Monday, April 23, 2012

basic commands

RHEL Linux basic commands cat bzip gzip pwd cd mkdir

In our last assignment you perform some basic task related to system administration from normal user. In this assignment we will extend this further. To complete this assignment login form our normal user Vinita.

How to redirect the matter of files in a new file

Create two file and write some text in them.


$cat > one
This is first file
$cat > second
This is second file

Now we will combine these two files in a single file. In standard linux its call redirection of output.


$cat one second > new
$cat new
This is first file
This is second files

Then what exactly this command did? As you know cat command is used to display the matter of file so it will first display the matter of first file and then it will display the matter of second file. But as you put a > sign at the end of command so despite of showing this output on screen command will redirect this matter to a file.

How to execute multiple commands in a single row


$[command] ; [command] ; [command] ;[command]……..

To execute multiple commands from single row use a ; between them form example


$cat new ; mkdir xyz ; mkdir rat ; ls
This is first file
This is second files
new xyz rat

this command will first execute the first command which is cat new so it will display the matter of new file, further is mkdir xyz so it will create a xyz directory , further is mkdir rat so will create a rat directory and in the end we use ls command so it will list the contain of current directory.

How to create multiple sub directory form single command

To create multiple sub directory from a single command use –p switch with mkdir command for example


$mkdir –p a/b/c/d/f/g/h/i/j

In this example we created 9 subdirectories form a single mkdir command. Now verify it by listing.


$ls
new  xyz rat a

now change the directory to verify the depth of directories.


$cd a/b/c/d/f/g/h/i/j
$pwd
/home/vinita/a/b/c/d/f/g/h/i/j

Come back to home directory. Simple cd command without passing any argument will do this.

$cd

How to move multiple file in directory with a single commands?

Give all files name one by one with a single space between them and in the end give the destination directory name for example
$mv new first second xyz

This command will move three files new, one, second to the xyz directory.


$cd xyz
$ls
New one second
$cd ..

how to take back-up and restore files and directories.

tar command is used to take the back up with –cvf switches and the same tar command is used to restore the matter with –xvf switches. For example


$tar –cvf backup.tar xyz
$ls
$rm –rf xyz

In linux you cannot restore the data once deleted unless you have backup. Now restore these files and directory.


$tar –xvf backup.tar
$ls
$cd xyz
$ls
new first second
$cd ..

How to compress files to save disk space?

Create a large file and check how much disk space is consumed by this file


$man ls > manoj
$du –h manoj
12k manoj

File manoj is using 12k space on hard disk. For exam prospective you should familiar with two compress utilities.


$bzip2 [file name]                      {command syntax}
$bzip2 manoj
$ls
$du -h  manoj.bz2
4k manoj.bz2

To decompress file
$bzip2 –d manoj.bz2 $ls manoj as you can show file has been decompressed. Now use other utility to compress the file.


$gzip manoj
$ls
manoj.gz
$du –h manoj.gz
4k manoj.gz
$gzip –d manoj.gz
$ls
manoj

Basic RHCE commands using help for commands

In this article I will show some basic system administration related task which a normal user can perform. To complete this assignment login from normal user which we created in our first assignment.

How to count line word and character form a file


$wc [file name]

This command is used to count line words and character of file. Out will first show the line number word and in the end characters.


$wc test
2  4 23 test

In this example there are 2 lines 4 words and 23 character in test file.

how to display top and bottom line form files


$head –n [number] [file name]

head command is used to display specific number of line from top for given file.


$head –n 4 test

For example this command will show the 4 top most line of file test.


$tail –n  [number] [file name]

tail command will display the specific number of line form bottom for given file.


$tail –n 3 test

This command will display the 3 line from bottom of file test



how to find wrong spelling and correct them
$spell [file name]

spell command will display the wrong spelling of files.


$spell test

This command will display the wrong spelling of test file. If there is no spelling mistake no out will show.


$aspell check [file]

This command is used to correct the spelling related mistake in any given files.


$aspell check test

This command will show the all wrong spelling from test file and there possible corrections. To use correction just press the number shown in front of words.

how display logged in user information and terminal number


$who am i

This command is use to display the username of currently logged.


$who

This command will display all the user currently logged in all terminals.


$tty

This command is used to display the terminal number of currently logged in terminals.

how to display date time and calendar


$cal

This command will display the calendar of current month. You can see the calendar of any specific year also.


