The Linux Kernel

The kernel is the core part of the operating system. It is the part that loads first, and it stays resident in memory as long as the computer is powered on and running.

The key functions of the kernel include:

• Process Management - creating, suspending, and terminating processes, and maintaining their states.

• Interprocess Communication (IPC) - managing the facilities for processes to communicate with each other. The two main types are:

  • Pipes: temporary channels to allow processes to communicate on the same computer.

  • Sockets: Network (including Internet) channels to allow processes to communicate across the network.

• File Management - the kernel manages the filesystem and the hardware such as disks that the filesystem uses.

• Memory Management - the kernel allocates RAM for itself and all the other processes.

On a typical GNU/Linux system, the kernel is stored in a file named /boot/vmlinux or /boot/vmlinuz. This file is loaded into memory when the computer boots.

The Unix Filesystem

A filesystem is the way that the operating system organizes stored information into files and keeps track of those files. The file is the basic unit of disk storage on a unix system. A file can hold any type of information including text, a program, or digitally encoded music.

On a unix system, all files are organized into directories. A directory is just a special file that stores information about other files. You may find it intuitive to think of a directory as a kind of container that holds files or other directories. Some other operating systems call a directory a folder. The unix filesystem has a hierarchical structure.

At the root of the filesystem is the root directory, written as a slash (/). In this directory are some files (not shown above) and a number of subdirectories such as bin, dev, etc. Each of these subdirectories of / are themselves directories which contain some files and some further subdirectories.

Every file (including directories, which are also files) can be specified in at least two ways. First, there is an absolute path name, which starts with the root and specifies every directory name along the way to the file. An example of this is /home/jbeck/workshop/index.html. All absolute pathnames begin with a leading slash.

Alternatively, you can use a name for a file that is relative to your current working directory. If your current working directory is /home/jbeck and you wish to refer to a file named index.html which is in the subdirectory workshop, you can refer to it as workshop/index.html. Finally, if your current working directory is /home/jbeck/src, you can refer to the file as ../workshop/index.html. The .. means the parent directory of where you are, one level up in the hierarchy. Notice that relative pathnames do not begin with a leading slash.

If most of your computer experience comes from the MSDOS/Windows world, you may notice the lack of drive letters (like C: or A:). In unix operating systems, there are no drive letters. Instead, different drives have different directory names. So /cdrom might refer to he CD-ROM drive and /floppy might refer to what you might normally consider A:. The mount command can show you what ``drives'' you can currently access.

The Shell

When you log into a unix system in text mode, or open a terminal window in GUI mode, you are running a program called a shell. The shell is a command interpreter. You type commands, and the shell carries out those commands (by running other programs, etc.). You primarily interact with the unix OS by entering commands at the shell's command prompt. If you have ever used MS-DOS, or a DOS Window in Microsoft Windows, then you have experienced a shell.

While there are many shells available, the standard shell for a GNU/Linux system is bash. The normal command prompt for bash is a dollar sign, often preceded by various information such as the current working directory and your username.

When you enter a command at the shell prompt, the shell interprets the command, passes it to the kernel, then displays the results of the command back to you.

Most commands are a combination of the command name, switches, and arguments. For example, in the command ls -l *.html, ls is the command name (the command for listing files). Minus ell, -l, is a switch meaning give a detailed (long) listing instead of the default brief listing. Finally, *.hmtl is the argument, in this case containing a wildcard (the star matches any string of letters).

The net effect of this command is to request a long-format directory listing of all files whose names end with .html. The result might look like:

$ ls -l *.html
-rw-r--r--    1 dbindner dbindner     5997 Mar 23 15:01 commands.html
-rw-r--r--    1 dbindner dbindner     1030 Mar 23 15:01 edit.html
-rw-r--r--    1 dbindner dbindner     1175 Mar 23 15:01 environment.html
-rw-r--r--    1 dbindner dbindner      780 Mar 23 15:01 files.html
-rw-r--r--    1 dbindner dbindner     2996 Mar 23 15:11 fs.html
-rw-r--r--    1 dbindner dbindner     1758 Mar 21 23:58 index.html
-rw-r--r--    1 dbindner dbindner      428 Mar 23 15:01 indx.html
-rw-r--r--    1 dbindner dbindner      627 Mar 23 15:01 install.html
-rw-r--r--    1 dbindner dbindner     1538 Mar 23 15:01 intro.html
-rw-r--r--    1 dbindner dbindner      550 Mar 23 15:01 jobs.html
-rw-r--r--    1 dbindner dbindner      687 Mar 22 21:33 kernel.html
-rw-r--r--    1 dbindner dbindner      473 Mar 23 15:01 links.html
-rw-r--r--    1 dbindner dbindner      511 Mar 23 15:01 network.html
-rw-r--r--    1 dbindner dbindner     1726 Mar 23 15:14 shell.html
-rw-r--r--    1 dbindner dbindner     1339 Mar 23 15:01 start.html
-rw-r--r--    1 dbindner dbindner      555 Mar 22 16:31 template.html

Logging in

Unix is a multi-user operating system. Thus, before you begin, you must login to authenticate yourself as an authorized user. On a Truman computer, your username is your TRUMAN domain network ID, and your password is your TRUMAN domain network password. On your own system, you may choose your username and password to be whatever you wish.

You may login using a completely text environment. This is common when connecting to a remote machine using a program like telnet or ssh. As soon as you authenticate, you are presented with a shell and can type commands.

You may also login to a graphical environment. This is common when sitting at a physical workstation. A graphical login prompt is presented which leads to a full graphical environment after you have authenticated. In this case, it is necessary to open a ``shell window,'' to type commands, and you'll generally find options to do that under a menu.. Sometimes a shell window is referred to as an xterm or rxvt window after the names of popular programs that provide a graphical means of running a shell.

