Basic Linux Usage

Preliminaries and Getting Help

First some preliminaries. The commands described here represent Linux/GNU system, i.e. the Linux kernel for interacting with the hardware, and several sets of basic user-level commands provided by the GNU project. Each of the commands typically implements an -? or --help option, which describes how to invoke it, e.g. ls --help. Each user-level command should have a manual page that can be viewed by the man command, e.g. man ls. In most cases these commands are binary executables or scripts that reside somewhere on the file system. To find the exact location use the which, e.g. which ls. You can use the file command to determine the type of the file. The pwd command prints the current directory. The whoami command prints your username (used for log-in). The hostname command prints the name of the machine you are working on. The uptime command prints for how long the machine has been running, since the last reboot. Some commands aren't executables but aliases to other command. Use alias to print them all. Some examples:

$ ls --help 
Usage: ls [OPTION]... [FILE]...
List information about the FILEs (the current directory by default).
...
$ man ls
... shows the man page ...
$ pinfo ls
... shows the more detailed info page (use q to exit) ...
$ which ls
/bin/ls
$ file /bin/ls
/bin/ls: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.6.33, BuildID[sha1]=0x88bca03aed9a306510a0bbcde1ea165e1d54c1ce, stripped

Both the man and pinfo command will open a text base document viewer in your terminal to read the man/info pages. Press the q key to exit.

	Online man pages
	Several web pages provide browsable, online versions of Linux man pages, e.g. Linux man pages . However, you need to check that the version of the command matches the one documented on these pages. Most commands provide a --version option for this purpose.

You are now ready to execute simple commands. Don't be scared. Follow the examples in the next section to view the contents of your home directory and to navigate in the file system.

First Steps: Files, Directories and Navigation

	Linux quick-reference:
	I recommend you open this Linux quick-reference in a separate window, to have an explanation for the most common commands at hand.

In most operating systems, documents are organised in a tree structure with directories as nodes in the tree and files as leaves in the tree. This means that each directory contains an arbitrary number of files and (sub-)directories. After login and launching a terminal, you'll be in your home directory. The command pwd will print the current directory. Use the ls command to list all files in the directory, add the options -la (-l for long and -a for all) to give more information for all files. Each line shows whether this is a directory (d) and the permissions in the first column, username and groupname of the owner in columns 3 and 4. Note that option -a shows all files, including hidden files that start with a dot. Without this option hidden files won't be shown. An important hidden file is .bashrc, which contains shell code that is executed whenever you open a new shell. For an explanation of the letters encoding permissions (first column) see the the Section called Security.

$ ls 
Documents/  Downloads/  public_html/  tmp/
$ ls -la 
total 20
drwxr-xr-x 1 newuser newuser 300 Aug 14 20:34 ./
drwxr-xr-x 1 root    root     80 Aug 14 20:33 ../
-rw-r--r-- 1 newuser newuser  24 May  9  2011 .bash_logout
-rw-r--r-- 1 newuser newuser 191 May  9  2011 .bash_profile
-rw-r--r-- 1 newuser newuser 124 May  9  2011 .bashrc
drwxrwxr-x 1 newuser newuser  60 Aug 14 20:34 Documents/
drwxrwxr-x 1 newuser newuser  60 Aug 14 20:34 Downloads/
-rw-r--r-- 1 newuser newuser  16 Jun  1 19:52 .draksnapshot
drwxr-xr-x 1 newuser newuser  60 Jun  1 19:47 .gnome2/
drwxr-xr-x 1 newuser newuser  80 Jun  1 19:52 .MgaOnline/
drwxr-xr-x 1 newuser newuser  60 Jun  1 19:47 .mozilla/
drwxrwxr-x 1 newuser newuser  60 Aug 14 20:34 public_html/
drwx------ 1 newuser newuser  60 Jan 11  2011 tmp/
-rw------- 1 newuser newuser  72 Aug 14 20:34 .xauthMvdoYh
$ls -ltr 
total 0
drwx------ 1 newuser newuser 60 Jan 11  2011 tmp/
drwxrwxr-x 1 newuser newuser 60 Aug 14 20:34 public_html/
drwxrwxr-x 1 newuser newuser 60 Aug 14 20:34 Downloads/
drwxrwxr-x 1 newuser newuser 60 Aug 14 20:34 Documents/

The exact contents that you see in your home directory, as well as the name of the home directory will differ from the output above. You should see the same structure, though. In particular, you should see a file public_html, which we will use in our running example of your own web page.

