UNIX tutorial

Meet the Terminal

The terminal is a program that allows you to talk to your computer by entering commands. No matter what operating system you use (Windows, MacOS, Linux), the terminal will be an essential tool for CS 61A.

Go ahead and open up a terminal. The terminal lets you give commands to your computer. Try this command:

echo "Hello world"

Your terminal will repeat "Hello world" back to the screen. The echo command just tells your terminal to repeat the words you typed. Not too impressive just yet, but it turns out the terminal can do a lot more!

The Filesystem

On Windows or MacOS, you are probably familiar with folders and files, with which you can interact by dragging and dropping icons. Today, we are getting rid of the icons and using just the terminal to manage our files and folders!

With the terminal, you can do everything that a graphical filesystem can do -- and more!

Directories

The first command we'll use is ls (the letter l and the letter s). Try typing it into the terminal!

user@computer:~$ ls
Desktop/  Documents/  Downloads/   ...

Depending on what computer you are using, the output that you see after typing ls might be different.

The ls command lists all the files and folders in the current directory. A directory is another name for a folder (such as the Documents folder).

When you open a terminal, you will start from the home directory. Notice that your prompt (e.g. user@computer:~$) has a tilde ~ in it. Your prompt helpfully tells you your current directory -- in this case, your current directory is ~, which stands for the home directory.

Making New Directories

Our next command is called mkdir. Try typing the following command into your terminal:

mkdir example

The mkdir command makes a new directory (i.e. makes a new folder). Notice that, unlike ls, we don't just type mkdir and press enter. We also need to specify an argument to the command (the argument is example in this example). For mkdir, the argument is the name of the directory we want to create.

Some commands always require arguments to work, like mkdir. Other commands can work just fine without supplying any arguments, like ls.

Now that we've made our example directory, let's make sure it is actually there. Use the ls command to verify that example shows up in our list of directories.

Moving to other directories

To move into another directory, we use the cd command. Try typing the following command into your terminal:

cd example

The cd command will change directories -- in other words, it moves you into the specified folder. In the example above, we chose to move into the example directory.

Notice that the ~ in your prompt turned into ~/example. Again, the prompt will tell you what your current directory is. In this case, we are in the example directory, which is located within the home directory (the ~).

If you use the ls command now, you'll notice that no output shows up. This makes sense, since we just created the example directory and we haven't added any files to it. We'll come back to this later.

For now, let's get back to our home directory. There are a few ways to do this:

Type cd .. (two dots). The .. means "the parent directory". In this case, the parent directory of example happens to be our home directory, so we can use cd .. to go up one directory.
Type cd ~ (the tilde). Remember that ~ means home directory, so this command tells your terminal to change to the home directory, no matter where you currently are.
Type cd (that is, the cd command with no arguments). In UNIX, typing just cd is a shortcut for typing cd ~.

Removing Directories

We now know how to see, create, and move to directories. Our last command involving directories will be to delete them using the rm command.

First, let's create a exampleorary directory:

mkdir tmp

If you use the ls command, you should now see tmp listed as a directory.

Next, let's delete the tmp directory:

rm -r tmp

The rm command will remove files and directories from your filesystem. By itself (that is, without the -r) the rm command only removes files. However, since we are removing a directory, we need to specify -r to recursively remove the tmp directory and any files that tmp might contain (the process is called "recursive" because, in order to remove tmp, we have to remove everything inside of tmp).

As you've seen, some commands require arguments, like mkdir. Other commands do not require any arguments in order to work, like ls. In addition, most commands can also be given flags, like the -r for rm. Flags are ways to specify modified behavior for commands -- for example, rm by itself only removes files; using -r tells rm to remove directories.

Summary of directories

So far, we have learned how to do the following with directories (folders):

ls: list the files and folders inside of the current directory
mkdir: make a new directory
cd: change directories
rm -r: remove a specified directory

Files

Directories are not very useful if they don't contain any files. In this section, we walk through some more commands that allow you to interact with files.

For this section, let's start back in our home directory. Recall that we can do this by simply typing

cd

into our terminal. Your prompt should now say ~.

Creating files

There are many different ways to create files. For this class, you will usually be using a text editor to directly write the file, much like how you would edit a Word document in Microsoft Word.

For now, we'll just download a file called unix.txt, which can be found here. In this class, you will start most homeworks and projects by downloading a file.

The default location for downloads on the school computers is in the Downloads directory. Let's change into that directory using our cd command.

cd ~/Downloads

You can use the ls command to verify your unix.txt is in this directory.

Moving files

On Windows and Mac, much of your interaction with files is likely spent dragging them from folder to folder. UNIX provides a way to move files with the mv command.

Remember that we created a directory called example. Let's move unix.txt into example:

mv unix.txt ~/example

The mv command moves one file/directory into another file/directory. Here, we are moving the unix.txt file into the example directory, which is inside the home directory.

To verify that the mv command work, do the following:

Use ls to check that unix.txt is no longer in our current directory (which is the Downloads directory).
Change into the example directory. Your prompt should now show ~/example.
Use ls to verify that unix.txt shows up in example.

Reading a file: the quick and easy way

Files are useful because they contain information. Let's see what unix.txt contains. Type in the following command:

cat unix.txt

This prints out a list of all the useful UNIX commands we've seen so far. The cat command prints the contents of a file to the screen. This is a fast way to verify that a file is correct or to read what a file contains. For example, if you forget any UNIX commands in in this article, you can quickly cat unix.txt to read about them.

Renaming files

To rename files on Windows or MacOS, you would click on the name of the file and type in the new name.