Logging out

To logout of a text environment, you must be at a command prompt. Type the command exit and press the Enter key, or press ^D (Ctrl-D).

To logout of a graphical environment you will need to find a logout or exit option on one of the menus. This will usually be found under a button on the lower left part of the screen (in Microsoft ``Start'' button fashion), or by clicking one of the mouse buttons while the pointer is on the desktop (a place where no windows are opened). Some graphical environments will also prompt you to logout when you press Ctrl-Alt-Del. A last-resort method of terminating a graphical environment is to press Ctrl-Alt-Backspace, which is sometimes referred to as ``zapping'' the server. It will generally close everything down, but programs may not get a chance to shut down nicely.

Starting the X Window System

If the workstation you login to presents a text environment, you can manually invoke the X Windows graphical environment with the command startx. In this case, logging out will involve a two step process. First you will exit the graphical environment, and then you will logout from the text environment.

Getting information about files

The most fundamental file command is ls which is a mnemonic for ``list.'' This command displays the contents of a directory, i.e., the names of all the files and directories in it. Using the command without arguments or switches gives a brief multi-column listing of all the files in the current working directory:

$ ls
commands.html  environment.html  fs.png        jobs.html     start.html
CVS            #files.html#      index.html    kernel.html   style.css
edit           files.html        indx.html     links.html    template.html
edit.html      fs.dia            install.html  network.html  vi.html
emacs.html     fs.html           intro.html    shell.html

Here we see a listing of all the files in this directory, in case-insensitive alphabetical order, with regular files and subdirectories (CVS and edit) intermingled.

The most common switches used with the ls command are -land -a. The -l switch (for long), and the -a switch (for all), expand the information shown. As with many commands, the switches may be combined:

$ ls -al
total 132
drwxrwxr-x    4 jbeck    jbeck        4096 Mar 25 12:55 .
drwxrwxr-x    4 jbeck    jbeck        4096 Mar 19 16:34 ..
-rw-rw-r--    1 jbeck    jbeck        5691 Mar 25 05:18 commands.html
drwxrwxr-x    2 jbeck    jbeck        4096 Mar 25 12:43 CVS
drwxrwxr-x    2 jbeck    jbeck        4096 Mar 23 17:36 edit
-rw-rw-r--    1 jbeck    jbeck         972 Mar 23 17:39 edit.html
-rw-rw-r--    1 jbeck    jbeck        1324 Mar 24 21:13 emacs.html
-rw-rw-r--    1 jbeck    jbeck        1119 Mar 23 17:39 environment.html
-rw-rw-r--    1 jbeck    jbeck        1819 Mar 25 12:54 #files.html#
-rw-rw-r--    1 jbeck    jbeck         724 Mar 23 17:39 files.html
-rw-rw-r--    1 jbeck    jbeck        2627 Mar 23 08:06 fs.dia
-rw-rw-r--    1 jbeck    jbeck        3042 Mar 24 17:47 fs.html
-rw-rw-r--    1 jbeck    jbeck        6563 Mar 23 08:06 fs.png
-rw-rw-r--    1 jbeck    jbeck           0 Mar 25 12:55 .htaccess
-rw-rw-r--    1 jbeck    jbeck        1700 Mar 23 17:39 index.html
-rw-rw-r--    1 jbeck    jbeck         372 Mar 23 17:39 indx.html
-rw-rw-r--    1 jbeck    jbeck        5722 Mar 25 05:18 install.html
-rw-rw-r--    1 jbeck    jbeck        1519 Mar 24 17:48 intro.html
-rw-rw-r--    1 jbeck    jbeck        8993 Mar 25 05:18 jobs.html
-rw-rw-r--    1 jbeck    jbeck        1653 Mar 24 17:44 kernel.html
-rw-rw-r--    1 jbeck    jbeck        1220 Mar 25 05:18 links.html
-rw-rw-r--    1 jbeck    jbeck        5797 Mar 25 12:43 network.html
-rw-rw-r--    1 jbeck    jbeck        2891 Mar 24 17:36 shell.html
-rw-rw-r--    1 jbeck    jbeck        1281 Mar 23 17:39 start.html
-rw-rw-r--    1 jbeck    jbeck         206 Mar 23 15:55 style.css
-rw-rw-r--    1 jbeck    jbeck         499 Mar 23 17:39 template.html
-rw-rw-r--    1 jbeck    jbeck        6334 Mar 25 05:18 vi.html

The -l switch is what causes the output to be listed in the columns. In order, the columns list the file type (first character), permissions (9 characters), number of hard links, owner name, group name, size in bytes, modification time, and file name. The -a switch means show all files, even the ``hidden'' ones whose names begin with the period character.

The output of ls may be sorted in many ways. For example, ls -ltr specifies a long-format listing, sorted by timestamp, reversed (i.e. oldest to most recent). It is also very common to specify arguments to ls using wildcards. The asterisk is used to specify 0 or more characters, and the question mark is used to specify exactly one character. For example:

$ ls -l f?.*
-rw-rw-r--    1 jbeck    jbeck        2627 Mar 23 08:06 fs.dia
-rw-rw-r--    1 jbeck    jbeck        3042 Mar 24 17:47 fs.html
-rw-rw-r--    1 jbeck    jbeck        6563 Mar 23 08:06 fs.png

While the ls command can tell you a lot about files, it does not ``look inside them,'' so it can't really tell you what a file is for. Luckily, the file command can often help you figure out what kind of information is contained in a file:

$ file fs.png edit
fs.png: PNG image data, 625 x 280, 8-bit/color RGB, non-interlaced
edit:   directory

This command was used with no switches and two arguments. The output says that the file fs.png is Portable Network Graphics image data, while edit is a subdirectory.

