basic redhat enterprise linux (1)

8/8/2019 Basic Redhat Enterprise Linux (1)

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REDH

rainer

AT E: Mr.

MSc.IICT MPart ‐T

TERP

Kao Se (SMU, Indinager, Sam

ime Compu

RISE

reyrata), BSc.IT (Nic Microfiner lecturer,

INUX

U) nce Norton Uni

5 T

versity

AINI G

Updated Ye

ar 2010



3

HISTORY OF REDHAT ENTERPRISE LINUX

Starting in 2001 , Red Hat, Inc. began offering Red Hat Enterprise Linux in addition to their original

consumer operating system, Red Hat Linux. In 2003 , Red Hat started the Fedora Project to release

the Fedora Core operating system instead of Red Hat Linux. The Fedora Project progresses at a rapid rate, releasing a new version of Fedora every four to six

months. This allows new technologies to be tested by millions of users, which in turn decreases the

amount of time it takes for these technologies to stabilize into production ‐ ready software. Each

release of Red Hat Enterprise Linux is based on a Fedora operating system release. The kernel and all

of the other software in Red Hat Enterprise Linux are specifically configured and tested for

enterprise ‐level usage.

Both Red Hat Enterprise Linux and Fedora are based on open source software developed by the open

source community, some of whom are members of the Red Hat engineering team. The term open

source means that the programming code is freely available to anyone and that anyone can submit

code to an existing open source project as long as the code stays open source. New projects or

programs can be created based on a different open source project or program. Open source

developers live all over the world, and they collaborate on projects every day together.



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12

UNDERSTANDING LINUX CONCEPT

FILESYSTEM HIERARCHY

The location of the files and directories in a Red Hat Enterprise Linux system are based on the

Filesystem Hierarchy System (FHS) guidelines. The purpose of the FHS is to provide guidelines for file

and directory locations for UNIX‐based operating systems such as Linux.

Instead of an administrator searching the entire filesystem for a particular type of file, he can know

that it will be in one of a few established locations. For example, most configuration files are in the

/etc/ directory, and log files are in the /var/log/ directory .

Because the FHS defines the /var/log/ directory as the location for log files, it is easy for an

administrator to find the log files she is looking for because they are all in one common directory.

Some common directory

Directory FHS Purpose

/bin/ Essential commands for admins and users

/usr/bin/ Common commands for admins and users

/sbin/ Essential commands for admins

/usr/sbin/ Common commands for admins

/tmp/ Temporary files for all users

/usr/local/ Location for locally‐installed software independent of operating

system updates /usr/share/man/ Manual pages

/usr/src/ Source code

/var/ Variable data files such as spool files and log files

/var/log/ Log files, can include subdirectories

/etc/ Configuration files, can include subdirectories

/proc/ Kernel virtual filesystem

/dev/ Device files



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14

Some basic command you should know:

To change to a different directory, use the cd <directory> command.

cd ../httpd takes you up one directory and then down into the httpd directory.

To create a directory, use the mkdir <directory> command. The directory will be created in

current directory. If you want to create project1 directory in /home/rath you can type mkdir

/home/rath/project1 To remove an empty directory, invoke the rmdir <directory> command.

To force the removal of a directory with all the files and subdirectories within that directory,

use the rm ‐rf <directory> command.

To remove a file, use the rm <file> command.

Use the ls <directory> command to view the contents of <directory>.

To list all the OpenOffice.org text documents, use the ls *.odt command, or the ls status* to

find all files whose filename begins with status. Multiple wildcards can be used such as ls

*status* to list all files that have status somewhere in their name.

To copy a file from one location to another, use the cp <from> <to>. For example: cp

status.txt dir1/.

The mv <from> <to> command is similar to the cp command. The only difference is that the

original <from> file will no longer exist after the move operation.

Instead of typing the entire command, you can type the first few characters of it and press the

Tab key, then it will show the possibilities message as below:

Display all 112 possibilities? (y or n)

Press the Y key to display all the results, or press the N key to go back to the prompt and

type a few more characters to the desired command.

