Anilrgowda

Basic Linux concepts.

Linux is a multitasking, multiuser operating system, which means that many people can run many different applications on one computer at the same time. This differs from MS-DOS, where only one person can use the system at any one time. Under Linux, to identify yourself to the system, you must log in, which entails entering your login name (the name the system uses to identify you), and entering your password, which is your personal key for logging in to your account. Because only you know your password, no one else can log in to the system under your user name.
On traditional UNIX systems, the system administrator assigns you a user name and an initial password when you are given an account on the system. However, because in Linux tt you are the system administrator, you must set up your own account before you can log in. For the following discussions, we'll use the imaginary user name, ``larry.''
In addition, each system has a host name assigned to it. It is this host name that gives your machine a name, gives it character and charm. The host name is used to identify individual machines on a network, but even if your machine isn't networked, it should have a host name. For our examples below, the system's host name is ``mousehouse''.

3.2.1 Creating an account.

Before you can use a newly installed Linux system, you must set up a user account for yourself. It's usually not a good idea to use the root account for normal use; you should reserve the root account for running privileged commands and for maintaining the system as discussed below.
In order to create an account for yourself, log in as root and use the useradd or adduser command. See Section 4.6 for information on this procedure.

3.2.2 Logging in.

At login time, you'll see a prompt resembling the following:



Enter your user name and press the Enter key. Our hero, larry, would type:



Next, enter your password. The characters you enter won't be echoed to the screen, so type carefully. If you mistype your password, you'll see the message

and you'll have to try again.
Once you have correctly entered the user name and password, you are officially logged in to the system, and are free to roam.

3.2.3 Virtual consoles.

The system's console is the monitor and keyboard connected directly to the system. (Because Linux is a multiuser operating system, you may have other terminals connected to serial ports on your system, but these would not be the console.) Linux, like some other versions of UNIX, provides access to virtual consoles (or VCs), that let you have more than one login session on the console at one time.
To demonstrate this, log in to your system. Next, press Alt-F2. You should see the login: prompt again. You're looking at the second virtual console. To switch back to the first VC, press Alt-F1. Voila! You're back to your first login session.
A newly-installed Linux system probably lets you to access only the first half-dozen or so VCs, by pressing Alt-F1 through Alt-F4, or however many VCs are configured on your system. It is possible to enable up to 12 VCs--one for each function key on your keyboard. As you can see, use of VCs can be very powerful because you can work in several different sessions at the same time.
While the use of VCs is somewhat limiting (after all, you can look at only one VC at a time), it should give you a feel for the multiuser capabilities of Linux. While you're working on the first VC, you can switch over to the second VC and work on something else.

3.2.4 Shells and commands.

For most of your explorations in the world of Linux, you'll be talking to the system through a shell, a program that takes the commands you type and translates them into instructions to the operating system. This can be compared to the COMMAND.COM program under MS-DOS, which does essentially the same thing. A shell is just one interface to Linux. There are many possible interfaces--like the X Window System, which lets you run commands by using the mouse and keyboard.
As soon as you log in, the system starts the shell, and you can begin entering commands. Here's a quick example. Larry logs in and is waiting at the shell prompt.

The last line of this text is the shell's prompt, indicating that it's ready to take commands. (More on what the prompt itself means later.) Let's try telling the system to do something interesting:



Well, as it turns out, make is the name of an actual program on the system, and the shell executed this program when given the command. (Unfortunately, the system was being unfriendly.)
This brings us to the burning question: What is a command? What happens when you type ``make love''? The first word on the command line, ``make'', is the name of the command to be executed. Everything else on the command line is taken as arguments to this command. Example:

The name of this command is ``cp'', and the arguments are ``foo'' and ``bar''.
When you enter a command, the shell does several things. First, it checks the command to see if it is internal to the shell. (That is, a command which the shell knows how to execute itself. There are a number of these commands, and we'll go into them later.) The shell also checks to see if the command is an alias, or substitute name, for another command. If neither of these conditions apply, the shell looks for a program, on disk, having the specified name. If successful, the shell runs the program, sending the arguments specified on the command line.
In our example, the shell looks for a program called make, and runs it with the argument love. Make is a program often used to compile large programs, and takes as arguments the name of a ``target'' to compile. In the case of ``make love'', we instructed make to compile the target love. Because make can't find a target by this name, it fails with a humorous error message, and returns us to the shell prompt.
What happens if we type a command to a shell and the shell can't find a program having the specified name? Well, we can try the following:

Quite simply, if the shell can't find a program having the name given on the command line (here, ``eat''), it prints an error message. You'll often see this error message if you mistype a command (for example, if you had typed ``mkae love'' instead of ``make love'').

