The Unix filesystem

Unix shells allow to give instructions to a Unix operating system. The first thing you will need to know is how storage is organized on such a system.

Structure

The Unix filesystem is a rooted tree of directories. The root is denoted by /.

Several directories exist under the root. Here are a few:

• /bin     This is where binaries are stored.
• /boot    There, you can find the files necessary for booting the system.
• /home    This directory contains all the users’ home directories.

These directories in turn can contain other directories. /home for instance contains the directories:

• /home/user01
• /home/user02
• /home/user03
• …

The home directory of each user in turn contains their files and directories.

Navigation

Working directory

The current working directory can be obtained with:

pwd

Stands for print working directory.

Your turn:

What is your current working directory?

Changing directory

To navigate to another directory, use cd (change directory) followed by the path of the directory.

Example:

cd /home

Because /home was the parent directory of our working directory (one level above in the rooted tree), we could have also gotten there with cd .. — the two dots represent one level up (a single dot represents the working directory).

Your turn:

• What will happen if you run cd .. from /home?
• What will happen if you run cd . from /home?

From any location, you can always go back to your home directory (e.g. /home/user09) by running cd without argument. Alternatively, you can use cd ~. This is because ~ gets expanded by the shell into the path of your home. Finally, you can use cd $HOME. $HOME is an environment variable representing the path of your home.

Your turn:

Try using cd - (that’s the minus sign) a few times. What does this command do?

Absolute and relative paths

• Absolute paths give the full path from the root (e.g. /bin, /home/user09/file).
• Relative paths give the path relative to the working directory (e.g. ../dir/file, dir/subdir).

Your turn:

Is ~ an absolute or relative path?

In the filesystem below, the current working directory is /home/user01.

• What is the output of ls?
• What is the output of ls ../..?
• The output of ls /thesis/src is:

ls: cannot access ‘/thesis/src’: No such file or directory

  Why?

• What are 2 ways to navigate to the results directory?
• From the results directory, what are 2 ways to print the content of the src directory?

Creating files and directories

Files can be created with a text editor.

Example using nano, a simple text editor available on many systems:

nano newfile.txt

This opens the text editor “nano” with a blank file. The file actually gets created when you save it from within the text editor.

or with the command touch:

touch newfile.txt

Your turn:

• What does the command touch do?
  
• How did you figure it out?
  
• What is the content of newfile.txt?
  
• How did you figure it out?

touch can create multiple files at once:

touch file1 file2 file3

New directories can be created with mkdir. This command can also accept multiple arguments to create multiple directories at once:

mkdir dir1 dir2

Deleting

Files can be deleted with the command rm followed by their paths:

rm file1 file2

Directories can be deleted with rm -r (“recursive”) followed by their paths or—if they are empty—with rmdir:

rm -r dir1
rmdir dir2   # only works if dir2 is empty

Be careful that these commands are irreversible. By default, there is no trash in Linux systems.

Copying, moving, and renaming

Copying is done with the cp command:

cp thesis/src/script1 thesis/ms

Moving and renaming are both done with the mv command:

# rename script1 to script
mv thesis/src/script1 thesis/src/script

# move graph1 to the ms directory
mv thesis/results/graph1 thesis/ms
# this also works:
# mv thesis/results/graph1 thesis/ms/graph1

Your turn:

Why is there only one command to move and rename?