$cal 2010 |more

This will display the calendar of year 2010. As output will be more than a page so use more switch with commands.


$date

This will the current system times and date.

how to use calculator

$bc

This command will launch the calculator at command prompt. Write down your calculations and press enter to get the answer. Use CTRL+D key combination to exit from calculator.

how to get help about commands


$info [command]

info command is used to get help about any commands.


$info cat

This will display the help about cat commands. Generally output of info commands is more a then a page. You can quit form this out by just pressing q.


$command - - help

This help options is really very useful when do not want to be read full manual page for help. This will provide very basic help like which switch will work with this command.


$cat - - help

This will show the available switch cat command and a very brief descriptions about these switches.


$man [command]

If want to read the detail about any command use this command. This will give you the complete detail about commands.


$man cat

This command will give the complete details about the cat commands including switches and their usages. Use q to quit from the output of this commands.


$less [file]

When you have a file more than one pages use less command to read the output of file despite of using cat command with more switch. As with more switch you cannot scroll the text in both directions.


$cat [file] |more

If you have a file more than one page than use |more switch with cat to read the output. Without this switch matter of file will scroll too fast that you will see only texts of last pages

how to use

Vi editor guide command reference

Critical to a Linux administrator is knowledge of one or more text editors to manage the many configuration files on a Linux system. The Linux file system hierarchy organizes hardware, drivers, directories, and of course, files. You need to master a number of basic commands to manage Linux. Printer configuration can be a complex topic. Shell scripts enable you to automate many everyday processes. Security is now a huge issue that Linux can handle better than other operating systems; locally, and on larger networks such as the Internet.

The VIsual Editor

Linux and Unix are managed through a series of text files. Linux administrators do not normally use graphical editors to manage these configuration files. Editors such as WordPerfect, starOffice, and yes, even Microsoft Word normally save files in a binary format that Linux can't read. Popular text editors for Linux configuration files include emacs, pico, joe, and vi.
While emacs may be the most popular text editor in the world of Linux, every administrator needs at least a basic knowledge of vi. While emacs may be more popular and flexible, vi may help you save a broken system. If you ever have to restore a critical configuration file using an emergency boot floppy, vi is probably the only editor that you’ll have available. You need to know how to restore your system from a rescue floppy, which does not have enough room to carry any editor other than vi.So should know how to use vi editor.

$ vi /tmp/test
If this is a new file, you should see something similar to the following:
~
~
~
~
~
“/tmp/test” [New File]

The box at the top represents where your cursor is. The bottom line keeps you informed about what is going on with your editing (here you just opened a new file). In between, there are tildes (~) as filler because there is no text in the file yet. Now here's the intimidating part: There are no hints, menus, or icons to tell you what to do. On top of that, you can't just start typing. If you do, the computer is likely to beep at you. And some people complain that Linux isn't friendly.
The first things you need to know are the different operating modes: command and input. The vi editor always starts in command mode. Before you can add or change text in the file, you have to type a command (one or two letters and an optional number) to tell vi what you want to do. Case is important, so use uppercase and lowercase exactly as shown in the examples! To get into input mode, type an input command. To start out, type either of the following:

a-The add command. After it, you can input text that starts to the right of the cursor. i-The insert command. After it, you can input text that starts to the left of the cursor.

Type a few words and then press Enter. Repeat that a few times until you have a few lines of text. When you’re finished typing, press Esc to return to command mode. Now that you have a file with some text in it, try moving around in your text with the following keys or letters: Remember the Esc key! It always places you back into command mode.
Arrow keys-Move the cursor up, down, left, or right in the file one character at a time. To move left and right you can also use Backspace and the space bar, respectively. If you prefer to keep your fingers on the keyboard, move the cursor with h (left), l (right), j (down), or k (up).

w-Moves the cursor to the beginning of the next word. b-Moves the cursor to the beginning of the previous word. 0 (zero)-Moves the cursor to the beginning of the current line. $-Moves the cursor to the end of the current line. H-Moves the cursor to the upper-left corner of the screen (first line on the screen). M-Moves the cursor to the first character of the middle line on the screen. L-Moves the cursor to the lower-left corner of the screen (last line on the screen).