Displaying information in a file

There are two main commands for displaying information in a text file, cat and less. The first, cat, is short for ``concatenate.'' Its primary function is for combining multiple files together, but when only one file is specified it writes the entire contents of the file to the screen. By contrast, less writes the file to the screen one page at a time interactively. When in less, press h to get help with its commands. Press q to quit out of less.

Creating and using directories

The mkdir command creates new directories. For example, mkdir coursework creates a directory named coursework in the current working directory. Once this directory is created, the command cd coursework changes the working directory one level down, to the newly created directory. To navigate back up one level, to the parent directory of the current working directory, use cd .. which uses the .. indicator for the parent of the current working directory. To remove an empty directory, employ the rmdir command. The pwd command prints the current working directory name, to let you know where you are.

File security

Because of its multiuser nature, it is expected that many different people will store their files on the same computer. It is understood that some files may be shared while others are private, and the operating system has a mechanism to prevent others from reading your private files.

$ groups
faculty

$ ls -l
total 8
drwxr-xr-x    2 dbindner faculty      4096 Jul  8 20:38 stuff
-rw-r-----    1 dbindner faculty       186 Jul  8 20:38 test.pl

$ chmod 755 test.pl
$ ls -l test.pl
-rwxr-xr-x    1 dbindner faculty       186 Jul  8 20:38 test.pl

$ chmod 700 stuff
$ ls -ld stuff
drwx------    2 dbindner faculty      4096 Jul  8 20:38 stuff

$ umask
0022
$ touch file1.txt
$ ls -l file1.txt
-rw-r--r--    1 dbindner faculty         0 Jul  9 11:49 file1.txt
$ umask 0077
$ touch file2.txt
$ ls -l file2.txt
-rw-------    1 dbindner faculty         0 Jul  9 11:50 file2.txt

Environment variables

The unix shells are programming environments, and because of this they have ways to define variables (names that can store data). Simple assignments can be made using the equals sign (=), and variables can be printed by preceding the name with a dollar sign ($). Variable assignments can be erased with unset. For example:

$ stuff=1
$ echo $stuff
1
$ unset stuff
$ echo $stuff

$

In addition to the variables that the shell supports for programming, there are variables that the user can use to customize the working environment. These are called environment variables. In bash, environment variables begin life as regular variables and are ``exported'' into the environment with the export command. Unlike regular variables, exported variables are visible to every program you run from the shell, so anything you customize has ``persistence'' as you run programs.

Common environment variables

Any program can make use of environment variables (check its man page or documentation to determine what variables it supports). However, there are some environment variables that are understood by many programs. They include:

In bash, you can use the echo command to display the current settings of these variables, and the export command to set new values.

$ echo $EDITOR
nano
$ export EDITOR=vim
$ echo $PAGER

$ PAGER=less
$ export PAGER

These settings would change the default editor from nano to vim, and define less as a default pager. Notice that the export command can be separate or combined with a variable assignment on one line.

Changing .bashrc

Changes to environment variables survive as long as the shell runs. When you exit the shell (or logout) these settings are lost. If you want to customize the environment and have those changes take effect every time you login, you need to record those settings in your startup scripts. If you are using bash, that means you want to edit the .bashrc file in your home directory. Simply add the relevant variable assignments (and the export commands) to this file. Since .bashrc is read every time you login to the computer, your customizations will become permanent.

Modal editing

The Vi editor is the quintessential unix text editor. It is relatively small in terms of program size, and is generally the one editor guaranteed present on any unix system. Anyone wishing to use a unix system seriously should obtain at least minimal skills with Vi.

The most difficult aspect for beginners to understand about Vi is that it is a modal editor. What that means is that there is a distinct mode the editor must be in for typing text, and a different mode for editing, pasting, and cursor movement. The typing mode is called insert mode, and the other mode is called command mode.

Other editors have modes, but not in the obvious way that Vi does. If Alt-F brings down a File menu in your favorite editor, you have implicitly switched from insert mode to command mode by pressing the Alt key. The same is true when you click a menu selection with the mouse. Historically, Alt keys and Ctrl keys have not worked well in all unix environments, or have worked differently from computer to computer. Because of this, Vi does not make use of them the same way that other editors do.

With Vi, changing modes requires an explicit act, and somewhat unintuitively the editor begins in command mode. In command mode, the letters of the alphabet (that you would normally use to type data into a document) perform editing functions instead. Some letters move the cursor around (you can quickly move by characters, words, sentences, lines and paragraphs). Other letters delete text (you can easily delete characters, words, sentences, and so on). Since every computer has alphabet keys that work the same way, the Vi editor is guaranteed to work well everywhere.

Starting Vi

Since Vi begins in command mode, generally the first thing a person learns is how to enter insert mode. Press i (lower case I), and begin typing. For example: iThis is my first line of text. You can type as much as you like, pressing Enter to break lines, and Backspace to correct mistakes. During insert mode, the letter keys work exactly as in any other editor. If you typed the text above, your editor should show:

This is my first line of text.

To return to command mode, press Esc. All of the keys return to their ``editing'' functions. For example, you can check that the letter b moves the cursor back one word at a time. Similarly the letter w moves the cursor forward by words. The arrow keys should work (which is not always true during insert mode for many Vi editors).

General practice is to enter command mode for cursor movement or editing and then return to insert mode. For example, we could use b and w to move the cursor to the beginning of the word ``text'' and type iplain Esc to insert the word ``plain'' before returning to command mode.