If you have ever forgotten a recently used command or forgotten which command line

arguments you used for a particular command, you can use history command. Or if you want

to filter some part of command you can use: history | grep ‘part of the command’ The command clear to clear the screen and place the prompt at the top of the screen.

locate .odt to find all OpenOffice.org text files or locate compare to find all filenames that

contain the word compare. The only catch to this command is that it relies on the generation

of a database file so it can quickly display results. The locate command is provided by the

mlocate package, which also provides the cron script /etc/cron.daily/mlocate.cron to

automatically generate this database daily.



15

The find command is a bit more complicated to use and takes longer to produce results

because it does not rely on a database to produce results. Because it takes longer, it is

possible to specify a specific directory to look in. The basic syntax is as follows:

find <directory> ‐name <filename>. To search in the current directory and below, replace

<directory> with a dot (.) character such as find . ‐name guidelines.txt

If you know a command exists on the system but keep getting the error message command not found, check to make sure you are typing the command correctly. Otherwise, it might not

be in your PATH environment variable. To view the value of your PATH, execute the command

echo $PATH from the command line. As you can see, your PATH is a list of directories. When

you execute a command without providing its full path, it must be in one of the directories

listed in your PATH.

You can provide the full path to the command if you know it, such as /sbin/lspci to execute

the command to list the PCI devices.

To add the /usr/sbin/ and /sbin/ directories to your PATH, add the following line:

export PATH=:$PATH:/usr/sbin:/sbin

To verify which command you are executing, type the command which <command> . If a

match to the command is found in the directories from your PATH, the full path to the

command is displayed.

To read a text file such as a configuration file without having to open a text editor. This is

possible with the less, more, and cat command ‐line utilities.

You can type less output.txt , more /var/log/messages, or cat /etc/sysconfig/network.

With the less command, the Page Up and Page Down keys can be used to scroll up and down

the contents of the file. The more command only allows you to scroll down the file using the

spacebar to advance. The cat command outputs the contents of the file to the command line and then exits, so if the file is longer than the number of lines in your terminal, you will only

see the last part of the file.

For example, to view only the kernel messages in the system log file, use the following

command:

cat /var/log/messages | grep kernel



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17

FILE PERMISSION

Every file on your system has an accompanying set of permissions based on ownership.

You can examine the default permissions for a file you create by:

$ touch file

$ ls ‐l file ‐rw ‐r‐‐r‐‐ 1 root root 0 2009 ‐04‐01 23:46 file

The first character of the field is the type of file created: A dash is for a plain file, d is for

directory , c for character device (such as a serial communications Ex: /dev/ttys0) and b for

block device (a device that transfers and caches data in block Ex: blocks/dev/hda).

Permissions —Read, Write, and eXecute permission for the owner , group , and other users on

the system.

Number of links to the file—The number one (1) designates that there is only one file,

whereas any other number indicates that there might be one or more linked files. Links are

created with the ln command.

The owner —The account that created or owns the file; you can change this designation by

using the chown command.

The group —The group of users allowed to access the file; you can change this designation by

using the chgrp command.

File size and creation/modification date— The last two elements indicate the size of the file in

bytes and the date the file was created or last modified.

Assigning Permissions

Under Linux, permissions are grouped by owner, group, and others, with read, write, and execute

permission assigned to each, like so:

Owner Group Others

rwx rwx rwx

4+2+1=7 4+2+1=7 4+2+1=7

r = open and read file = 4

w = open and write file = 2

x = execute the file or read directory = 1



18

Using chmod command to modify permission

$chmod a‐w readme.txt

$ ls ‐l readme.txt

‐r‐‐r‐‐r‐‐ 1 andrew andrew 12 Jan 2 16:48 readme.txt

Some option you could use with chmod command: u—Adds or removes user (owner) permission

g—Adds or removes group permission

o—Adds or removes for others not in a file’s group

a—Adds or removes for all users

r—Adds or removes read permission

w—Adds or removes write permission

x—Adds or removes execution permission

Ex:

$ chmod u+rw readme.txt

$ ls ‐l readme.txt

‐rw‐‐‐‐‐‐‐1 andrew andrew 0 2007 ‐10‐23 19:08 readme.txt

Or

$ chmod 600 readme.txt

INITIALIZATION SCRIPT

Network services such as the Apache HTTP Server and DHCP along with other programs such as cron

and syslog require a daemon to be running at all times. The daemon performs actions such as

listening for connections to a service on specific ports, making sure commands are executed at

specific times, and capturing data such as log messages when they are sent out by other

programs.Programs that require a daemon to be started have an initialization script in the

/etc/rc.d/init.d/ directory.