The only other editing you need to know is how to delete text. Here are few vi commands for deleting text:

x-Deletes the character under the cursor. X-Deletes the character directly before the cursor. dw-Deletes from the current character to the end of the current word. d$-Deletes from the current character to the end of the current line. d0-Deletes from the previous character to the beginning of the current line.

To wrap things up, use the following keystrokes for saving and quitting the file:

ZZ-Save the current changes to the file and exit from vi. :w-Save the current file but continue editing. :wq-Same as ZZ. :q-Quit the current file. This works only if you don’t have any unsaved changes. :q!-Quit the current file and don’t save the changes you just made to the file.

If you've really trashed the file by mistake, the :q! command is the best way to exit and abandon your changes.

The file reverts to the most recently changed version. So, if you just did a :w, you are stuck with the changes up to that point. If you just want to undo a few bad edits, press u to back out of changes.
You have learned a few vi editing commands. I describe more commands in the following sections. First, however,
here are a few tips to smooth out your first trials with vi:

Esc-Remember that Esc gets you back to command mode. (I’ve watched people press every key on the keyboard trying to get out of a file.) Esc followed by ZZ gets you out of command mode, saves the file, and exits. u-Press u to undo the previous change you made. Continue to press u to undo the change before that, and the one before that. Ctrl+R-If you decide you didn’t want to undo the previous command, use Ctrl+R for Redo. Essentially, this command undoes your undo. Caps Lock-Beware of hitting Caps Lock by mistake. Everything you type in vi has a different meaning when the letters are capitalized. You don’t get a warning that you are typing capitals-things just start acting weird. :! command-You can run a command while you are in vi using :! followed by a command name. For example, type :!date to see the current date and time, type :!pwd to see what your current directory is, type :!jobs to see if you have any jobs running in the background. INSERT-When you are in insert mode, the word INSERT appears at the bottom of the screen. Ctrl+G-If you forget what you are editing, pressing these keys displays the name of the file that you are editing and the current line that you are on at the bottom of the screen. It also displays the total number of lines in the file, the percentage of how far you are through the file, and the column number the cursor is on.

Moving Around the File

Besides the few movement commands described earlier, there are other ways of moving around a vi file. To try these out, open a large file that you can’t do much damage to. (Try copying /var/log/ messages to /tmp and opening it in vi.) Here are some movement commands you can use:

Ctrl+F-Page ahead, one page at a time. Ctrl+B-Page back, one page at a time. Ctrl+D-Page ahead one-half page at a time. Ctrl+U-Page back one-half page at a time. G-Goto the last line of the file. 1G-Go to the first line of the file. (Use any number to go to that line in the file.)

Searching for Text

To search for the next occurrence of text in the file, use either the slash (/) or the question mark (?) character. Follow the slash or question mark with a pattern (string of text) to search forward or backward, respectively, for that pattern. Within the search, you can also use metacharacters. Here are some examples:

/hello-Searches forward for the word hello. ?goodbye-Searches backward for the word goodbye. /The.*foot-Searches forward for a line that has the word The in it and also, after that at some point, the word foot. ?[pP]rint-Searches backward for either print or Print. Remember that case matters in Linux, so make use of brackets to search for words that could have different capitalization.

The vi editor was originally based on the ex editor, which didn’t let you work in full-screen mode. However, it did enable you to run commands that let you find and change text on one or more lines at a time. When you type a colon and the cursor goes to the bottom of the screen, you are essentially in ex mode. Here is an example of some of those ex commands for searching for and changing text. (I chose the words Local and Remote to search for, but you can use any appropriate words.)

:g/Local-Searches for the word Local and prints every occurrence of that line from the file. (If there is more than a screenful, the output is piped to the more command.) :s/Local/Remote-Substitutes Remote for the word Local on the current line. :g/Local/s//Remote-Substitutes the first occurrence of the word Local on every line of the file with the word Remote. :g/Local/s//Remote/g-Substitutes every occurrence of the word Local with the word Remote in the entire file. :g/Local/s//Remote/gp-Substitutes every occurrence of the word Local with the word Remote in the entire file, and then prints each line so that you can see the changes (piping it through more if output fills more than one page).

Using Numbers with Commands

You can precede most vi commands with numbers to have the command repeated that number of times. This is a handy way to deal with several lines, words, or characters at a time. Here are some examples:

3dw-Deletes the next three words. 5cl-Changes the next five letters (that is, removes the letters and enters input mode). 12j-Moves down 12 lines.

