Basic Linux Commands

You can click below on some Basic Commands    :  Basic Commands   & Basic Commands 2

Features in RHEL6

1. ext4 file system is introduced.
2. xen is removed and kernel virtualization machine (KVM) is introduced.
3. neat command is removed.
4. portmap service is removed.
5. iscsi is introduced, which supports for SAN.
6. rpmbuild is available, which is used to create our own rpms.
7. File encyption is added.
8. palimpsest is available for disk management.
9. Virtual machine will run only on 64bit processors.
10. postfix service is recommended instead of sendmail service

Linux Disk Quota Implementation

Linux User Disk Quota Implementation

What is disk quota?
Ans : Disk quota is  restricting the disk-space usage to the users. We have to remember one  thing when we are dealing with disk quota i.e. Disk Quota can be applied only on disks/partitions not on files and folders.

how we can implement disk quota?
Disk quota can be implemented in two ways

a. On INODE
b. On BLOCK

What is an INODE?
Ans : In Linux every object is consider as file, every file will be having an inode number associated and this is very much easy for computer to recognize where the file is located.

Inode stands for Index Node, and is the focus of all file activities in the UNIX file-system.
Each file has one inode that defines the file’s type (regular, directory, device etc), the location on disk, The size of the file, Access permissions, Access times.

Note that the file’s name is not stored in the inode.

So how to know what is your file Inode number?

Ans : Its just simple execute ls -i on your file.

ls -i test.txt

13662 test.txt

I think now you got what is INODE? Lets move on to BLOCK.

BLOCK A block usually represents one least size on a disk, usually one block equal to 1kb. Some terms in Disk quota.

Soft limit: This is the disk limit where the user gets just a warning message saying that your disk quota is going to expire. This is just a warning, no restriction on data creation will occur at this point.

Hard limit : This is the disk limit where user gets error message, I repeat user gets error message stating that unable to create data.

Implementing QUOTA :
Step1 : Select/prepare the partition for quota, most of the time disk quota is implemented for restricting users not to create unwanted data on servers, so we will implement disk quota on /home mount point.

#vi /etc/fstab

Edit the /home mount point as follows
Before editing

/dev/hda2 /home ext3 defaults 0 0

after editing

/dev/hda2 /home ext3 defaults,usrquota 0 0

Step2 : Remounting the partition(this is done because the mount table should be updated to kernel). Other wise you can reboot the system too for updating of mount table, which is not preferred for live servers.

#mount -o remount,rw /home

Here -o specifies options, with remounting /home partition with read and write options.

Step3 : Creating quota database

#quotacheck -cu /home

The option -c for creating disk quota DB and u for user
Check for user database is created or not when you give ls /home you have to see auota.user file in /home directory,which contains user database.

Step4 : Switching on quota

#quotaon /home

Now get the report for default quota values for user sadeek

#repquoata -a | grep sadeek

sadeek_mohd --    4       0               1     0     0

sadeek_m --       4       0       0       1     0     0

sadeek_test --   16       0       0       4     0     0

Step5 : Now implementing disk quota for user sadeek_moh on /home mount point(/dev/hda2)

#setquota -u sadeek_mohd 100 110 0 0 /dev/hda2

Step6 : Checking quota is implemented or not login to user sadeek_mohd and execute this command

#repquota -a

or

#quota

Step7 : Keep creating data, once 100MB is reached user will get an warning message saying, and when he reaches 110MB he cannot create any more data.

Hint : To create a data file you can use seq command as below

#seq 1 10000 > test.txt

this command will create a file with 10000 lines with numbers in it.

Removing quota :
To do this one, all the users should log out from the system so better do it in run level one.

Step8 : Stop the disk quota

#quotaoff /home

Step9 : Removing quota database which is located /home

#rm /home/aquota.user

Step10 : Edit fstab file and remove usrdata from /home line

#vi /etc/fstab

Before editing

/dev/hda2 /home ext3 defaults,usrquota 0 0

After editing

/dev/hda2 /home ext3 defaults 0 0

Step11 : Remount the /home partition

#mount -o remount,rw /home

That’s it you are done with Disk Quota Implementation in Linux. Now test yourself in creating Linux user disk quota on your own.

