Unix Shell Tutorial

This is a modified version of Tutorials 1 and 2 of a Unix tutorial from the University of Surrey.

Listing Files and Directories

When you first open a terminal window, your current working directory is your home directory. To find out what files are in your home directory, type:

$ ls

(As with all examples in these pages, the $ is not part of the command. It is meant to evoke the shell’s prompt, and you should type only the characters that come after it.)

There may be no files visible in your home directory, in which case you’ll just see another prompt.

By default, ls will skip some hidden files. Hidden files are not special: they just have filenames that begin with a . character. Hidden files usually contain configurations or other files meant to be read by programs instead of directly by humans. To see everything, including the hidden files, use:

$ ls -a

ls is an example of a command which can take options, a.k.a. flags. -a is an example of an option. The options change the behavior of the command. There are online manual pages that tell you which options a particular command can take, and how each option modifies the behavior of the command. (See later in this tutorial.)

Making Directories

We will now make a subdirectory in your home directory to hold the files you will be creating and using in the course of this tutorial. To make a subdirectory called “unixstuff” in your current working directory type:

$ mkdir unixstuff

To see the directory you have just created, type:

$ ls 

Changing Directories

The command cd [directory] changes the current working directory to [directory]. The current working directory may be thought of as the directory you are in, i.e., your current position in the file-system tree.

To change to the directory you have just made, type:

$ cd unixstuff

Type ls to see the contents (which should be empty).

Exercise. Make another directory inside unixstuff called backups.

The directories . and ..

Still in the unixstuff directory, type

$ ls -a

As you can see, in the unixstuff directory (and in all other directories), there are two special directories called . and ... In UNIX, . means the current directory, so typing:

$ cd .

(with is a space between cd and .) means stay where you are (the unixstuff directory). This may not seem very useful at first, but using . as the name of the current directory will save a lot of typing, as we shall see later in the tutorial.

In UNIX, .. means the parent directory. So typing:

$ cd ..

will take you one directory up the hierarchy (back to your home directory). Try it now!

Typing cd with no argument always returns you to your home directory. This is very useful if you are lost in the file system.

Pathnames

Pathnames enable you to work out where you are in relation to the whole file-system. For example, to find out the absolute pathname of your home-directory, type cd to get back to your home-directory and then type:

$ pwd

pwd means “print working directory”. The full pathname will look something like this:

/home/youruser/unixstuff

which means that unixstuff is inside youruser (your home directory), which is in turn in a directory called home, which is in the “root” top-level directory, called /.

Exercise. Use the commands ls, cd, and pwd to explore the file system.

Understanding Pathnames

First, type cd to get back to your home-directory, then type

$ ls unixstuff

to list the conents of your unixstuff directory.

Now type

$ ls backups

You will get a message like this -

backups: No such file or directory

The reason is, backups is not in your current working directory. To use a command on a file (or directory) not in the current working directory (the directory you are currently in), you must either cd to the correct directory, or specify its full pathname. To list the contents of your backups directory, you must type

$ ls unixstuff/backups

You can refer to your home directory with the tilde ~ character. It can be used to specify paths starting at your home directory. So typing

$ ls ~/unixstuff

will list the contents of your unixstuff directory, no matter where you currently are in the file system.

Summary

Copying Files

cp [file1] [file2] makes a copy of file1 in the current working directory and calls it file2.

We will now download a file from the Web so we can copy it around. First, cd to your unixstuff directory:

$ cd ~/unixstuff

Then, type:

$ curl -O https://www.cs.cornell.edu/robots.txt

The curl command puts this text file into a new file called robots.txt. Now type cp robots.txt robots.bak to create a copy.

Moving Files

mv [file1] [file2] moves (or renames) file1 to file2.

To move a file from one place to another, use the mv command. This has the effect of moving rather than copying the file, so you end up with only one file rather than two. It can also be used to rename a file, by moving the file to the same directory, but giving it a different name.