Stopping Vi

Quitting the editor is accomplished via command mode (press Esc if you aren't sure what mode you're in). From command mode, you can always quit by typing the ``colon'' command :q! (and pressing Enter). Many of the commands that operate on files begin with colon this way. For example :w file.txt writes everything to a file named file.txt, and :e file.txt loads file.txt and ``edits'' it.

Common tasks

Here are some of the more common things you might wish to do with an editor, with the commands to accomplish them in Vi. Naturally, all of these assume command mode.

• Search for a word using slash: /word or backward using question: ?word

• Search again for the same word: n

• Search again, but reverse direction: N

• Go to particular line. Type the line number and hit capital G: 20G

• Go to the end of the file. Use G without the line number: G

• Delete some lines of text. Type the number of lines to delete followed by dd: 3dd

• Paste some previously deleted text: capital P to paste in front of the cursor or little p to paste after.

• Search for word1 and replace with word2 (note: this is a colon command): :%s/word1/word2/g

• Undo an edit: u

Gaining proficiency

Becoming really comfortable and proficient with Vi requires an investment. The more editing commands that you have memorized, the more you will enjoy Vi and the faster you will be at accomplishing edits, particularly complex ones. Watching a skilled Vi user is like watching a magician do sleight of hand. Text disappears and reappears as if by magic. Like magic, skill with Vi comes from both practice and work.

To get started learning commands, a popular and extensive Vi reference card is available online in both pdf and postscript formats.

Endorsement

It is the author's personal opinion that the Vim editor is the superior Vi editor. There are text and graphical versions for various platforms (including Windows) available from the website: www.vim.org. If you are using Linux, Vim is undoubtedly packaged for your system, and you may already have it installed.

Important Emacs commands

Retrieving and saving files with Samba

Most of the files that students use on the Truman network are stored on Windows shares, sometimes called SMB shares or Samba shares. SMB is the name of the protocol used to transfer files to and from these shares, and Samba is the name of the program for Linux that speaks the SMB protocol.

Using Samba, Linux machines can create shares that can be read by the Windows machines on the network. More important for most users, however, is that Samba is the program that allows access to the Truman U: and Y: drives.

To use the Samba shares, it is necessary to know their ``real'' name. The drive letter assignments we generally refer to are just arbitrary names, but the real name specifies where on the network the data is stored. Under Windows, you can learn these names in My Computer. For example, the U: drive is described as Student_files on 'hydrogen'. This tells you the real name of the U: drive in Windows terms is \\hydrogen\Student_files. In a unix environment, we would normally reverse the slashes. The available shares currently at Truman are:

• The S: drive: //Ss1/Win_apps

• The T: drive: //xenon/user

• The U: drive: //hydrogen/Student_files

• The Y: drive: //hydrogen/user

$ smbclient //hydrogen/Student_Files -U dbindner -W truman
added interface ip=150.243.160.42 bcast=150.243.191.255 nmask=255.255.224.0
Password: 
Domain=[TRUMAN] OS=[Windows 5.0] Server=[Windows 2000 LAN Manager]
smb: \>

smb: \> dir
  .                                  DR        0  Fri Aug 30 15:08:00 2002
  ..                                 DR        0  Fri Aug 30 15:08:00 2002
  _BU Student File Area               D        0  Mon Mar 17 17:26:41 2003
  _ED Student File Area               D        0  Wed Feb 26 07:42:55 2003
  _FN Student File Area               D        0  Tue Dec 10 13:12:32 2002
  _HES Student File Area              D        0  Wed Feb  5 16:28:11 2003
  _ITS Student File Area              D        0  Fri Feb 28 14:20:15 2003
  _LL Student File Area               D        0  Tue Dec  3 17:08:22 2002
  _MT Student File Area               D        0  Mon Feb  3 18:39:54 2003
  _NU Student File Area               D        0  Fri Feb 21 11:53:03 2003
  _PL Student File Area               D        0  Wed Mar 19 18:17:48 2003
  _SC Student File Area               D        0  Fri Mar 21 16:42:10 2003
  _SS Student File Area               D        0  Mon Nov 11 15:39:28 2002
  _UB Student File Area               D        0  Tue Dec 17 09:51:32 2002

                19200 blocks of size 4096. 19200 blocks available
smb: \> cd "_MT Student File Area"
smb: \_MT Student File Area\> cd dbindner
smb: \_MT Student File Area\dbindner\> dir
  .                                   D        0  Tue Apr  9 14:05:05 2002
  ..                                  D        0  Tue Apr  9 14:05:05 2002
  brian                               D        0  Tue May  1 05:45:30 2001
  CalcIII                             D        0  Tue May  1 05:45:30 2001
  cs120                               D        0  Tue Apr  9 13:50:21 2002
  gvim.lnk                                   509  Tue Apr  9 14:04:44 2002
  Office2000                          D        0  Tue Apr  9 13:49:38 2002
  Sound                               D        0  Tue May  1 05:44:57 2001
  vim                                 D        0  Wed Oct 16 16:04:40 2002

                19200 blocks of size 4096. 19200 blocks available
smb: \_MT Student File Area\dbindner\> get gvim.lnk
getting file gvim.lnk of size 509 as gvim.lnk (99.4 kb/s) (average 99.4 kb/s)
smb: \_MT Student File Area\dbindner\> quit

$ mkdir u_drive
$ smbmount //Ss1/Student_files u_drive -o username=dbindner,workgroup=TRUMAN
Password:
$

$ smbumount u_drive
$

Connecting to other computers using SSH

It is common practice to work with unix computers remotely, i.e. over a modem or from the Internet. Historically, telnet or rlogin would have been used to do this. Neither was ever secure, and today both have been supplanted by a secure alternative, the Secure Shell ssh.