Configure linux samba server step by step

Configure Linux samba server step by step guide example and implementation

Exam question There are mixed lots of System running on Linux and Windows OS. Some users are working on Windows Operating System. There is a /data directory on Linux server should make available on windows to only sadeek should have right to connect with samba server . Configure to make available.

Configure samba server

How to make a Samba Server on RHEL6

Install Samba packages in your machine by yum command:

[root@sadeek~]$ yum install samba* -y

Open Samba's configuration file:

[root@sadeek ~]$ vi /etc/samba/smb.conf

# at line 58 add the following

unix charset = UTF-8

dos charset = CP932

# at line 75 change to the windows workgroup.

workgroup =  WORKGROUP

# at line 81 uncomment and change the ip addresses.

hosts allow = 127. 192.168. 

(127 is for local host and your machine virtual ip if you are using a VM).

# at line 102 change the parameters.

security = user

[root@sadeek ~]$ vi /etc/sysconfig/iptables

Add the line at the last before commit.

-A INPUT -i vmnet6 -j ACCEPT  (If using VMPlayer).

-A INPUT -i virbr0 -j ACCEPT (If using Virt-Manager).

(The values of vmnet6 and virbr0 may change as per your machine's config.).

Restart service iptables:

[root@sadeek ~]$ service iptables restart

Enable sharing of home directories:

[root@sadeek~]$ setsebool -P samba_enable_home_dirs=on

Now set a password for the user who is going to access that in windows machine:

[root@sadeek~]$ smbpasswd -a sadeek

New SMB password:

Retype new SMB password:

Now restart samba services:

[root@sadeek ~]$ service smb restart

[root@sadeek~]$ service nmb restart

Now make these services to be enabled at boot time so that you need not to start those again and again.

[root@sadeek~]$ chkconfig on smb

[root@sadeek~]$ chkconfig on nmb

Task to be performed on Windows Box:

Double-click the "My Computer" icon on the desktop.

In the address bar, type "\\192.168.122.123" [Your server ip]without the quotes and hit Enter

Username: sadeek (Username to be used when you did smbpasswd -a USERNAME).

Password: Which you used while setting smbpasswd.

Check "Remember my password" so you need not to enter password again and again.

Click on ok.

Right-click the folder named sadeek [USERNAME].

Choose "Map Network Drive".

Ensure that "Reconnect at Logon" is checked

ssh-keygen: password-less SSH login

SSH is often used to login from one machine to another machine, There are number of methods to achieve this but mostly in every method it requires authentication..... It also does require authentication but for one time only i.e. for the first time you need to do a setup and for rest of the times when you will try to login via ssh it will not ask for any password.

you just need to generate your own personal set of private/public key pair. ssh-keygen is used to generate that key pair.

How to generate public/private key

root@server:/other_part# ssh-keygen -t rsa Press  (enter)
generating public/private rsa key pair.
Enter file in which to save the key (/root/.ssh/id_rsa):   (save your default  on default location)
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /root/.ssh/id_rsa.  (enter)
Your public key has been saved in /root/.ssh/id_rsa.pub.   (enter)
The key fingerprint is:
dc:20:59:db:43:6a:8c:52:7a:08:08:10:0b:eb:da:3d root@server
The key's randomart image is:
+--[ RSA 2048]----+
|Bo.   . . .      |
|oo . + = =       |
|o   + = * o      |
|.    o + o .     |
| .      S .      |
|.. .             |
|. . E            |
|     .           |
|                 |
+-----------------+
|++E...           |
|o.oo.            |
+-----------------+

Now copy private key on remote machine with below mention command

root@server:/other_part# ssh-copy-id -i /root/.ssh/id_rsa 172.27.24.75
root@172.27.24.75's password:  (Enter Password)
Now try logging into the machine, with "ssh '172.27.24.75'", and check in:
  ~/.ssh/authorized keys

to make sure we haven't added extra keys that you weren't expecting.