Putting a number in front of most commands just repeats those commands. At this point, you should be fairly proficient at using the vi command. Once you get used to using vi, you will probably find other text editors less efficient to use.

vi editor

RHCE vi editor switches options descriptions

In our last assignment your learnt about cat command. cat is a very basic command you cannot depend on cat command in exam to create files as cat is a very essential command. In cat you can neither see the matter of file nor modify the material. In this assignment we are going to use world's most powerful editor vi.
Before we go further just learn how to create a hidden file in linux and see it.


$cat > [.name of file]

A single dot in front of the file will make it hidden. For example to make a file named secret to hidden use this command


$cat > .secret
This is a secret file

Now normal ls command will not list this file. Do a ls form current directory

$ls

As you can see in output .secret file is not shown here. But can see hidden file with –a switch.


$ls –a
.secret

Now rename and make it unhidden. Use mv command to rename the file


$mv .secret  test
$ls
test

If we want to change the matter of file cat will not do it for us. So we will use vi editor to change matter of file.


$vi test

This will open a window with bilking cursor. vi editor work in three different mode.


Esc         Command mode [press Esc key before giving any command]
Insert      Insert  mode [ to start editing use press Insert key ]
Exit        Exit mode [ Exit mode can be access via press Ecs key and :]

Beside it there are various command which can be used to control the behavior of vi editor, some of them most command are there to remember


Esc +:+w+q                    save and exit form file
Esc+:+q+!                     exit without saving
Esc+:+set nu                  to show hidden line
Esc+:+/test                   to find test word in forward directions
Esc+:+21                      to move cursor in line number 21
Esc+:+2+yy                    to copy 2 line form cursor
Esc+:+p                       to paste the copied line below the crusor
Esc+:+dd                      to remove the entire line
Esc+:+4+dd                    to remove 4 line below of cursor
Esc+:+x                       to remove single character
Esc+:+e                       to go to end of the word
Esc+:+h                       to go one character back

We have written a complete article about Vi editor. You can read it for more information about vi editor.

commands examples

RHCE Linux system administrations commands examples

halt
This command shuts down the operating system, but can only be run by the root user.

#halt

reboot
This command shuts down and restarts the operating system. It also can only be run by root.

#reboot           [will perform simple reboot]
#reboot -f        [will perform fast reboot ]

init 0
This command also shuts down the operating system, and can only be run by your root user.

#init 0

init 6 This command also shuts down and restarts the operating system. It also can only be run by root

#init 6

man
This command opens the manual page for the command or utility specified. The man utility is a very useful tool. If you are unsure how to use any command, use man to access its manual page. For example, you could enter man ls at the shell prompt to learn how to use the ls utility.

#man ls

info
The info utility also displays a help page for the indicated command or utility. The information displayed with info command will be in-depth than that displayed in the man page for the same command.

info ls

su
This command switches the current user to a new user account. For example, if you’re logged in as vickey and need to change to user account to vinita, you can enter su vinita at the shell prompt. This command is most frequently used to switch to the superuser root account.
In fact, if you don’t supply a username, this utility assumes that you want to change to the root account. If you enter su -, then you will switch to the root user account and have all of root’s environment variables applied.
This command require password of the user you want switch.

Looking for Files

There are two basic commands used for file searches: find and locate

find

The find command searches through directories and subdirectories for a desired file. For example, if you wanted to find the directory with the grub.conf linux boot loader file, you could use the following command, which would start the search in the top-level root (/) directory:

# find / -name grub.conf

But this search took several minutes to get it task done. Alternatively, if you know that this file is located in the /etc subdirectory tree, or /boot/grub/grub.conf you could start in that directory with the following command:

# find /etc -name grub.conf

locate

If this is all too time-consuming, RHEL 5 includes a default database of all files and directories. Searches with the locate command are almost instantaneous. And locate searches don't require the full file name. The drawback is that the locate command database is normally updated only once each day, as documented in the /etc/cron.daily/mlocate.cron script.