To navigate between directories use the cd (change directory). To copy files use cp and to move files use mv. Either specify files and directories, giving the full path, or use . to refer to the current directory and .. to the parent directory. For example, the following enters the public_html directory, makes a copy of the index.html in it (so that we can overwrite it later in this intro), and then goes back to the parent directory.

$ cd public_html
$ cp index.html index-org.html
$ ls 
index.html  index-org.html  
$ cd ..

If the cp above returns an error message like this: cp: cannot stat `index.html': No such file or directory don't worry: it means that there was no initial file to backup, to avoid overwriting it below. You can proceed with the next commands below.

	Side remark: Filename completion
	To save typing, in particular if you need to refer to long filenames or deep pathes, use filename completion: start typing the filename, e.g. ind, and then hit TAB, which will complete the filename if it is unique, or show all possible matches. You can also use it to show all commands starting with the typed letters, e.g. ma TAB, will print all commands starting with ma.

For frequently changing between directories, it is often useful to keep a stack of recently visited directories while navigating through the filesystem. To do that use pushd, which behaves like cd but remembers the directory you go to. Later, popd can be used to go back to that directory, or more precisely it jumps to the directory that's top of the stack of remebered directories. To show all directories on that stack, type dirs. For example, the following jumps to your Desktop directory, containing all files that you see on your graphical desktop, then lists all files (most recent last), prints the directory stack and then jumps back to the directory you started from. Note that ~ stands for your home directory, and you can use it jump directly to your home directory doing cd ~.

$ pushd Desktop/
~/Desktop ~
$ ls -ltr
total 3
-rwxr-xr-x 1 live live 1444 Jun  1 19:52 register.desktop*
-rwxr-xr-x 1 live live 1234 Jun  1 19:52 draklive-install.desktop*
$ dirs
~/Desktop ~
$ popd

	Linux Practical
	If this is confusing to you, work through this practical before you continue with the Linux Introduction: Linux Practical.

At this point you should be able to run basic commands from the command line.

	Exercises
	To exercise this knowledge, go to your home directory and show all files. Answer these questions: How many files do you have in the current directory? Which of these files are directories? What's the name of the most recent file? Which combination of options would you use, to list all files in a directory, sorted by size with largest file last, showing full information for each file?

	More on file access:
	A detailed discussion of the most important commands for accessing files and directories is given in the early sections of Chapter four in Sobell's textbook. Most online tutorials on Linux cover these commands in quite some detail as well (e.g. UNIX Tutorial for Beginners).

Our Running Example: Build your Own Web Page

As a simple example, let's create your own home page. By convention, all your files visible within a web browser are stored in the directory public_html in your home directory. Such a directory should exist already. Check that by typing

$ ls public_html

$ mkdir public_html

The following commands will copy a template for a web page from the departmental server jove to a new directory public/LinuxIntro in your home directory. This should work both on a departmental Linux machine and also in the MACS Linux VM image.

The first step is to go to your home directory, create a directory with name public and then enter this new directory:

$ pushd ~
$ mkdir public
$ cd public 

Now create another new directory called LinuxIntro and enter this directory:

$ mkdir LinuxIntro
$ cd LinuxIntro

Finally, copy over the template files from a central location into the current directory. The command scp performs a secure copy over the network, using a format of: uaername@machinename:directory. For a local copy of files you would use the command cp.

Don't forget the . ('dot') at the end of all cp commands below!

$ scp -q -r `whoami`@jove.macs.hw.ac.uk:/home/msc/public/LinuxIntro/www .

Finally, you want to go back to the place (directory) where you started from. The popd retrives the directory of the most recent pushd, and goes to this directory.

$ popd

Executing scp will ask you for a password: use your departmental user-name and password for that. You may have to replace `whoami` with your user-name on the departmental Linux machines, if your user-name on the MACS VM is different from the one on the departmental Linux machines

Now, the following, main steps will bring you into the directory for the web page (public_html), using the command cd, copy these sample files for your home page (using the command cp), and rename them so that they are used as your home page.