This program first executes the /etc/rc.d/rc.sysinit script to perform actions such as loading kernel

modules for hardware support, loading the default keymap, and setting the hostname. The



19

/etc/inittab script is run next, which then tells init which runlevel to start. The runlevel defines which

services to start at boot time, or which initialization scripts to execute.

Lastly, the /etc/rc.d/rc.local script is executed. Commands can be added to this file for custom

initialization.

The initialization scripts can also be used to start, stop, and restart services after the system has booted . These actions are performed with the service command as the root user.

To perform an action, use the following syntax:

service <service> <action>

For example, the following starts the OpenSSH service:

service sshd start

RUNLEVELS

How does the system know which initialization scripts to run so that only the desired services are

started at boot time? Linux uses the concept of runlevels to define which services to start at boot

time.

There are 7 runlevels , with each having its own general purpose:

0 Halt the system

1 Single‐user mode or rescue mode

2 Not used

3 Multi ‐user mode with text login

4 Not used

5 Multi ‐user mode with graphical login

6 Reboot Each runlevel has its own directory named rcX.d in /etc/rc.d/, where X is the runlevel number . Each

of these directories contains symbolic links to the actual initialization scripts in /etc/rc.d/init.d/ . Each

symbolic link start with the letter S or K followed by a number. The S stands for start, and the K

stands for kill, which means to stop a process. When a runlevel is initialized, all the services starting

with K are stopped first, and then all the services starting with S are started.



20

Changing the Default Runlevel

By default, Red Hat Enterprise Linux boots into runlevel 5 with a graphical login screen and a

graphical desktop once the user successfully authenticates. Runlevel 3 is essentially the same except

the text login is used.

The default runlevel is configured on the following line from the /etc/inittab file: id:5:initdefault:

To change to a different runlevel without rebooting the system, please type the command:

init <runlevel>

Configuring runlevel

To configuring which services are started for a runlevel, use one of three programs: chkconfig

(command line only), ntsysv (simple text ‐based application that doesn’t require a graphical desktop),

or the Service Configuration Tool (graphical application).

The chkconfig command can be used to configure runlevels and list the current runlevel

configuration. It must be run as root if modifying a runlevel.

To list the status of all services, execute the chkconfig ‐‐list command.

httpd 0:off 1:off 2:off 3:off 4:off 5:off 6:off

To list the status for just one service, provide the name of the service:

chkconfig ‐‐list <service>

To modify whether the service is turned on or off for the runlevel, specify the service name and then

on, off, or reset . Set it to on to have the service started at boot time. Set it to off to have the service

stopped at boot time. Setting it to reset resets the values of all runlevels to the defaults from the initialization script. The syntax is as follows:

chkconfig <service> [on|off|reset]

OR

chkconfig ‐‐level <levels> <service> [on|off|reset]



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24

Optional rpm Arguments When Installing or Updating

Argument Description

‐‐nodeps Install or upgrade the package without checking for dependencies.

The software will most likely not function properly without the

software dependencies installed. If you contact Red Hat support with problems, they will most likely ask you to reproduce the problem on

a system where all package dependencies have been satisfied.

‐‐excludedocs Do not install packages marked as documentation files such as man

pages.

‐‐oldpackage Allow a package to be replaced with an older version.

‐‐test Check for potential conflicts such as package dependencies but do

not install the package.

To remove a package, issue the following command:

rpm ‐e <packagename>

Notice that this time, only <packagename> is used, not the full name of the file used to install the

software.

If multiple versions of a package are installed, you can use:

rpm ‐e <packagename> ‐<version> ‐<release>

If the package that depends on the package you are trying to remove is still needed on the system,

you should not try to remove the package. If the package that depends on the package you are trying

to remove is also not needed, both must be removed at the same time to resolve the dependency:

rpm ‐e <packagename1> <packagename1>

If a configuration file is part of the package being removed but it has been modified, the file will be

renamed instead of removed with the .rpmsave extension, and a message similar to the following is

displayed:

warning: /etc/sysconfig/samba saved as /etc/sysconfig/samba.rpmsave

What if you want to verify that the files associated with a package haven’t been corrupted?

if you suspect your system has been accessed by a non ‐authorized user, you can verify that the files

from a package have not been changed with the RPM verify feature.