The ssh program allows you to work interactively on a remote computer. You can type commands and see their output. In addition, all of the data transfered between the local and remote computer is encrypted. No one can eavesdrop on the text you type, nor see the output that is sent back to you. It its easiest form, you just type ssh remotehost.

$ ssh kronos
Last login: Tue Mar 25 10:21:42 2003 from limestone.truman.edu on pts/0
Linux vh222005 2.4.20 #2 SMP Sun Jan 26 20:13:46 CST 2003 i686 unknown

You can access your home directory from a Windows computer
on the Truman network by sharing \\kronos\user where user is
your account name.

Daily filesystem backups can be found in /snapshot.

A new graphical statistics program is installed, called "salstat".

Here the "message of the day" for kronos has been displayed, and you can see on the last line that the prompt indicates a hostname of vh222005 (the true name of kronos). Any commands typed from this point on, will be executed on kronos. When you are finished, simply type exit or press Ctrl-D to exit the shell and close the connection.

The ssh program is the preferred method for creating secure connections to the unix servers at Truman, including xenon, gold, kronos, and gaia.

Sometimes you may need to login as a different user on the remote host than the username you use on the local host. In that case, you may specify the username and hostname together as user@remote.

$ ssh root@kronos

Transferring files with SSH tools

The most common file transfer program of old was a program called ftp, short for ``file transfer program.'' Like telnet and rlogin, the ftp program is unsecure and has been replaced by ssh tools. There are two tools you might use to copy files between machines. They are sftp and scp.

$ sftp kronos
Connecting to kronos...
sftp> get hello.cc
Fetching /home/ldap/dbindner/hello.cc to hello.cc
sftp> put goodbye.cc
Uploading goodbye.cc to /home/ldap/dbindner/goodbye.cc
sftp> quit

$ scp hello.cc kronos:hello.cc
hello.cc             100% |*****************************|   344       00:00    
$ scp kronos:hello.o .
hello.o              100% |*****************************|  3528       00:00    
$ scp dbindner@kronos:hello.cc /tmp/hello.cc
hello.cc             100% |*****************************|   344       00:00

Using the Truman VPN from Linux

IPSec gateway vpn.truman.edu
IPSec ID GroupName
IPSec secret GroupPwd
Xauth username Username

# vpnc-connect
Enter password for dbindner@vpn.truman.edu:
VPNC started in background (pid: 25872)...

# ip route show
192.168.1.0/24 dev eth0  proto kernel  scope link  src 192.168.1.2
default via 192.168.1.1 dev eth0

# ip route add 192.168.1.0/24 dev eth0  proto kernel  scope link  src 192.168.1.2 table 1
# ip route add default via 192.168.1.1 dev eth0 table 1
# ip rule add from 192.168.1.2/32 table 1 priority 501
# ip route flush cache

Directory structure on a Linux system

Many of the files on a Linux system go in specific locations. For example, you will find most installed software in the /usr (pronounced ``user'') directory. In particular, most Linux distributions install their software in this directory. Under /usr there are directories for various parts of applications. Executable programs (the part that you would run) are often referred to as binaries, and can be found in /usr/bin. The ``man'' pages, containing online manuals are found in /usr/man, and so on.

By convention, applications that were not pre-packaged with your Linux distribution are stored in /usr/local, so they do not conflict with other programs. Like /usr, it is subdivided into /usr/local/bin, /usr/local/man, and so forth.

Hint: Although Linux beginners are usually aware that there are hundreds of commands that they might type in their shell, they often don't know the names of many commands. One very enlightening way to learn about the commands on a system is to peruse the /bin, /usr/bin, and /usr/local/bin directories (a la ls /bin | less). Type man command to read the manual page about each command and learn what it does.

Other directories that will be of interest are /etc (pronounced ``et-see'') where configuration information is kept for many programs, /tmp (``temp'') where temporary files may be placed, and /mnt (``mount'') where new filesystems are attached (mounted). Unread email and files that are in the process of being printed are both stored in /var, as are the system logs and other files which change frequently.

The /home directory is where personal files are stored. The typical place for a user's home directory is named after their login account, /home/user, although this may differ from system to system. You can learn the location of your home directory by printing the $HOME variable.

$ echo $HOME
/home/ldap/dbindner

Programs for finding files

$ grep "Work experience" *
...

$ locate printcap
/etc/printcap
/etc/printcap.dpkg-dist
/usr/share/doc/lpr/examples/printcap
/usr/share/man/man5/printcap.5.gz
$ locate .bashrc | grep dbindner
/home/ldap/dbindner/.bashrc
/home/ldap/dbindner/.bashrc.bak

$ whereis chfn
chfn: /usr/bin/chfn /usr/local/bin/chfn /usr/share/man/man1/chfn.1.gz
$ which chfn
/usr/local/bin/chfn

$ find . -name .bashrc -print
./.bashrc
$ find . -iname "*Hello*" -print
./octave/hello.cc
./octave/hello.o
./octave/hello.oct
$ find / -name .bashrc -print 2>/dev/null | grep dbindner
/home/ldap/dbindner/.bashrc