Getting into the Files

Now that you see how to find and get around different files, it's time to start reading, copying, and moving the files around. Most Linux configuration files are text files. Linux editors are text editors. Linux commands are designed to read text files. If in doubt, you can check the file types in the current directory with the
file * command.

cat

The most basic command for reading files is cat. The cat filename command scrolls the text within the filename file. It also works with multiple file names; it concatenates the file names that you might list as one continuous output to your screen. You can redirect the output to the file name of your choice.

more and less

Larger files demand a command that can help you scroll through the file text at your leisure. Linux has two of these commands:
more and less.
With the more filename command, you can scroll through the text of a file, from start to finish, one screen at a time. With the less filename command, you can scroll in both directions through the same text with the PAGE UP and PAGE DOWN keys. Both commands support vi-style searches.

head and tail

The head and tail commands are separate commands that work in essentially the same way. By default, the head filename command looks at the first 10 lines of a file; the tail filename command looks at the last 10 lines of a file. You can specify the number of lines shown with the -nx switch. Just remember to avoid the space when specifying the number of lines; for example, the

# tail -n15 /etc/passwd

command lists the last 15 lines of the /etc/passwd file.

cp

The cp (copy) command allows you to take the contents of one file and place a copy with the same or different name in the directory of your choice. For example, the cp file1 file2 command takes the contents of file1 and saves the contents in file2. One of the dangers of cp is that it can easily overwrite files in different directories, without prompting you to make sure that's what you really wanted to do.

mv

While you can't rename a file in Linux, you can move it. The mv command essentially puts a different label on a file. For example, the mv file1 file2 command changes the name of file1 to file2. Unless you're moving the file to a different partition, everything about the file, including the inode number, remains the same.

ln

You can create a linked file.
linked files are common with device files such as /dev/dvdwriter and /dev/par0. They're also useful for making sure that multiple users have a copy of the same file in their directories. Hard links include a copy of the file. As long as the hard link is made within the same partition, the inode numbers are identical. You could delete a hard-linked file in one directory, and it would still exist in the other directory. For example, the following command creates a hard link from the actual Samba configuration file to smb.conf in the local directory:

# ln smb.conf /etc/samba/smb.conf

On the other hand, a soft link serves as a redirect; when you open up a file created with a soft link, you're directed to the original file. If you delete the original file, the file is lost. While the soft link is still there, it has nowhere to go. The following command is an example of how you can create a soft link:

# ln -s smb.conf /etc/samba/smb.conf

sort

You can sort the contents of a file in a number of ways. By default, the sort command sorts the contents in alphabetical order depending on the first letter in each line. For example, the sort /etc/passwd command would sort all users (including those associated with specific services and such) by username.

grep and egrep

The grep command uses a search term to look through a file. It returns the full line that contains the search term. For example, grep 'vickey' /etc/passwd looks for my name in the /etc/passwd file.
The egrep command is more forgiving; it allows you to use some unusual characters in your search, including +, ?, |, (, and). While it's possible to set up grep to search for these characters with the help of the backslash, the command can be awkward to use.

wc

The wc command, short for word count, can return the number of lines, words, and characters in a file. The wc options are straightforward: for example, wc -w filename returns the number of words in that file.

sed

The sed command, short for stream editor, allows you to search for and change specified words or even text streams in a file. For example, the following command changes the first instance of the word Windows to the word Linux in each line of the file data, and writes the result to the file newdata:

# sed 's/Windows/Linux/' data > newdata

However, this may not be enough. If a line contains more than one instance of Windows, the above sed command does not change the second instance of that word. But you can make it change every appearance of Windows by adding a "global" suffix:

# sed 's/Windows/Linux/g' data > newdata

awk

The awk command, named for its developers (Aho, Weinberger, and Kernighan), is more of a database manipulation utility. It can identify lines with a keyword and read out the text from a specified column in that line. Again, using the /etc/passwd file, for example, the following command will read out the username of every user with a vickey in the comment column:

# awk '/vickey/ {print $1}' /etc/passwd

ps

It's important to know what's running on your Linux computer. The ps command has a number of critical switches. When trying to diagnose a problem, it's common to get the fullest possible list of running processes, and then look for a specific program. For example, if the Firefox Web browser were to suddenly crash, you'd want to kill any associated processes. The ps aux | grep firefox command could then help you identify the process(es) that you need to kill.

who and w

If you want to know what users are currently logged into your system, use the who command or the w command. This can help you identify the usernames of those who are logged in, their terminal connections, their times of login, and the processes that they are running.

