OPS145 Lab 3 Newversion

Filesystems

A filesystem is a system of organizing files on a storage device. Such a system consists of:

• A standard specifying the meaning and order of bytes on the storage device, and
• Driver-like software which hides the filesystem implementation details, and allows user-level software to perform generic operations (e.g. copy this file to that directory)

Files and directories are very different things from a human point of view, but on a filesystem they are almost the same thing. Each has a name, permissions, modification date, and the location of its contents. The contents of a file are whatever you'd expect they are, it's different for each type of file. The contents of a directory are the list of its contents.

In this course we're only going to look at the user level of Linux filesystems.

We'll start exploring these ideas by paying attention to the directories and files in the following diagram:

You should have all of these if you finished lab 2.

Absolute paths start at root

In Linux there are no drive letters. All the storage which is accessible is accessible via a path which starts at the root (a single forward slash).

For example your first.txt file is in the Documents directory. The Documents directory is in the asmith15 directory. The asmith15 directory is in the home directory, and the home directory is in the root directory. The root directory does not have a parent directory, technically it is its own parent.

There might be other first.txt files on the system. In order to specify the path to this specific first.txt you use an absolute path: /home/youruserid/Documents/first.txt

The directories in a path are separated by forward slashes (/). This is different from Windows where backslashes are used.

If a path starts with a slash: it's an absolute path.

• Open a terminal and use ls, or ls -l to confirm that first.txt exists and you have the correct path to it:

  ls /home/youruserid/Documents/first.txt
  

• Read the contents of first.txt using an absolute path:

  cat /home/youruserid/Documents/first.txt
  

Relative paths start at PWD

In the last lab you used relative paths when you ran your ls and cat commands. All that means is that the path you used (e.g. first.txt), because it didn't start with a slash, was automatically appended to the present working directory in your terminal (e.g. /home/youruserid/Documents/), resulting in an unambiguous path (e.g. /home/youruserid/Documents/first.txt).

A file or directory name on its own is the simplest type of relative path - you used that last week.

This week we can look at slightly more complicated relative paths so you can navigate the filesystem more efficiently.

1. Use pwd to confirm you're in /home/youruserid
2. Change your PWD to the Documents directory using a relative path, and confirm that worked:

   cd Documents
   pwd
   ls
   

3. Now go back to your home directory (/home/youruserid, not /home):

   cd ..
   pwd
   ls
   

   Two dots (..) means "parent directory" in the shell. Since /home/youruserid is the parent directory of /home/youruserid/Documents: that does what you want in this case. .. is also a relative path.
4. From your home directory change directly into the SampleFiles directory:

   cd Downloads/SampleFiles
   pwd
   ls
   

   This time you had a path with two directories, but it's still a relative path, not an absolute path. It will get appended to the PWD (which was /home/youruserid), resulting in /home/youruserid/Downloads/SampleFiles/
5. You can use .. more than once in the same path. It always means "parent directory". Go back to your home directory from where you are now:

   cd ../..
   pwd
   ls
   

6. Here's a screenshot of the commands from this section so far:

Go places, find things

Using the basic knowledge of absolute and relative paths introduced above you can navigate to any directory on your system, open any file, and run any program you have permissions for.

• Go to the directory which has most of the applications on your system (/bin):

  cd /bin
  ls
  

  This is vaguely similar to "C:\Program Files\" on Windows, with one big difference: it only has the executables in it, not the other data files which the programs need. In Linux an application is rarly installed in one directory, an application's files are spread over several system directories (e.g. /usr/bin, and /usr/lib, and /usr/share).
• In this directory look for the programs you've run so far (cat, less, ls, pwd). Notice that they're coloured green. That's because they're executable programs.
• You can also open the /bin directory in your graphical file manager, but only as a curiosity (you don't typically run graphical applications from a file manager in Linux):
  
• From /bin in the graphical file manager navigate back to your home directory and keep a mental note of the steps you took to get there:
  • Go up one directory
  • Go into the "home" directory
  • Go into the "yourusername" directory
• Do the equivalent in the terminal:

  cd ../home/yourusername
  pwd
  

• Now do the reverse. In the graphical file manager:
  • Go up one directory, you'll end up in /home
  • Go up one more directory, you'll end up in /
  • Go to the "bin" directory
• And do the equivalent in the terminal:

  cd ../../bin
  pwd
  

• You'll need a lot of practice to get used to these ideas. Choose some number of random directories in the graphical file manager, and navigate to/between them in the terminal using absolute and relative paths.