3.2.5 Logging out.

Before we delve much further, we should tell you how to log out of the system. At the shell prompt, use the command

to log out. There are other ways of logging out, but this is the most foolproof one.

3.2.6 Changing your password.

You should also know how to change your password. The command passwd prompts you for your old password, and a new password. It also asks you to reenter the new password for validation. Be careful not to forget your password--if you do, you will have to ask the system administrator to reset it for you. (If you are the system administrator, see page .)

3.2.7 Files and directories.

Under most operating systems (including Linux), there is the concept of a file, which is just a bundle of information given a name (called a filename). Examples of files might be your history term paper, an e-mail message, or an actual program that can be executed. Essentially, anything saved on disk is saved in an individual file.
Files are identified by their file names. For example, the file containing your history paper might be saved with the file name history-paper. These names usually identify the file and its contents in some form that is meaningful to you. There is no standard format for file names as there is under MS-DOS and some other operating systems; in general, a file name can contain any character (except the / character--see the discussion of path names, below) and is limited to 256 characters in length.
With the concept of files comes the concept of directories. A directory is a collection of files. It can be thought of as a ``folder'' that contains many different files. Directories are given names, with which you can identify them. Furthermore, directories are maintained in a tree-like structure; that is, directories may contain other directories.
Consequently, you can refer to a file by its path name, which is made up of the filename, preceded by the name of the directory containing the file. For example, let's say that Larry has a directory called papers, which contains three files: history-final, english-lit, and masters-thesis. Each of these three files contains information for three of Larry's ongoing projects. To refer to the english-lit file, Larry can specify the file's pathname, as in:


As you can see, the directory and filename are separated by a single slash (/). For this reason, filenames themselves cannot contain the / character. MS-DOS users will find this convention familiar, although in the MS-DOS world the backslash (\) is used instead.
As mentioned, directories can be nested within each other as well. For example, let's say that there is another directory within papers, called notes. The notes directory contains the files math-notes and cheat-sheet. The pathname of the file cheat-sheet would be


Therefore, a path name is really like a path to the file. The directory that contains a given subdirectory is known as the parent directory. Here, the directory papers is the parent of the notes directory.

3.2.8 The directory tree.

Most Linux systems use a standard layout for files so that system resources and programs can be easily located. This layout forms a directory tree, which starts at the ``/'' directory, also known as the ``root directory''. Directly underneath / are important subdirectories: /bin, /etc, /dev, and /usr, among others. These directories in turn contain other directories which contain system configuration files, programs, and so on.
In particular, each user has a home directory, which is the directory set aside for that user to store his or her files. In the examples above, all of Larry's files (like cheat-sheet and history-final) are contained in Larry's home directory. Usually, user home directories are contained under /home, and are named for the user owning that directory. Larry's home directory is /home/larry.
The diagram on page shows a sample directory tree, which should give you an idea of how the directory tree on your system is organized.
=1.0pt

Figure 3.1: A typical (abridged) Linux directory tree.


3.2.9 The current working directory.

At any moment, commands that you enter are assumed to be relative to your current working directory. You can think of your working directory as the directory in which you are currently ``located''. When you first log in, your working directory is set to your home directory--/home/larry, in our case. Whenever you refer to a file, you may refer to it in relationship to your current working directory, rather than specifying the full pathname of the file.
Here's an example. Larry has the directory papers, and papers contains the file history-final. If Larry wants to look at this file, he can use the command

The more command simply displays a file, one screen at a time. However, because Larry's current working directory is /home/larry, he can instead refer to the file relative to his current location by using the command

If you begin a filename (like papers/final) with a character other than /, you're referring to the file in terms relative to your current working directory. This is known as a relative path name.
On the other hand, if you begin a file name with a /, the system interprets this as a full path name--that is, a path name that includes the entire path to the file, starting from the root directory, /. This is known as an absolute path name.