Wildcards

Sometimes you may not know the exact name of the file or the exact search term. This is when a wildcard is handy. The basic wildcards are shown

Wildcard	Description
*	Any number of alphanumeric characters (or no characters at all). For example, the ls ab* command would return the following file names, assuming they exist in the current directory: ab, abc, abcd.
?	One single alphanumeric character. For example, the ls ab? command would return the following file names, assuming they exist in the current directory: abc, abd, abe
[ ]	A range of options. For example, the ls ab[123] command would return the following file names, assuming they exist in the current directory: ab1, ab2, ab3. Alternatively, the ls ab[X-Z] command would return the following file names, assuming they exist in the current directory: abX, abY, abZ.

env

This command displays the environment variables for the currently logged-in user.

echo

This command is used to echo a line of text on the screen. It’s frequently used to display environment variables. For example, if you wanted to see the current value of the PATH variable, you could enter
echo $PATH

top

This command is a very useful command that displays a list of all applications and processes currently running on the system. You can sort them by CPU usage, memory usage, process ID number, and which user owns them

which

This command is used to display the full path to a shell command or utility. For example, if you wanted to know the full path to the ls command, you would enter
which ls

whoami

This command displays the username of the currently logged-in user.

netstat

This command displays the status of the network, including current connections, routing tables, etc

route

This command is used to view or manipulate the system’s routing table.

ifconfig

This command is used to manage network boards installed in the system. It can be used to display or modify your network board configuration parameters. This command can only be run by the root user.

accessing virtual terminals

Login linux terminal pwd ls useradd commands accessing virtual terminals

In this assignment I will instruct you about some basic commands of Linux. You will get seven virtual terminal when you perform full installations. Although you can use graphics for daily task but here we are preparing for RHCE exam so you must use command line interface. Because all question are based on command line in RHCE Exam.

Virtual Consoles

A virtual console is a command line where you can log into and control Linux. As RHEL is a multi terminal operating system, you can log into Linux, even with the same user ID, several times. It's easy to open a new virtual console. Just use the appropriate ALT-function key combination. For example, pressing ALT-F2 brings you to the second virtual console. You can switch between adjacent virtual consoles by pressing ALT-RIGHT ARROW or ALT-LEFT ARROW. For example, to move from virtual console 2 to virtual console 3, press ALT-RIGHT ARROW.
You can switch between virtual terminals by just press the ALT+CTRL+Funcation key combinations.

ALT + CTRL + F1  for terminal 1
ALT + CTRL + F2  for terminal 2
ALT + CTRL + F3  for terminal 3
ALT + CTRL + F4  for terminal 4
ALT + CTRL + F5  for terminal 5
ALT + CTRL + F6  for terminal 6
ALT + CTRL + F7  for terminal 7

Terminal 7 is by default graphic mode beside it all six terminal are CLI based. Open any terminial by press ALT+CTRL+F1 key combinations. root account is automatically created when we install Linux.

Type root on login name and press enter key, now give password ( no asterisk character like window to guess the password length) When you login from root account you will get # sign at command prompt , and when you login from normal user you will get $ prompt.

#clear

This command is used to clear the screen.You have three options to logout .


Press CTRL+D
#exit
#logout

All three commands perform same task.


#pwd
/root

Print working directory command will tell you about current location from / partition.

#ls

ls command will list the object in directory. All directory are listed in blue color while files are shown white color.


#ls –a

Normal ls command will not list the hidden files. If you want to list the hidden file use –a switch with ls command to list the hidden files.


#ls –l

Ls command with –l switch will list the objects in long formats . we will discuss more about –l switch in coming sections.

#ll

Same as ls –l . First and major task for any system administrator is user managements. For testing purpose you can perform all task with root account but in real life root account is used for administrative purpose only. Lest create a normal user account for further practical.


#useradd  [user name ]

Useradd command is used to create user. Several advance options are used with useradd command but you will learn about them in coming article.


#passwd  [user name]

In linux no user can be login without password. passwd command is used to assign password for any user. Do not execute this command without user name otherwise it will change root password.

Linux Filesystem

Linux Unix Filesystem Hierarchy and Structure

Overview of the FHS

Everything in Linux can be reduced to a file. Partitions are associated with files such as /dev/hda1. Hardware components are associated with files such as /dev/modem. Detected devices are documented as files in the /proc directory. The Filesystem Hierarchy Standard (FHS) is the official way to organize files in Unix and Linux directories.

