verify where we were on the filesystem.
• We ran ls providing it with a relative path. Desktop is a directory in our current location. We would get different results depending on where we are.
• We finally ran ls providing and absolute path. This will provide the same output regardless of the location we ran it from.

Here are a few more tips that can help build on your new knowledge on paths.

• ~ (tilde) - This is a shortcut for your home directory. So if the user fkayiwa above wanted to head to their home directory. Typing ~/Desktop in the example above is the equivalent of typing /home/fkayiwa/Desktop
• . (dot) - This is a reference to your current directory. We will see more on this later.
• . . (dotdot) - This is a reference to the parent directory. You can use this several times to keep going up the heirarchy.

A few more examples to explain this.

username@hostname:~$ pwd
/home/fkayiwa
username@hostname:~$
username@hostname:~$ ls ~/Desktop
test.mrc
usename@hostname:~$ ls ./Desktop
test.mrc
username@hostname:~$ ls /home/fkayiwa/Desktop
test.mrc
username@hostname:~$ ls ../../
bin boot lib lost+found proc selinux usr boot home lib32
...
username@hostname:~$ ls /
bin boot lib lost+found proc selinux usr boot home lib32
...

Spend time listing the content of various directories on your filesystem and familiarize yourself on how the elements of building a path.

Moving around your filesystem

In order to move around your filesystem we use the cd command which stands for change directory.

A handy tip to remember is if you run the command cd without any arguments then it will always take you back to your home directory.

Otherwise the cd command can be used much like the ls before to change into absolute or relative paths as some of the examples below will show.

username@hostname:~$ pwd
/home/fkayiwa
username@hostname:~$ cd Desktop
username@hostname:~$ ls
test.mrc
username@hostname:~$ cd /
username@hostname:~$ pwd
/
username@hostname:~$ ls
bin boot lib lost+found proc selinux usr boot home lib32
...
username@hostname:~$ cd ~/Desktop
username@hostname:~$ pwd
/home/fkayiwa/Desktop
username@hostname:~$ cd ../../
username@hostname:~$ pwd
/home
username@hostname:~$ cd
username@hostname:~$ pwd
/home/fkayiwa

Another handy tip when navigating is Tab Completion. Typing out these paths can become tedious. If you’re like me, you’re also prone to making typos. The command line has a nice little mechanism to help us in this respect. It’s called Tab Completion.

When you start typing a path (anywhere on the command line, you’re not just limited to certain commands) you may hit the Tab key on your keyboard at any time which will invoke an auto complete action. If nothing happens then that means there are several possibilities. If you hit Tab again it will show you those possibilities. You may then continue typing and hit Tab again and it will again try to auto complete for you.

It’s kinda hard to demonstrate here so it’s probably best if you try it yourself. If you start typing cd /hTab/<beginning of your username>Tab you’ll get a feel for how it works.

Summary

Commands we learned about

pwd - Present Working Directory
ls - List the contents of a directory
cd - Change directories