While you are copying the private key then it will ask password.. Now try login again on remote machine it will not ask the password

root@server:/other_part# ssh 172.27.24.75

Last login: Wed Feb  5 12:36:46 2014 from server.oasis.com

[root@virtual ~]# 

njoy :)

6 Stages of Linux Boot Process

Press the power button on your system, and after few moments you see the Linux login prompt.

Have you ever wondered what happens behind the scenes from the time you press the power button until the Linux login prompt appears?

The following are the 6 high level stages of a typical Linux boot process.

1. BIOS

BIOS stands for Basic Input/Output System

Performs some system integrity checks

Searches, loads, and executes the boot loader program.

It looks for boot loader in floppy, cd-rom, or hard drive. You can press a key (typically F2 of ESC, but it depends on your system) during the BIOS startup to change the boot sequence.

Once the boot loader program is detected and loaded into the memory, BIOS gives the control to it.

So, in simple terms BIOS loads and executes the MBR boot loader.

2. MBR

MBR stands for Master Boot Record.

It is located in the 1st sector of the bootable disk. Typically /dev/hda, or /dev/sda

MBR is less than 512 bytes in size. This has three components 1) primary boot loader info in 1st 446 bytes 2) partition table info in next 64 bytes 3) mbr validation check in last 2 bytes.

It contains information about GRUB (or LILO in old systems).

So, in simple terms MBR loads and executes the GRUB boot loader.

3. GRUB

GRUB stands for Grand Unified Bootloader.

If you have multiple kernel images installed on your system, you can choose which one to be executed.

GRUB displays a splash screen, waits for few seconds, if you don’t enter anything, it loads the default kernel image as specified in the grub configuration file.

GRUB has the knowledge of the filesystem (the older Linux loader LILO didn’t understand filesystem).

Grub configuration file is /boot/grub/grub.conf (/etc/grub.conf is a link to this). The following is sample grub.conf of CentOS.

#boot=/dev/sda

default=0

timeout=5

splashimage=(hd0,0)/boot/grub/splash.xpm.gz

hiddenmenu

title CentOS (2.6.18-194.el5PAE)

          root (hd0,0)

          kernel /boot/vmlinuz-2.6.18-194.el5PAE ro root=LABEL=/

          initrd /boot/initrd-2.6.18-194.el5PAE.img

As you notice from the above info, it contains kernel and initrd image.

So, in simple terms GRUB just loads and executes Kernel and initrd images.

4. Kernel

Mounts the root file system as specified in the “root=” in grub.conf

Kernel executes the /sbin/init program

Since init was the 1st program to be executed by Linux Kernel, it has the process id (PID) of 1. Do a ‘ps -ef | grep init’ and check the pid.

initrd stands for Initial RAM Disk.

initrd is used by kernel as temporary root file system until kernel is booted and the real root file system is mounted. It also contains necessary drivers compiled inside, which helps it to access the hard drive partitions, and other hardware.

5. Init

Looks at the /etc/inittab file to decide the Linux run level.

Following are the available run levels

0 – halt

1 – Single user mode

2 – Multiuser, without NFS

3 – Full multiuser mode

4 – unused

5 – X11

6 – reboot

Init identifies the default initlevel from /etc/inittab and uses that to load all appropriate program.

Execute ‘grep initdefault /etc/inittab’ on your system to identify the default run level

If you want to get into trouble, you can set the default run level to 0 or 6. Since you know what 0 and 6 means, probably you might not do that.

Typically you would set the default run level to either 3 or 5.

6. Runlevel programs

When the Linux system is booting up, you might see various services getting started. For example, it might say “starting sendmail …. OK”. Those are the runlevel programs, executed from the run level directory as defined by your run level.

Depending on your default init level setting, the system will execute the programs from one of the following directories.