Programs for finding people

$ who
thammond pts/0        May 28 10:14 (vh225002.truman.edu)
agarvey  pts/1        May 30 11:44 (vh216601.truman.edu)
hammond  vh225002.truman.edu:1 Jun  5 17:23         
thammond pts/3        May 28 12:26 (vh225002.truman.edu)
dbindner pts/7        Jun 11 13:13 (modem65.ipv6.truman.edu)
hammond  pts/9        Jun 11 11:08 (vh225002.truman.edu)
$ w
 13:14:20 up 14 days, 19:31,  6 users,  load average: 2.07, 2.00, 1.98
USER     TTY      FROM              LOGIN@   IDLE   JCPU   PCPU  WHAT
thammond pts/0    vh225002.truman. 28May03  2:06m  0.33s  0.33s  -bash 
agarvey  pts/1    vh216601.truman. 30May03  9days  6.51s  6.51s  -bash 
hammond  vh225002 -                Thu17    0.00s 13days  0.13s  sshd           
thammond pts/3    vh225002.truman. 28May03  2:06m  0.55s  0.55s  -bash 
dbindner pts/7    modem65.ipv6.tru 13:13    0.00s  0.03s  0.02s  w 
hammond  pts/9    vh225002.truman. 11:08    1:58m  0.01s  0.01s  -bash 
$ finger
Login     Name               Tty      Idle  Login Time   Office     Office Phone
agarvey   Alan Garvey        pts/1      9d  May 30 11:44 (vh216601.truman.edu)
dbindner  Donald J Bindner   pts/7          Jun 11 13:13 (modem65.ipv6.truman.edu)
hammond   Todd Hammond      *vh22500        Jun  5 17:23
hammond   Todd Hammond       pts/9    1:59  Jun 11 11:08 (vh225002.truman.edu)
thammond  Todd Hammond       pts/0    2:07  May 28 10:14 (vh225002.truman.edu)
thammond  Todd Hammond       pts/3    2:06  May 28 12:26 (vh225002.truman.edu)

$ last -10
dknudson pts/8        0-1pool245-222.n Wed Jun 11 13:14 - 13:14  (00:00)    
dbindner pts/7        modem65.ipv6.tru Wed Jun 11 13:13 - 13:24  (00:10)    
dgarth   vh222005.tru                  Wed Jun 11 12:23 - 12:26  (00:02)    
dknudson pts/7        0-1pool245-222.n Wed Jun 11 12:20 - 12:46  (00:25)    
hammond  vh222005.tru                  Wed Jun 11 11:53 - 12:14  (00:21)    
cmj489   pts/7        :0.0             Wed Jun 11 11:34 - 11:35  (00:00)    
cmj489   :0                            Wed Jun 11 11:34 - 11:35  (00:00)    
hammond  vh222005.tru                  Wed Jun 11 11:09 - 11:15  (00:05)    
hammond  pts/9        vh225002.truman. Wed Jun 11 11:08   still logged in   
cmj489   pts/9        :0.0             Wed Jun 11 09:41 - 09:41  (00:00)    

wtmp begins Sun Jun  1 18:51:35 2003

$ who am i
dbindner pts/7        Jun 11 13:13 (modem65.ipv6.truman.edu)

$ finger dbindner
Login: dbindner                         Name: Donald J Bindner
Directory: /home/ldap/dbindner          Shell: /bin/bash
Office: Violette Hall 2244, 785-4258
On since Wed Jun 11 13:13 (CDT) on pts/7 from modem65.ipv6.truman.edu
No mail.
No Plan.
$ finger dbindner@limestone
[limestone.truman.edu]
Login: dbindner                         Name: Donald J Bindner
Directory: /home/dbindner               Shell: /bin/bash
Office: 2244 Violette Hall, 785-4258
On since Tue May 13 10:41 (CDT) on tty1    29 days 2 hours idle
     (messages off)
Last login Wed Jun 11 12:15 (CDT) on pts/1 from modem65.ipv6.truman.edu
Mail last read Wed Jun 11 13:07 2003 (CDT)
Plan:

    Public key: http://limestone.truman.edu/~dbindner/public.txt
    Key fingerprint = 95B9 6D7A 3457 E0C7 4636  A9F9 2EE7 86CE 1B23 F31F

$ grep "dbindner" /etc/passwd
dbindner:x:1000:1000:Donald J Bindner,2244 Violette Hall,785-4258,:/home/dbindner:/bin/bash

Processes

Unix is a multiuser, multitasking operating system, which means that many users can run many programs, all at the same time. It is the responsibility of the kernel to allocate CPU time to each of the running programs of a system.

Processes are what the kernel uses to handle this accounting. You can usually think of each process as a separate program that is running on the system. However, some large programs, like a web browser, may actually be comprised of many separate tasks (processes) that run simultaneously.

The ps command

The ps command is used to list the processes running on the computer. It can be used to list both your processes and the processes of other users. It its simplest form, you just type ps.

$ ps
  PID TTY          TIME CMD
 3155 tty2     00:00:00 -bash
 3190 tty2     00:00:00 ps

Here two processes are listed. The first process, whose process ID number (PID) is 3155, is bash. The second process refers to the ps program itself, which also uses a process when it is running.

The ps command can accept many options to alter the way it lists processes and to alter which processes are listed. Among these are a which tells ps to report processes for all users, u which tells ps to report the user that owns each process, and x which indicates to list all processes, even those you ``might normally not be interested in.'' For example:

$ ps x
  PID TTY      STAT   TIME COMMAND
  545 ?        SN     0:00 ssh-agent -s
 2660 ?        SN     0:00 ssh-agent -s
 2875 ?        SN     0:00 ssh-agent -s
 2961 ?        S      0:00 ssh-agent -s
 2989 tty1     S      0:00 -bash
 3117 tty1     S      0:01 vim jobs.html
 3155 tty2     S      0:00 -bash
 3206 tty2     R      0:00 ps -x

Here, ps has listed the processes that are running on terminal 2 (tty2), but has also listed the processes running on tty1. It has also listed several ssh-agent processes that aren't associated with any terminal at all. On a graphical system, this command would have shown even more processes, including every graphical program being run by the user (at the very least). You can get an idea of all the processes running on a computer with ps aux.

Job control

Before graphical environments were common-place, job control was essential for working with multiple programs at the same time. Even in a graphical environment, job control can be a huge convenience for working with multiple programs without the need for a large number of open windows.