We are now going to move the file robots.bak to your backup directory.

First, change directories to your unixstuff directory (can you remember how?). Then, inside the unixstuff directory, type:

$ mv robots.bak backups/robots.bak

Type ls and ls backups to see if it has worked.

Removing files and directories

To delete (remove) a file, use the rm command. As an example, we are going to create a copy of the robots.txt file then delete it.

Inside your unixstuff directory, type:

$ cp robots.txt tempfile.txt
$ ls
$ rm tempfile.txt
$ ls

You can use the rmdir command to remove a directory (make sure it is empty first). Try to remove the backups directory. You will not be able to since UNIX will not let you remove a non-empty directory.

Exercise. Create a directory called tempstuff using mkdir , then remove it using the rmdir command.

Displaying the contents of a file on the screen

Before you start the next section, you may like to clear the terminal window of the previous commands so the output of the following commands can be clearly understood. At the prompt, type:

$ clear

This will clear all text and leave you with the $ prompt at the top of the window.

The command cat can be used to display the contents of a file on the screen. Type:

$ cat robots.txt

As you can see, the file is longer than than the size of the window, so it scrolls past making it unreadable.

The command less writes the contents of a file onto the screen a page at a time. Type:

$ less robots.txt

Press the [space-bar] if you want to see another page, and type [q] if you want to quit reading.

The head command writes the first ten lines of a file to the screen.

First clear the screen, then type:

$ head robots.txt

Then type:

$ head -5 robots.txt

What difference did the -5 do to the head command?

The tail command writes the last ten lines of a file to the screen. Clear the screen and type:

$ tail robots.txt

Exercise. How can you view the last 15 lines of the file?

Searching the Contents of a File

Using less, you can search though a text file for a keyword (pattern). For example, to search through robots.txt for the word “jpeg”, type

$ less robots.txt

then, still in less, type a forward slash [/] followed by the word to search

/jpeg

As you can see, less finds and highlights the keyword. Type [n] to search for the next occurrence of the word.

grep is one of many standard UNIX utilities. It searches files for specified words or patterns. First clear the screen, then type:

$ grep jpeg robots.txt

As you can see, grep has printed out each line containing the word “jpeg”.

To search for a phrase or pattern, you must enclose it in single quotes (the apostrophe symbol). For example to search for spinning top, type

$ grep 'web crawlers' robots.txt

Some of the other options of grep are:

• -v: display those lines that do NOT match
• -n: precede each matching line with the line number
• -c: print only the total count of matched lines

Summary

Don’t stop here! We highly recommend completing the online UNIX tutorial, beginning with Tutorial 3.

Manual Pages

Unix has a built-in “help system” for showing documentation about commands, called man. Try typing this:

$ man grep

That command launches less to read more than you ever wanted to know about the grep command. If you want to know how to use a given command, try man <that_command>.

Saving Time on the Command Line

Tab completion is an extremely handy service available on the command line. It can save you time and frustration by avoiding retyping filenames all the time. Say you want to run this command to find all the occurrences of “gif” in robots.txt:

$ grep gif robots.txt

Try just typing part of the command first:

$ grep gif ro

Then hit the [tab] key. Your shell should complete the name of the robots.txt file.

History

Type history at the command line to see your command history.

$ history

The Up Arrow

Use the up arrow on the command line instead of re-typing your most recent command. Want the command before that? Type the up arrow again!

Try it out! Hit the up arrow! If you’ve been stepping through these tips, you’ll probably see the command history.

Ctrl+r

If you need to find a command you typed 10 commands ago, instead of typing the up arrow 10 times, hold the [control] key and type [r]. Then, type a few characters contained within the command you’re looking for. Ctrl+r will reverse search your history for the most recent command that has that string.

Try it out! Assuming you’ve been working your way through all these tutorials, typing Ctrl+r and then grep will show you your last grep command. Hit return to execute that command again.