Renaming files with the terminal can be a little confusing at first. Try the following command in the terminal (from the example directory)

mv unix.txt unix_commands.txt

Using ls, you'll see that unix.txt is gone -- in its place is a file called unix_commands.txt. Furthermore, typing cat unix_commands.txt will print out the same list of UNIX commands.

It appears that we renamed unix.txt to unix_commands.txt by using the mv command! Here's how to think about it:

mv will move the contents of a file/directory into another file/directory. In the previous section, we moved a file into a directory.
This time, we are moving the contents of a file (unix.txt) into another file (unix_commands.txt). While we are technically moving file contents, this is effectively the same thing as renaming a file!

This can be a bit confusing if you're seeing it for the first time, so make sure you understand it before you move onto the next section.

Note: Suppose you already have two files, alice.txt and bob.txt, and you issue the command:
mv alice.txt bob.txt
This will overwrite the old contents of bob.txt with the contents of alice.txt! UNIX won't warn you about overwriting, so be careful when using the mv command!

Copying files

Sometimes, it is useful to have multiple copies of a file. Try the following command:

cp unix_commands.txt new_file.txt

The cp command copies the contents of one file into another file. Using ls, you will see that the example directory now contains two files, unix_commands.txt and new_file.txt. Using cat will verify that both files have the same contents.

Suppose we also wanted to copy the unix_commands.txt file to our home directory. Here's one way to do it:

Change back to the home directory. (challenge: try doing this without looking up the command!)
Next, use the following command:
```
cp example/unix_commands.txt .
```
Don't forget the dot at the end!

The first argument (example/unix_commands.txt) tells the terminal to look in the example directory to find unix_commands.txt.

The second argument . tells the terminal to copy unix_commands.txt to the directory .. Just as two dots (..) represents the parent directory, a single dot (.) represents the current directory (the directory we're in right now).

Now that we're in the home directory, we can use ls to verify that there is a copy of unix_commands.txt:

user@computer:~$ ls
Desktop/ ... unix_commands.txt ...

Using cat unix_commands.txt will show the same output of UNIX commands.

Recap: we've seen two special directories: two dots .. represents the parent directory (one directory up), while a single dot . represents the current directory. You can use these special expressions with any command that deals with directories. For example, you can mv a file to the current directory with the command
mv some_file .

Removing files

Recall the rm command we introduced earlier. We originally used the -r flag to remove directories. Now we will use rm without the -r to remove a file. Type this into your terminal:

rm unix_commands.txt

This will delete the copy of unix_commands.txt that is in our current directory (which is the home directory). A quick ls will show you that unix_commands.txt is gone.

Warning: Unlike on Windows and MacOS, there is no friendly Recycle Bin or Trash from which you can restore deleted files. In UNIX, when you rm a file, it's gone. You can't "undo" rm, so think twice (and thrice!) before using the rm command!.

Summary of files

In this section, we learned the basics of manipulating files:

cat: displays the contents of a file on the screen
mv: moves a file/directory to another file/directory. When moving one file to another, we are effectively renaming the file!
cp: copies a file to another file/directory.
rm: removes a file. When using the -r flag, rm will delete directories.

In addition, we learned about two special directories: .. (the parent directory) and . (the current directory).

Getting Help

If you ever come across a terminal command with which you are unfamiliar, you can use a command called man:

man ls

The man command will show the manual pages (reference pages) for another command. In the example above, we ask the terminal to show the manual pages for the ls command. As you skim through the manual pages, you'll notice that ls can do a lot more than just list the contents of a directory! man is a great way to learn more about new commands and even commands that you think you already know.

Note: Some school computers do not have the man command installed, so you might get an error. That's okay -- if man ever fails, Google is your friend!

Conclusion

While the primary programming language in CS 61A is Python, it is important to know how to navigate through the UNIX filesystem to manage your class assignments. You will also be interacting with the Python interpreter from your terminal, whether you are using a school computer or your home computer.

In addition, if you continue with computer science after 61A, you will definitely interact more with UNIX and the terminal.

Appendix

Terminology

Terminal: a program that allows users to enter commands to control the computer
Prompt: displays certain information every time the terminal is ready to receive new commands. For example, your prompt might look something like this:
```
user@computer~$
```
Usually, prompts will tell you your current directory (in the example above, the current directory is ~)
Directory: the same thing as a folder. Directories can contain files as well as other directories
Parent directory: the directory that is immediately above the current directory (i.e. one directory up). This is represented in UNIX as two dots, ..
Current directory: the directory that we are currently looking at. This is represented in UNIX as a single dot, .
Home directory: the top-level directory that contains all of your files and sub-directories. This is represented in UNIX as a tilde, ~.

UNIX commands

Directories:
- ls: list the files and folders inside of the current directory
- mkdir: make a new directory. For example, mkdir example creates a directory called example
- cd: change directories. For example, cd example changes directories to example
- rm -r: recursively remove a specified directory. For example, rm -r example removes the example directory and all files and subdirectories inside it.
Files:
- cat: displays the contents of a file on the screen. For example, cat unix.txt shows the contents of the file unix.txt
- mv: moves a file/directory to another file/directory. For example, mv file1 file2 moves the contents of file1 into a (possibly new) file called file2. When moving one file to another, we are effectively renaming the file!
- cp: copies a file to another file/directory. For example, cp file1 file2 copies the contents of file1 into a file named file2.
- rm: removes a file. For example, rm file1 deletes the file called file1.
Miscellaneous:
- echo: displays words on the screen
- man: displays manual pages for a specified command