Run level 0 – /etc/rc.d/rc0.d/

Run level 1 – /etc/rc.d/rc1.d/

Run level 2 – /etc/rc.d/rc2.d/

Run level 3 – /etc/rc.d/rc3.d/

Run level 4 – /etc/rc.d/rc4.d/

Run level 5 – /etc/rc.d/rc5.d/

Run level 6 – /etc/rc.d/rc6.d/

Please note that there are also symbolic links available for these directory under /etc directly. So, /etc/rc0.d is linked to /etc/rc.d/rc0.d.

Under the /etc/rc.d/rc*.d/ direcotiries, you would see programs that start with S and K.

Programs starts with S are used during startup. S for startup.

Programs starts with K are used during shutdown. K for kill.

There are numbers right next to S and K in the program names. Those are the sequence number in which the programs should be started or killed.

For example, S12syslog is to start the syslog deamon, which has the sequence number of 12. S80sendmail is to start the sendmail daemon, which has the sequence number of 80. So, syslog program will be started before sendmail.

That is what happens during the Linux boot process.

Linux Filesystem Management

badblocks		Used to search a disk or partition for badblocks.
cfdisk		Similar to fdisk but with a nicer interface.
debugfs		Allows direct access to filesystems data structure.
df		Shows the disk free space on one or more filesystems.
dosfsck		Check and repair MS-Dos filesystems.
du		Shows how much disk space a directory and all its files contain.
dump		Used to back up an ext2 filesystem. Complement is restore.
dumpe2fs		Dump filesystem superblock and blocks group information. Ex: dumpe2fs /dev/hda2
e2fsck		Check a Linux second extended filesystem.
e2label		Change the label on an ext2 filesystem.
exportfs		Used to set up filesystems to export for nfs (network file sharing).
fdisk		Used to fix or create partitions on a hard drive.
fdformat		Formats a floppy disk.
fsck		Used to add new blocks to a filesystem. Must not be run on a mounted file system.
hdparm		Get/set hard disk geometry parameters, cylinders, heads, sectors.
mkfs		Initializes a Linux filesystem. This is a front end that runs a separate program depending on the filesystem's type.
mke2fs		Create a Linux second extended filesystem.
mkswap		Sets up a Linux swap area on a device or file.
mount		Used to mount a filesystem. Complement is umount.
rdev		Query/set image root device, swap device, RAM disk size of video mode. What this does is code the device containing the root filesystem into the kernel image specified.
rdump		Same as dump.
rmt		Remote magtape protocol module.
restore		Used to restore an ext2 filesystem.
setfdprm		Set floppy drive parameters.
swapoff(8)		Used to de-activate a swap partition.
swapon(8)		Used to activate a swap partition.
sync		Forces all unwritten blocks in the buffer cache to be written to disk.
tune2fs		Adjust tunable filesystem parameters on second extended filesystems.
umount		Unmounts a filesystem. Complement is mount.