Linux/Unix Filesystems and Directories

Several major directories are associated with all modern Unix/Linux operating systems. These directories organize user files, drivers, kernels, logs, programs, utilities, and more into different categories. The standardization of the FHS makes it easier for users of other Unix-based operating systems to understand the basics of Linux. Every FHS starts with the root directory, also known by its label, the single forward slash (/). All of the other directories shown in Table are subdirectories of the root directory. Unless they are mounted separately, you can also find their files on the same partition as the root directory.

/	The root directory, the top-level directory in the FHS. All other directories are subdirectories of root, which is always mounted on some partition. All directories that are not mounted on a separate partition are included in the root directory's partition.
/bin	Essential command line utilities. Should not be mounted separately; otherwise, it could be difficult to get to these utilities when using a rescue disk.
/boot	Includes Linux startup files, including the Linux kernel. Can be small; 16MB is usually adequate for a typical modular kernel. If you use multiple kernels, such as for testing a kernel upgrade, increase the size of this partition accordingly.
/etc	Most basic configuration files.
/dev	Hardware and software device drivers for everything from floppy drives to terminals. Do not mount this directory on a separate partition.
/home	Home directories for almost every user.
/lib	Program libraries for the kernel and various command line utilities. Do not mount this directory on a separate partition.
/mnt	The mount point for removable media, including floppy drives, CD-ROMs, and Zip disks.
/opt	Applications such as WordPerfect or StarOffice.
/proc	Currently running kernel-related processes, including device assignments such as IRQ ports, I/O addresses, and DMA channels.
/root	The home directory of the root user.
/sbin	System administration commands. Don't mount this directory separately.
/tmp	Temporary files. By default, Red Hat Linux deletes all files in this directory periodically.
/usr	Small programs accessible to all users. Includes many system administration commands and utilities.
/var	Variable data, including log files and printer spools.

Types of Files Used by Linux

When working with Linux, you need to be aware of the fact that there are a number of different file types used by the file system. This is another area where the Linux file system differs significantly from the Windows file system. With a Windows file system you basically have two entry types in the file system:

Directories
Files

Granted, you can have normal files, hidden files, shortcut files, word processing files, executable files, and so on. However, these are all simple variations of the basic file when working with Windows.
With Linux, however, there are a variety of different file types used by the file system. These include the file types shown in Table

File Type	Description
Regular files	These files are similar to those used by the file systems of other operating systems—for example, executable files, OpenOffice.org files, images, text configuration files, etc.
Links	These files are pointers that point to other files in the file system.
FIFOs	FIFO stands for First In First Out. These are special files used to move data from one running process on the system to another. A FIFO file is basically a queue where the first chunk of data added to the queue is the first chunk of data removed from the queue. Data can only move in one direction through a FIFO.
Sockets	Sockets are similar to FIFOs in that they are used to transfer information between sockets. With a socket, however, data can move bi-directionally.

Some of the Configuration Files in /etc Directory that you should remember

File	Function
/etc/fstab	Lists the partitions and file systems that will be automatically mounted when the system boots.
/etc/group	Contains local group definitions.
/etc/grub.conf	Contains configuration parameters for the GRUB bootloader (assuming it’s being used on the system).
/etc/hosts	Contains a list of hostname-to-IP address mappings the system can use to resolve hostnames.
/etc/inittab	Contains configuration parameters for the init process.
/etc/init.d/	A subdirectory that contains startup scripts for services installed on the system. On a Fedora or Red Hat system, these are located in /etc/rc.d/init.d.
/etc/modules.conf	Contains configuration parameters for your kernel modules.
/etc/passwd	Contains your system user accounts.
/etc/shadow	Contains encrypted passwords for your user accounts.
/etc/X11/	Contains configuration files for X Windows

RHCE EXAMS

Red Hat Certified Engineer-RHCE EXAMS INFORMATIONS

RHCE exam assesses the candidates ability to do the following:

Install and configure Red Hat Linux

Configure file systems and networking

Configure X, the graphical user interface used on Unix and Linux systems

Configure basic security

Configure network services

Perform routine maintenance

Perform diagnostics and troubleshooting

Because Red Hat regularly updates its Linux distribution, RHCE certification is not valid indefinitely. However, according to Red Hat policy, RHCE certification remains valid for at least one year.