3.2.10 Referring to home directories.

Under both tcsh and bash you can specify your home directory with the tilde character (~). For example, the command

is equivalent to

The shell replaces the ~ character with the name of your home directory.
You can also specify other user's home directories with the tilde character. The pathname ~karl/letters translates to /home/karl/letters by the shell (if /home/karl is karl's home directory). Using a tilde is simply a shortcut; there is no directory named ~--it's just syntactic sugar provided by the shell.


3.3 First steps into Linux.

Before we begin, it is important to know that all file and command names on a Linux system are case-sensitive (unlike operating systems such as MS-DOS). For example, the command make is very different from Make or MAKE. The same is true for file and directory names.

3.3.1 Moving around.

Now that you can log in, and you know how to refer to files using pathnames, how can you change your current working directory, to make life easier?
The command for moving around in the directory structure is cd, which is short for ``change directory''. Many often-used Linux commands are two or three letters. The usage of the cd command is

where directory is the name of the directory which you wish to become the current working directory.
As mentioned earlier, when you log in, you begin in your home directory. If Larry wanted to switch to the papers subdirectory, he'd use the command

As you can see, Larry's prompt changes to reflect his current working directory (so he knows where he is). Now that he's in the papers directory, he can look at his history final with the command


Now, Larry is stuck in the papers subdirectory. To move back up to the next higher (or parent) directory, use the command

(Note the space between the ``cd'' and the ``..''.) Every directory has an entry named ``..'' which refers to the parent directory. Similarly, every directory has an entry named ``.'' which refers to itself. Therefore, the command

gets us nowhere.
You can also use absolute pathnames with the cd command. To cd into Karl's home directory, we can use the command


Also, using cd with no argument will return you to your own home directory.



3.3.2 Looking at the contents of directories.

Now that you know how to move around directories, you might think, ``So what?'' Moving around directories is fairly useless by itself, so let's introduce a new command, ls. The ls command displays a listing of files and directories, by default from your current directory. For example:



Here we can see that Larry has three entries in his current directory: Mail, letters, and papers. This doesn't tell us much--are these directories or files? We can use the -F option of the ls command to get more detailed information.


From the / appended to each filename, we know that these three entries are in fact subdirectories.
Using ls -F may also append ``*'' to the end of a filename in the resulting list which would indicate that the file is an executable, or a program which can be run. If nothing is appended to the filename using ls -F, the file is a ``plain old file'', that is, it's neither a directory nor an executable.
In general, each UNIX command may take a number of options in addition to other arguments. These options usually begin with a ``-'', as demonstrated above with the -F option. The -F option tells ls to give more information about the type of the files involved--in this case, printing a / after each directory name.
If you give ls a directory name, the system will print the contents of that directory.


Or, for a more interesting listing, let's see what's in the system's /etc directory.



If you're a MS-DOS user, you may notice that the filenames can be longer than 8 characters, and can contain periods in any position. You can even use more than one period in a filename.
Let's move to the top of the directory tree, and then down to another directory with the commands


You can also move into directories in one step, as in cd /usr/bin.
Try moving around various directories, using ls and cd. In some cases, you may run into the foreboding ``Permission denied'' error message. This is simply UNIX security kicking in: in order to use the ls or cd commands, you must have permission to do so. We talk more about this starting on page .

3.3.3 Creating new directories.

It's time to learn how to create directories. This involves the use of the mkdir command. Try the following:



Congratulations! You made a new directory and moved into it. Since there aren't any files in this new directory, let's learn how to copy files from one place to another.

3.3.4 Copying files.

To copy files, use the command cp, as shown here:



The cp command copies the files listed on the command line to the file or directory given as the last argument. Notice that we use ``.'' to refer to the current directory.

3.3.5 Moving files.

The mv command moves files, rather than copying them. The syntax is very straightforward:



Notice that the termcap file has been renamed sells. You can also use the mv command to move a file to a completely new directory.
Note: mv and cp will overwrite a destination file having the same name without asking you. Be careful when you move a file into another directory. There may already be a file having the same name in that directory, which you'll overwrite!

3.3.6 Deleting files and directories.

You now have an ugly rhyme developing with the use of the ls command. To delete a file, use the rm command, which stands for ``remove'', as shown here:



We're left with nothing but shells, but we won't complain. Note that rm by default won't prompt you before deleting a file--so be careful.
A related command to rm is rmdir. This command deletes a directory, but only if the directory is empty. If the directory contains any files or subdirectories, rmdir will complain.