$ find / -name needle.txt -print >list.txt 2>/dev/null &

$ cat > calc.txt
The sum 1+1 is equal to 2.
Zero plus any number is itself.
^Z
[1]+  Stopped                 cat >calc.txt

$ calc
C-style arbitrary precision calculator (version 2.11.5t4.5)
Calc is open software. For license details type:  help copyright
[Type "exit" to exit, or "help" for help.]

> 10!
        3628800
> ^Z
[2]+  Stopped                 calc

$ jobs
[1]-  Stopped                 cat >calc.txt
[2]+  Stopped                 calc

$ fg %1
cat >calc.txt
The value for 10! is 3628800.
^D

$ xterm
^Z
[1]+  Stopped                 xterm
$ bg %1
[1]+ xterm &
$

The X Window System as a network application

$ X -query kronos.truman.edu :0

$ echo $DISPLAY
limestone.truman.edu:0.0

$ ssh -X dbindner@kronos
dbindner@kronos's password:
kronos$ echo $DISPLAY
localhost:11.0
kronos$ xlogo

Customizing the keyboard map

It might at first seem a bit strange to think that you would want to change the keyboard map, i.e. the symbols that the various keys on the keyboard generate. However, the way key presses are converted into characters under X11 is completely customizable, and this can be surprisingly convenient.

For example, if you enroll in a foreign language class, you may find yourself typing papers that contain characters or diacritical marks not used in English. In French, it is common to need a `c' with a cedilla or an `e' with a grave accent. In German, the eszett is required. All of these characters can be conveniently mapped to keys on the keyboard so they are available in programs like OpenOffice.

Before creating new key definitions, it is important to realize that there are two kinds of keys on a keyboard. There are keys that ``do something'' and there are keys that modify the behavior of other keys.

Shift, Ctrl, Alt, Caps Lock, Num Lock, and Scroll Lock are all modifier keys. On some keyboards you may also find Meta, Mode switch, and Multi keys. In fact, the Meta key is so important, that on many keyboards without a separate Meta key, the key labeled Alt is actually mapped to Meta.

There is a program that can tell you all of the key bindings under X called xev which reports ``X events.'' When run from an xterm window, xev opens a new small window. Move the mouse into the window and numerous events will be reported back in the xterm, mouse events in this case. Hit a key, however, and you see events corresponding to the key press and key release. For our use, the keycode number and keysym are the important parts to note.

$ xmodmap -e "keycode 117 = Mode_switch"

$ xmodmap -e "keysym e = e E egrave Egrave"

$ xmodmap -pke > keyboard.orig

$ cp keyboard.orig keyboard.new
$ vi keyboard.new
$ xmodmap keyboard.new

Man pages

The most traditional form of documentation on a unix system is the collection of man, short for manual, pages. The man page for a program can be accessed using the man command. To learn about a program, type ``man'' and the name of the program, and a browsable set of directions is displayed (shown below in the example). For example, to learn about the ls program, type man ls. Almost all commands have man pages. Even the man program itself has a man page (to read it, do man man).

LS(1)                          FSF                          LS(1)

NAME
       ls - list directory contents

SYNOPSIS
       ls [OPTION]... [FILE]...

DESCRIPTION
       List information about the FILEs (the current directory by
       default).  Sort entries alphabetically if none of -cftuSUX
       nor --sort.

       -a, --all
              do not hide entries starting with .

       -A, --almost-all
              do not list implied . and ..

       -b, --escape
              print octal escapes for nongraphic characters

       --block-size=SIZE
              use SIZE-byte blocks

       -B, --ignore-backups
              do not list implied entries ending with ~

       -c     with  -lt:  sort  by, and show, ctime (time of last
              modification of file status information)  with  -l:
              show  ctime  and  sort  by  name otherwise: sort by
              ctime
lines 1-34 

Since the man environment is interactive, it is helpful to know some of the keystrokes for navigating through it. As with most unix pagers (programs that show data one page at a time), Space or f moves one page forward. Use b to move one page backward, / to search for a word, and n to repeat the last search. Finally, use q to quit. To learn other useful keys, see the man pages for more or less, two popular pagers. Also check out the Section called Environment variables in Chapter 4 to learn how to set a default pager.

Sometimes you will need help, but you will not know the name of the command to ask for. When this happens, the apropos command is very helpful. For example, to find help related to web browsers, try apropos browser for a list of possible topics. Apropos will do a keyword search through the description sections of man pages and list the results, in this case including the names of web browsers on your system. Sometimes a little creativity is required as well. For example, to learn how to delete a file, you might be tempted to try apropos delete without success. However, apropos remove lists, among other things, the desired rm command.

One note about man pages. Most beginners find them daunting at first. A good man page is very detailed, usually containing more detail than a beginner wants. Amazingly enough, one day you'll find yourself wishing man pages had more detail. When that happens you will know you are on your way to becoming a skilled unix user.

Another note is in order. More than just programs are documented by man pages. Many programming topics also have man pages, a real convenience for programmers. For example, there is a mkdir command for making a new directory, but there is also a mkdir() function that can be used in C programs to make a new directory. Both have man pages. You may, on some occasions, find man returning a page other than the one you seek. You can get man to return all of the pages in turn by passing the -a switch, as in man -a mkdir.

The Texinfo system

The Texinfo system is a more recent development for program documentation than man pages, and it is the standard documentation format for many GNU programs (the GNU project is an important source of free software). It was designed so that the same file could be used to generate printed documentation with indexes as well as hyperlinked online documentation (Texinfo predates the web but works similarly). The most common tool for browsing Texinfo documentation is the info command, and Texinfo documentation is often referred to as info pages.