RHCE Courses and Exam

as part of its RHCE program, Red Hat offers several courses designed to prepare candidates for the RHCE exam. Candidates must take course RH302, which is the exam itself. However, candidates can take none, some, or all of the other courses.
This section describes the RHCE courses offered by Red Hat, as well as the RH302 exam.

RH033: Introduction to Red Hat Linux I, II

RH033 is a four-day course intended for users who have no previous Unix or Linux experience. Students should, however, have previous experience with a computer, including use of a mouse and graphical user interface. Upon completion of the course, the student should be able to use and customize the GNOME desktop and be able to use the Linux command shell.

RH133: Red Hat Linux System Admin I, II

RH133 is a four-day course intended for users who are familiar with Red Hat Linux. Before taking this course, students should complete RH033 or possess equivalent experience. Upon completion of the course, students should be able to install and configure Red Hat Linux, X, and various network services and clients, such as DHCP, NIS, NFS, and Samba. Students should also be able to perform basic troubleshooting and rebuild the Linux kernel from source code.

RH253: Red Hat Linux Networking and Security Admin

RH253 is a four-day course intended for Unix or Linux system administrators. Before taking this course, students should complete RH133 or possess equivalent experience. Upon completion of the course, students should be able to install and configure network services such as Apache, DHCP, DNS, FTP, Samba, NFS, sendmail, and IMAP4/POP3 mail. Students should also be able to establish and administor a security policy that includes such elements as password security, kernel security, public/private key encryption, Kerberos, secure shell, and firewalls.

RH300: RHCE

RH300 is a five-day course that includes RH302, the RHCE exam, as an integral part of the course. Before taking this course, students should complete RH253 or possess equivalent experience and have experience as a Unix or Linux system or network administrator. The course consists of four days of instruction, and the fifth day is devoted to the RHCE exam. Upon completion of the course, students should be prepared to manage a Red Hat Linux system that offers common TCP/IP services, such as FTP and HTTP. The course includes eight units of instruction, each of which has one or more hands-on labs associated with the following topics:

Hardware and Installation (x86 Architecture)

Configuration and Administration

Alternate Installation Methods

Kernel Services and Configuration

Standard Networking Services

X Window System

User and Host Security

Routers, Firewalls, Clusters, and Troubleshooting

RH302: RHCE Exam

Though styled by Red Hat as a course, RH302 is not a course in the ordinary sense of the word. Instead, RH302 is the RHCE exam, which has duration of one day. RH302 is the only course that RHCE candidates must take. The exam consists of the following three closed-book components:

Diagnosis and troubleshooting lab (2 1/2 hours)
Installation and configuration lab (2 1/2 hours)
Multiple choice exam (1 hour)

Effective May 1, 2009, the RHCE exam is a single section lasting 3.5 hours. Previously, it had been two sections lasting a combined 5.5 hours. The content has be consolidated and reorganized into a single section in which time is used more efficiently. The RHCE exam consists of RHCT components (essentially the RHCT exam) plus RHCE-specific components. It is possible to earn RHCT in an RHCE exam if one has met the RHCT requirements but not the RHCE ones.
The components are not always presented in the same sequence. The course instructor, who acts as the exam proctor, will determine the sequence and announce it early in the day. The three components are equally weighted at 100 points each. The minimum passing score is 240 points, or 80 percent. However, a candidate must score at least 50 points (50 percent) on each exam component to pass the exam. Red Hat advises candidates of their exam scores by e-mail within 10 business days of taking the exam.

Practice

Because the RHCE exam is performance-based, it's crucial that you have, or develop, experience in working with Unix or Linux and related network services. This step by step guide is focus to get your RHCE certification by present practical exercises.
These exercises can be time-consuming, and you may feel that they’re unnecessary. However, unless you have extensive practical experience, you should perform every exercise. Often, exercises will lead you to discover subtle points not evident merely from reading the exercise. If your practical experience with Unix and Linux is small, you should construct and perform your own additional exercises. The RHCE exam tests for the equivalent of about two years of experience with Linux and networking. With diligent effort, you can accumulate that experience in a matter of weeks. However, doing so will require that you skimp nowhere.

Basic Commands