3.3.7 Looking at files.

The commands more and cat are used for viewing the contents of files. more displays a file, one screenful at a time, while cat displays the whole file at once.
To look at the file shells, use the command


In case you're interested what shells contains, it's a list of valid shell programs on your system. On most systems, this includes /bin/sh, /bin/bash, and /bin/csh. We'll talk about these different types of shells later.
While using more, press Space to display the next page of text, and b to display the previous page. There are other commands available in more as well, these are just the basics. Pressing q will quit more.
Quit more and try cat /etc/termcap. The text will probably fly by too quickly for you to read it all. The name ``cat'' actually stands for ``concatenate'', which is the real use of the program. The cat command can be used to concatenate the contents of several files and save the result to another file. This will be again in section 3.14.1.

3.3.8 Getting online help.

Almost every UNIX system, including Linux, provides a facility known as manual pages. These manual pages contain online documentation for system commands, resources, configuration files and so on.
The command used to access manual pages is man. If you're interested in learning about other options of the ls command, you can type

and the manual page for ls will be displayed.
Unfortunately, most manual pages are written for those who already have some idea of what the command or resource does. For this reason, manual pages usually contain only the technical details of the command, without much explanation. However, manual pages can be an invaluable resource for jogging your memory if you forget the syntax of a command. Manual pages will also tell you about commands that we don't cover in this book.
I suggest that you try man for the commands that we've already gone over and whenever I introduce a new command. Some of these commands won't have manual pages, for several reasons. First, the manual pages may not have been written yet. (The Linux Documentation Project is responsible for manual pages under Linux as well. We are gradually accumulating most of the manual pages available for the system.) Second, the the command might be an internal shell command, or an alias (discussed on page ), which would not have a manual page of its own. One example is cd, which is an internal shell command. The shell itself actually processes the cd--there is no separate program that implements this command.

Accessing MS-DOS files.

If, for some twisted and bizarre reason, you want to access files from MS-DOS, it's easily done under Linux.
The usual way to access MS-DOS files is to mount an MS-DOS partition or floppy under Linux, allowing you to access the files directly through the file system. For example, if you have an MS-DOS floppy in /dev/fd0, the command

will mount it under /mnt. See Section 4.8.4 for more information on mounting floppies.
You can also mount an MS-DOS partition of your hard drive for access under Linux. If you have an MS-DOS partition on /dev/hda1, the command

mounts it. Be sure to umount the partition when you're done using it. You can have a MS-DOS partition automatically mounted at boot time if you include the entry in /etc/fstab. See Section 4.4 for details. The following line in /etc/fstab will mount an MS-DOS partition on /dev/hda1 on the directory /dos.


You can also mount the VFAT file systems that are used by Windows 95:

This allows access to the long filenames of Windows 95. This only applies to partitions that actually have the long filenames stored. You can't mount a normal FAT16 file system and use this to get long filenames.
The Mtools software may also be used to access MS-DOS files. The commands mcd, mdir, and mcopy all behave like their MS-DOS counterparts. If you install Mtools, there should be manual pages available for these commands.
Accessing MS-DOS files is one thing; running MS-DOS programs is another. There is an MS-DOS Emulator under development for Linux; it is widely available, and included in most distributions. It can also be retrieved from a number of locations, including the various Linux FTP sites listed in Appendix B. The MS-DOS Emulator is reportedly powerful enough to run a number of applications, including WordPerfect, from Linux. However, Linux and MS-DOS are vastly different operating systems. The power of any MS-DOS emulator under UNIX is limited. In addition, a Microsoft Windows emulator that runs under X Windows is under development.

3.5 Summary of basic UNIX commands.

This section introduces some of the most useful basic commands of a UNIX system, including those that are covered in the previous section.
Note that options usually begin with ``-'', and in most cases you can specify more than one option with a single ``-''. For example, rather than use the command ls -l -F, you can use ls -lF.
Rather than listing every option for each command, we only present useful or important commands at this time. In fact, most of these commands have many options that you'll never use. You can use man to see the manual pages for each command, which list all of the available options.
Also note that many of these commands take as arguments a list of files or directories, denoted in this table by ``file1 ...fileN''. For example, the cp command takes as arguments a list of files to copy, followed by the destination file or directory. When copying more than one file, the destination must be a directory.