Shortcut for your home directory: ~

Your home directory (/home/yourusername) is so frequently used that there's a special character you can use as a shortcut to get to it: the tilde ~ (top left of your keyboard).

No matter what your PWD is in your terminal: ~ will always be automatically converted to /home/yourusername. You can use it alone, or in combination with other path components.

Sometimes it's complicated

As a challenge look at the diagram, and the relative paths, and figure out what the PWD will be after you execute each of the following commands. Then run the commands one at a time to check your mental gymnastics:

cd ~
cd Downloads/../Documents
cd ../Documents/../Downloads
cd ../../../bin

Then write four cd commands using absolute paths instead of relative paths to do exactly the same operations as the four cd commands above.

Create directories with mkdir

For the rest of this lab you'll use the graphical interface to do stuff, and then do the equivalent thing in the command-line.

• Use the graphical file manager to create a lab3 directory in your home directory.
• In a terminal cd to your home directory first, then create the lab3-cmd directory:

  cd ~
  mkdir lab3-cmd
  

• You can open multiple file managers and multiple terminals to convince yourself that other than the name there isn't any difference between the two directories you created.
• In the graphical file manager go into your lab3 directory and create a red subdirectory. A subdirectory is just a regular directory, using that word usually implies that it's inside some specific other directory.
• In your terminal create a red subdirectory under lab3-cmd and then cd into it:

  mkdir lab3-cmd/red
  cd lab3-cmd/red
  pwd
  

• In the graphical file manager create a green directory inside the lab3 directory.
• In your terminal use a relative path to create a green directory inside the lab3 directory and use ls to check that it was created:

  mkdir ../green
  pwd
  ls
  ls ..
  

• In the graphical file manager create a blue directory inside the lab3 directory.
• In your terminal use an absolute path to create a blue subdirectory of the lab3 directory. Check that it got created using an ls command and a relative-to-home path:

  mkdir /home/youruserid/lab3-cmd/blue
  ls ~/lab3-cmd
  

• In the graphical file manager try to create another blue directory inside the lab3 directory. You'll get an error message saying that it can't be done because it already exists.
• In your terminal cd to the lab3-cmd directory using an absolute path, then try to create another blue directory there. You'll get a similar error message, though not quite as easy to understand as the message in the graphical interface.

This is a screenshot of me running the commands in this section:

Removing empty directories with rmdir and rm -r

For some reason Linux has a command which can only be used to remove empty directories: rmdir. But it's used often enough that we'll learn it now.

• In the graphical file manager create a new subdirectory of lab3/red: garbage. Inside garbage create two subdirectories called stuff and things.
• Select the garbage directory and press Delete on your keyboard. It will go to your Trash (you might know it as a "recycling bin" on Windows). If you feel like it: you can restore what you deleted by clicking Go/Trash and finding your deleted files there.
• In your terminal create the garbage, stuff, and things directories at the same time by giving mkdir more than one argument:

  cd ~/lab3/red
  mkdir garbage garbage/stuff garbage/things
  

• Try to delete the garbage directory using rmdir:

  rmdir garbage
  

• It will fail with an error saying that the directory is not empty. Delete the empty stuff directory using rmdir:

  rmdir garbage/stuff
  

• You could also delete the things directory and then the garbage directory when it's empty, but instead you can use the rm command with the -r argument:

  rmdir -r garbage
  

• Note that things deleted on the command-line are not moved to anything resembling a recycling bin. For most practical purposes they are deleted permanently and irrecoverably.

Also

tree

ls -r

cp

mv

rm

ln -s