Linux Configuration Files

profile		System wide environment and startup script program.
/dev/MAKEDEV		The /dev/MAKEDEV file is a script written by the system administrator that creates local only device files or links such as device files for a non-standard device driver.
/etc/aliases		Where the user's name is matched to a nickname for e-mail.
/etc/bootptab		The configuration for the BOOTP server daemon.
/etc/crontab		Lists commands and times to run them for the cron deamon.
/etc/dhcpd.conf		The configuration file for the DHCP server daemon.
/etc/ethers		File for RARP mapping from hardware addresses to IP addresses. See the man page ethers(5).
/etc/exports		The file describing exported filesystems for NFS services.
/etc/fdprm		The floppy disk parameter table. Describes the formats of different floppy disks. Used by setfdprm.
/etc/filesystems		Can be used to set the filesystem probe order when filesystems are mounted with the auto option. The nodev parameter is specified for filesystems that are not really locally mounted systems such as proc, devpts, and nfs systems.
/etc/fstab		Lists the filesystems mounted automatically at startup by the mount -a command (in /etc/rc or equivalent startup file).
/etc/group		Similar to /etc/passwd but for groups rather than users.
/etc/groups		May contain passwords that let a user join a group.
/etc/gshadow		Used to hold the group password and group administrator password information for shadow passwords.
/etc/host.conf		Specifies how host names are resolved.
/etc/hosts		List hosts for name lookup use that are locally required.
/etc/HOSTNAME		Shows the host name of this host. Used for support of older programs since the hostname is stored in the /etc/sysconfig/network file.
/etc/inittab		Configuration file for init, controls startup run levels, determines scripts to start with.
/etc/inetd.conf		Sets up the services that run under the inetd daemon.
/etc/issue		Output by getty before the login prompt. Description or welcoming message.
/etc/issue.net		Output for network logins with LINUX version
/etc/ld.so.conf		Configuration file for ld.so, the run time linker.
/etc/lilo.conf		Configuration file for LILO.
/etc/limits		Limits users resources when a system has shadow passwords installed.
/etc/localtime		In Debian the system time zone is determined by this link.
/etc/login.defs		Sets user login features on systems with shadow passwords.
/etc/logrotate.conf		Configures the logrotate program used for managing logfiles.
/etc/magic		The configuration file for file types. Contains the descriptions of various file formats for the file command.
/etc/motd		The message of the day, automatically output by a successful login.
/etc/mtab		A list of currently mounted file systems. Setup by boot scripts and updated by the mount command.
/etc/named.conf		Used for domain name servers.
/etc/networks		Lists names and addresses of your own and other networks, used by the route command.
/etc/nologin		If this file exists, non-root logins are disabled. Typically it is created when the system is shutting down.
/etc/nsswitch.conf		Name service switch configuration file.
/etc/passwd		The user database with fields giving the username, real name, home directory, encrypted password and other information about each user.
/etc/printcap		A configuration file for printers.
/etc/profile, /etc/cshlogin, /etc/csh/cshrc		Files executed at login or startup time by the Bourne or C shells. These allow the system administrator to set global defaults for all users.
/etc/protocols		Describes DARPA internet protocols available from the TCP/IP subsystem. Maps protocol ID numbers to protocol names.
/etc/rc or /etc/rc.d or /etc/rc?.d		Scripts or directories of scripts to run at startup or when changing run level.
/etc/rc.d/rc0.d		Contains files used to control run level 0. Usually these files are softlink files.
/etc/rc.d/rc1.d		Contains files to control run level 1. Scripts beginning with an S are for start, K for kill.
/etc/rc.d/rc.sysinit		Init runs this when it starts.
/etc/resolv.conf		Configures the name resolver, specifying the address of your name server and your domain name.
/etc/securetty		Identifies secure terminals from which root is allowed to log in.
/etc/services		Lists the network services that the system supports.
/etc/shadow		Shadow password file on systems with shadow password software installed. Shadow passwords move the encrypted password files from /etc/passwd to /etc/shadow which can only be read by root.
/etc/shadow.group		Systems with shadow passwords may have this file.
/etc/shells		Lists trusted shells. The chsh command allows users to change their login shell to shells listed only in this file.
/etc/skel/.profile		Can be used by administrator to set the editor environment variable to some editor that is friendly to new users.
/etc/sudoers		A list of users with special privileges along with the commands they can execute.
/etc/smb.conf		The configuration file for setting up Samba services.
/etc/sysconfig/amd		Used to configure the auto mount daemon.