To see, for example, the info page for ls, type info ls. Naturally, info itself has info pages which can be displayed with info info. There are pages both about writing info documentation as well as browsing it. To learn about the ``stand-alone'' info browser type info info-stnd.

File: info-stnd.info,  Node: Top,  Next: What is Info,  Up: (dir)

GNU Info
********

   This file documents GNU Info, a program for viewing the on-line
formatted versions of Texinfo files, version 4.1.  This documentation
is different from the documentation for the Info reader that is part of
GNU Emacs.

   This manual is for Info version 4.1, updated 2 March 2002.

* Menu:

* What is Info::                What is Info?
* Invoking Info::               Options you can pass on the command line.
* Cursor Commands::             Commands which move the cursor within a node.
* Scrolling Commands::          Commands for reading the text within a node.
* Node Commands::               Commands for selecting a new node.
* Searching Commands::          Commands for searching an Info file.
* Xref Commands::               Commands for selecting cross references.
* Window Commands::             Commands which manipulate multiple windows.
* Printing Nodes::              How to print out the contents of a node.
* Miscellaneous Commands::      A few commands that defy categories.
* Variables::                   How to change the default behavior of Info.
* Custom Key Bindings::         How to define your own key-to-command
                                  bindings.
* Index::                       Global index containing keystrokes,
                                  command names, variable names,
                                  and general concepts.

--zz-Info: (info-stnd.info.gz)Top, 31 lines --All-- Subfile: info-stnd.info-1.gz
Welcome to Info version 4.1. Type C-h for help, m for menu item.

Info pages are browsed using different keys than man pages. As might be expected, PgDn and PgUp scroll by pages. Tab moves forward from one hyperlink to the next, and Esc-Tab moves backward through hyperlinks. The Enter key follows a hyperlink. Searching can be done with the s key and the last search is repeated with Ctrl-xn. Finally, pages are hierarchical, and u moves up one level of the hierarchy.

Other sources of help

$ gzip --help
gzip 1.3.2
(2001-11-03)
usage: gzip [-cdfhlLnNrtvV19] [-S suffix] [file ...]
 -c --stdout      write on standard output, keep original files unchanged
 -d --decompress  decompress
 -f --force       force overwrite of output file and compress links
 -h --help        give this help
 -l --list        list compressed file contents
 -L --license     display software license
 -n --no-name     do not save or restore the original name and time stamp
 -N --name        save or restore the original name and time stamp
 -q --quiet       suppress all warnings
 -r --recursive   operate recursively on directories
 -S .suf  --suffix .suf     use suffix .suf on compressed files
 -t --test        test compressed file integrity
 -v --verbose     verbose mode
 -V --version     display version number
 -1 --fast        compress faster
 -9 --best        compress better
 file...          files to (de)compress. If none given, use standard input.
Report bugs to <bug-gzip@gnu.org>.

  The Mutt E-Mail Client
  by Michael Elkins <me@cs.hmc.edu>
  version 1.3.28

  ``All mail clients suck.  This one just sucks less.'' -me, circa 1995
  ______________________________________________________________________

  Table of Contents

  1. Introduction
     1.1 Mutt Home Page
     1.2 Mailing Lists
     1.3 Software Distribution Sites
     1.4 IRC
     1.5 USENET
     1.6 Copyright

  2. Getting Started
     2.1 Moving Around in Menus
     2.2 Editing Input Fields
     2.3 Reading Mail - The Index and Pager
        2.3.1 The Message Index
           2.3.1.1 Status Flags
        2.3.2 The Pager
        2.3.3 Threaded Mode
        2.3.4 Miscellaneous Functions
     2.4 Sending Mail
        2.4.1 Editing the message header
        2.4.2 Using Mutt with PGP
        2.4.3 Sending anonymous messages via mixmaster.
     2.5 Forwarding and Bouncing Mail
     2.6 Postponing Mail
lines 1-32 

Distributions

Nearly all of the software that comes with a Linux system is free software. You are free to make copies of the programs for yourself. You are free to share copies with others. You are free to change the programs (if you have the skill) and give away the modified versions. Implicit in these statements is that you get the source code to the programs.

It is possible to build an entire Linux system from scratch using the source. However, it requires a great deal of work, more than most people want to do. To make Linux systems easier to install, various groups and companies have pre-built and tested complete systems which can be obtained on CD. These are called distributions.

Although the software is free, many people find it quite reasonable to pay real money for Linux distributions--to obtain the convenience they provide. Many distributions are also available for no cost. Some of the most popular Linux distributions are:

• Slackware Linux: The grandfather of Linux distributions. If you meet someone who has been using Linux for a long time, they started with this.

• Red Hat Linux: The most recognized commercial name in Linux. Several ITS servers run Red Hat; including the CourseInfo server, xenon, and gold.

• Debian GNU/Linux: Maintained by hundreds of volunteers, Debian is the ``most free'' of the Linux distributions. Many MTCS servers run this, including kronos.

• Mandrake Linux: A commercially produced Linux; from France. It claims a very polished install and relative ease for beginners.

• SuSE Linux: A commercially produced Linux; from Germany. SuSE is very popular in Europe.

• turbolinux: A commercially produced Linux. The most popular Linux in Asia.

Installation

Each distribution of Linux has a custom install program. Some are text-based, some are graphical. All can install from a CD. Some can install from floppy disks or over a network. Despite the differences, all Linux installs must complete the same basic steps:

Keeping current

Like any system on the Internet, Linux systems are subject to attack and exploitation if they are not maintained. Because of this, many distributions provide downloadable security updates. This may be at no cost or for a fee, depending on the distribution. It is probably worth investigating the update mechanism when choosing a distribution. After all, with luck you'll only install once. You'll be downloading security updates forever.