$ cd ~
$ # go into the directory containing your web page
$ cd public_html
$ # copy the template files for your web page: main page (HTML), stylesheets (CSS) and images
$ cp ~/public/LinuxIntro/www/samples/redandgreen/index.html .
$ cp ~/public/LinuxIntro/www/samples/redandgreen/style.css .
$ cp -r ~/public/LinuxIntro/www/samples/redandgreen/images .

To make the file visible to a web server, you need to set its access permissions appropriately using the chmod command: in this case it gives all users (a) read (r) access to this file. See the Section called Security for more details on how to set permissions.

$ # make the files accessible to the web server
$ chmod a+r index.html 
$ chmod go-w index.html 

Finally, launch a web browser, such as firefox, and point it to the local file.

$ # view the web page (see alternatives below)
$ firefox file://`pwd`/index.html

Note the . ('dot') at the end of all cp commands above! If you see an error message like this $ cp: missing destination file operand after `~/public/LinuxIntro/www/samples/redandgreen/index.html' you have forgotten the dot at the end of the line.

If you are working on one of the Linux Lab machines, and have put all your files into your public_html directory, you can type the URL into the address field of a running browser, or point the web browser to your home page when you launch it, typing

$ firefox http://www2.macs.hw.ac.uk/~`whoami`

Congratulations, you now have a home page, visible to everyone on the web!

At this point you should have a web browser running and see a red-and-green web page, showing the sample page you just copied over into your public_html directory.

All editing of your web pages can be done directly in this directory.

Alternative (minimal) sample files for your home page

The example above uses a web page with a style-sheet file and images for a pretty design. Alternatively, you can go through the same example using a minimal, single-file web page. The following command should work both on the Linux lab machines and on the MACS VM: $ scp -q `whoami`@jove.macs.hw.ac.uk:/home/msc/public/LinuxIntro/www/samples/index-minimal.html index.html You may have to replace `whoami` with your user-name on the departmental Linux machines.

Editing Files

Now, you will want to fill in some contents. To do so, you will need to edit the file index.html. The editor of choice on most Linux system is emacs, so you can start editing by typing (use the option -nw to start the editor in the terminal rather than in an own window):

$ emacs index.html 

	Editing Files: Emacs cheat sheet
	You can find an emacs cheat sheet, summarising its most important commands, here.

Now, in the editor modify the index.html file and change title and top-level heading to indicate that this is your web page: search for the text This is my home page and modify it. Add a 1 paragraph description about yourself, replacing the text Some text. You need only very basic knowledge about HTML to do this. Essentially, you just have to fill in the blanks of the sample page.

	HTML cheat sheet
	If you want to do more complex changes, look up this HTML cheat sheet.

Now, go back to your web browser, the firefox window, and reload the page. You'll see that the text has changed, corresponding to your edits.

A note on editors: There are many more editors available on standard Linux systems: gedit (a graphical editor that is part of the Gnome infrastructure), vim (a very flexible text editor), pico (a small and simple text editor), nano (a pico clone and GNU software). You can call each editor from the command-line, and some can be used in batch mode for repeated processing of text files. Also, check the Accessoires section in the top-level menu of your window-manager (typically, bottom left menu) to get list of available editors.

In most cases, a text editor is all you need to edit text files using ASCII encoding. In some cases, such as Coursework 1 of F21CN, you need to edit individual bytes in a non-ASCII files. There are specialised editors, hex-editors, to do this. You can start the shed hex-editor directly from the command line: $ shed sample.bmp

At this point, you should have your own web page, created from the red-and-green sample page. If your user name is xyz12, you can view this page by typing the following URL in your web browser:

http://www2.macs.hw.ac.uk/~xyz12

Some Exercises

Basic Exercises

Answer the following questions by using and interpreting the appropriate command on the commandline: How many files and directories do you have in your home directory (top level only)? Which files in your top directory can be read by any other user on the system? Which directories in your top directory can be entered (with the cd command) by any other user on the system? How much disk space is available on the current partition and on the partition that hosts your home directory?