/etc/sysconfig/clock		Used to configure the system clock to Universal or local time and set some other clock parameters.
/etc/sysconfig/i18n		Controls the system font settings.
/etc/sysconfig/init		This file is used to set some terminal characteristics and environment variables.
/etc/sysconfig/keyboard		Used to configure the keyboard.
/etc/sysconfig/mouse		This file is used to configure the mouse.
/etc/sysconfig/network-scripts/ifcfg-interface		Defines a network interface.
/etc/sysconfig/pcmcia		Used to configure pcmcia network cards.
/etc/sysconfig//routed		Sets up dynamic routing policies.
/etc/sysconfig/static-routes		Configures static routes on a network.
/etc/sysconfig/tape		Used for backup tape device configuration.
/etc/X11/XF86Config		The configuration file for the X server.
/etc/syslog.conf		Configuration file for the syslogd daemon.
/etc/termcap		The terminal capability database. Describes by what "escape sequences" various terminals can be controlled. See terminfo, termcap, curs_termcap man pages.
/etc/terminfo		Details for terminal I/O.
/etc/usertty		This file is used to impose special access restrictions on users.
$HOME/.bashrc		User aliases, path modifier, and functions.
$HOME/.bash_profile		Users environment stuff and startup programs.
$HOME/.bash_logout		User actions to be done at logout.
$HOME/.hushlogin		When this file exists in the user's home directory, it will prevent check for mail, printing of the last login time, and the message of the day when the user logs in.
$HOME/.inputrc		Contains keybindings and other bits.
$HOME/Xrootenv.0		Has networking and environment info.
/proc/cpuinfo		Information about the processor such as its type, make and performance.
/proc/devices		A list of devices configured into the currently running kernel.
/proc/dma		Shows which DMA channels are being used at the moment.
/proc/filesystems		Filesystems that are configured into the kernel. The file used to detect filesystems if the /etc/filesystems does not exist.
/proc/ioports		Shows which I/O ports are in use at the moment.
/proc/interrupts		Shows which interrupts are in use and how many of each there have been.
/proc/kcore		An image of the physical memory of the system.
/proc/kmsg		Messages output by the kernel. These are also routed to syslog.
/proc/ksyms		Symbol table for the kernel.
/proc/loadavg		The load average of the system.
/proc/meminfo		Information about memory usage, both physical and swap.
/proc/modules		Which kernel modules are currently loaded.
/proc/mounts		Contains information on filesystems currently mounted, similar to /etc/mtab
/proc/net		Contains status information about network protocols.
/proc/self		A symbolic link to the process directory of the program that is looking at /proc. When 2 process look at proc, they get different links.
/proc/stat		Various statistics about the system such as the number of page faults since the system was booted.
/proc/uptime		The time the system has been up.
/proc/version		The kernel version.
/tmp/fvwmrca01339		FVWM-M4 defines. Contains networking, Xwindows, other setup info.
/usr/lib/zoneinfo		Time zone datafiles are stored here on the Debian system
/var/log/lastlog		Used by finger to tell when a user was last logged in.
/var/log/wtmp		Binary info on users that have been logged on. The last command uses this info.
/var/run/utmp		Contains information about users currently logged in. Who and w commands use this file.
/var/named/root.hints		Used for domain name server. Placed here optionally, but this is the normal location.
/var/named/*		Files used by domain name server. Placed here optionally, but this is the normal location.
/var/log/btmp		Used to store information about failed logins. This file must be first created to activate it.
/var/log/lastlog		Contains information about the last time a login was done on the system. Works with lastb(1).
/var/log/maillog		The normal system mail log file.
/var/log/messages		The main system message log file.
var/log/secure		System tracking of user logins. Check this file periodically.
/var/spool/mail		Where mailboxes are usually stored.

What are the main differences between RHEL4 & RHEL5 and RHEL 6

#########RHEL4 and RHEL 5 ########################
XEN, YUM and improved SELinux
all the features updated with better options
Better GUI support then RHEL4
YUM over RPM package management
IPTables and SELinux for more secure environment
ext2 & ext3 file system
In RHEL 4 SELinux Block only 13 services, But on RHEL 5
SElinux Block 80 services
 ###############################################

Difference between RHEL 4 and RHEL 5 :
1) In RHEL 4 SELinux Block only 13 services, But on RHEL 5 SElinux Block 80 services.

2) RHEL 4 have ext2 filesystem, but RHEL 5 we have ext3 filesystem that support Journal.

3) RHEL 4 have no virtualization Feature, but in RHEL 5 we have virtualization with Hypervisor-V.

4) In RHEL 4 we have no Yum, But in RHEL 5 we have Yum available.

#####################################################

NMAP Network Mapper

You can click here for NMAP Linux / UNIX: Scanning network for open ports with nmap command