OPS145 Lab 3 Newversion: Difference between revisions

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The latest version is here: [[OPS145_Lab_3]]


= Filesystems =
[[Category:OPS145]]
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:[[File:FilesystemIntro.png|center|499x499px]]
 
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:<syntaxhighlight lang="bash">
ls /home/youruserid/Documents/first.txt
</syntaxhighlight>
* Read the contents of first.txt using an absolute path:<syntaxhighlight lang="bash">
cat /home/youruserid/Documents/first.txt
</syntaxhighlight>
 
= 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.
 
# Use '''pwd''' to confirm you're in /home/youruserid
# Change your PWD to the Documents directory using a relative path, and confirm that worked: <syntaxhighlight lang="bash">
cd Documents
pwd
ls
</syntaxhighlight>
# Now go back to '''your home''' directory (/home/youruserid, not /home):<syntaxhighlight lang="bash">
cd ..
pwd
ls
</syntaxhighlight>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.
# From your home directory change directly into the SampleFiles directory:<syntaxhighlight lang="bash">
cd Downloads/SampleFiles
pwd
ls
</syntaxhighlight>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/
# 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:<syntaxhighlight lang="bash">
cd ../..
pwd
ls
</syntaxhighlight>
# Here's a screenshot of the commands from this section so far:
[[File:SimplePaths.png|center]]
 
= 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):<syntaxhighlight lang="bash">
cd /bin
ls
</syntaxhighlight>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):[[File:BinCat.png|center|800x800px]]
* 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:<syntaxhighlight lang="bash">
cd ../home/yourusername
pwd
</syntaxhighlight>
* 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:<syntaxhighlight lang="bash">
cd ../../bin
pwd
</syntaxhighlight>
* 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:[[File:FilesystemIntro.png|center|499x499px]]<syntaxhighlight lang="bash">
cd ~
cd Downloads/../Documents
cd ../Documents/../Downloads
cd ../../../bin
</syntaxhighlight>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:<syntaxhighlight lang="bash">
cd ~
mkdir lab3-cmd
</syntaxhighlight>
* 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:<syntaxhighlight lang="bash">
mkdir lab3-cmd/red
cd lab3-cmd/red
pwd
</syntaxhighlight>
* 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:<syntaxhighlight lang="bash">
mkdir ../green
pwd
ls
ls ..
</syntaxhighlight>
* 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:<syntaxhighlight lang="bash">
mkdir /home/youruserid/lab3-cmd/blue
ls ~/lab3
</syntaxhighlight>
*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.
 
= Also =
mkdir
 
rmdir
 
tree
 
ls -r
 
cp
 
mv
 
rm
 
ln -s

Latest revision as of 23:28, 29 January 2024

The latest version is here: OPS145_Lab_3

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