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If the verify function is used, file properties such as file size, MD5 sum, file permissions, file type, and

file ownership are compared to the original values stored in the RPM database.

To verify that the files are associated with a package, use the following command:

rpm ‐V <packagename>

If no output is returned, the files from the package have not been modified since installation. If a file,

such as a configuration file, has been modified, the output is similar to: .M.....T /etc/httpd/conf/httpd.conf

RPM Verification Codes

Code Explanation

S File size has changed

M Mode has changed, including file permissions and file type

5 MD5 sum has changed

D Device major or minor number has changed

L The path of the symbolic link has changed

U The owner of the file has changed

G The group of the file has changed

T The last modified time has changed

Querying Package Files

How do you know which files are associated with which RPM packages? You can query the RPM database and find out:

rpm ‐qf <filename>

The <filename> must be the full path to the file. If the file is associated with an RPM package

installed, the name of the package and the version installed is displayed.

To query the package of the command, you could use:

rpm ‐qf `which <filename>`

To retrieve a list of configuration files from a package installed, use the following command: rpm ‐qc <packagename>

A similar query can be performed to list any documentation files installed with a package:

rpm ‐qd <packagename>

Note: Documentation files include man pages and text or HTML formatted files in /usr/share/doc/

installed by the package.



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SYSTEM ADMINISTRATION

Managing user and group

Each user on a Red Hat Enterprise Linux system is assigned a unique user identification number, also

known as a UID. UIDs below 500 are reserved for system users such as the root user.

By default in Red Hat Enterprise Linux, when a user is added, a private user group is created. By default, the directory /home/<username>/ is created as the user’s home directory.

Adding/modifying/deleting user

The basic syntax to create new user is useradd <option> <username> .

Some options you can use along with useradd command:

Command ‐Line option Description

‐c <fullname> Full name of the user (or a comment about the user). If more thanone word is needed, place quotation marks around the value

‐d <directory> Home directory for the user. The default value is /home/ <username>/

‐e <date> Date on which the user account will expire and be disabled. Use the format YYYY‐MM‐DD (default: never expire or disable).

‐g <group> Default group for the user specified as a group name or group IDnumber.

‐G <group> Comma ‐separated list of additional group names or GIDs to which the user will be a member.

‐p <password> Specify an encrypted password for the user

‐s <shell> Specify the user login shell for the user. The default shell if notspecified is /bin/bash.

‐u <uid> Integer to use for the user ID. Values less than 500 are reserved for system users.

To modify the user usermod <options> <username> .

To create a password for the user, use the command passwd <username> . To delete user userdel <username>. To remove the user’s home directory and mail spool, use the

userdel ‐r <username> command.



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Managing Group A unique integer known as a GID is associated with each group. GIDs below 500 are reserved for

system groups just like UIDs below 500 are reserved for system users.

The basic syntax to create new user is groupadd <groupname>.

To specify a GID, use the groupadd ‐g <gid> <groupname> command. The modify the group such as changing the GID of a group, use the

groupmod ‐g <gid> <groupname> command.

To change the name of the group, use the groupmod ‐n <newname> <groupname> command. To delete an existing group, use the groupdel <groupname> command.

Where they store?

A list of all local users is stored in the /etc/passwd file. This file is in plain text format and is readable by anyone logged in to the system.

Each user is listed on a separate line, with the following format: username:password:uid:gid:real_name:/home/directory:shell

Field Description

username Login name for the user. Can’t contain spaces or tabs.

password The x character that denotes the encrypted password is stored in /etc/shadow. If shadow passwords are not used, this field contains the encrypted user password.

uid Unique integer used as the user ID.gid Unique integer used as the group ID.real_name Full name of the user (not required). /home/directory Full path to the home directory of the user.shell Login shell for the user. /bin/bash is the default.

If shadow passwords are used (the default), the encrypted passwords are stored in the /etc/shadow file. All user’s groups are stored in the /etc/group file, readable by everyone but only writable by root. Each group is listed on a separate line in the following format:

groupname:password:gid:users

The default values used when adding a user are stored in the /etc/default/useradd file. if you edit the file /etc/default/useradd you will see:

# useradd defaults file GROUP=100 HOME=/home INACTIVE=‐1 EXPIRE= SHELL=/bin/bash SKEL=/etc/skel

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