Monday, September 27, 2021

Difference between RHEL 6 and RHEL 7

Features	RHEL 7	RHEL 6
Default File System	XFS	EXT4
Kernel Version	3.10.x-x kernel	2.6.x-x Kernel
Kernel Code Name	Maipo	Santiago
General Availability Date of First Major Release	2014-06-09 (Kernel Version 3.10.0-123)	2010-11-09 (Kernel Version 2.6.32-71)
First Process	systemd (process ID 1)	init (process ID 1)
Runlevel	runlevels are called as "targets" as shown below: runlevel0.target -> poweroff.target runlevel1.target -> rescue.target runlevel2.target -> multi-user.target runlevel3.target -> multi-user.target runlevel4.target -> multi-user.target runlevel5.target -> graphical.target runlevel6.target -> reboot.target /etc/systemd/system/default.target (this by default is linked to the multi-user.target OR graphical.target)	Traditional runlevels defined : runlevel 0 runlevel 1 runlevel 2 runlevel 3 runlevel 4 runlevel 5 runlevel 6 and the default runlevel would be defined in "/etc/inittab" file.
Host Name Change	In Red Hat Enterprise Linux 7, as part of the move to the new init system (systemd), the hostname variable is defined in "/etc/hostname" file.	In Red Hat Enterprise Linux 6, the hostname variable was defined in the "/etc/sysconfig/network" configuration file.
Change In UID Allocation	By default a new user created would get UIDs assigned starting from 1000. This could be changed in "/etc/login.defs" file if required.	Default UID assigned to users would start from 500. This could be changed in "/etc/login.defs" file if required.
Max Supported File Size	Maximum (individual) file size = 500TB Maximum filesystem size = 500TB (This maximum file size is only on 64-bit machines. Red Hat Enterprise Linux does not support XFS on 32-bit machines.)	Maximum (individual) file size = 16TB Maximum filesystem size = 16TB (This maximum file size is based on a 64-bit machine. On a 32-bit machine, the maximum files size is 8TB.)
File System Check	"xfs_repair" XFS does not run a file system check at boot time.	"e2fsck" File system check would gets executed at boot time.
Differences Between xfs_repair & e2fsck	"xfs_repair" - Inode and inode blockmap (addressing) checks. - Inode allocation map checks. - Inode size checks. - Directory checks. - Pathname checks. - Link count checks. - Freemap checks. - Super block checks.	"e2fsck" - Inode, block, and size checks. - Directory structure checks. - Directory connectivity checks. - Reference count checks. - Group summary info checks.
Difference Between xfs_growfs & resize2fs	"xfs_growfs" xfs_growfs takes mount point as arguments.	"resize2fs" resize2fs takes logical volume name as arguments.
Change In File System Structure	/bin, /sbin, /lib, and /lib64 are now nested under /usr.	/bin, /sbin, /lib, and /lib64 are usually under /
Boot Loader	GRUB 2 Supports GPT, additional firmware types, including BIOS, EFI and OpenFirmware. Ability to boot on various file systems (xfs, ext4, ntfs, hfs+, raid, etc)	GRUB 0.97
KDUMP	RHEL7 supports kdump on large memory based systems up to 3 TB	Kdump doesn’t work properly with large RAM based systems.
System & Service Manager	"Systemd" systemd is a system and service manager for Linux, and replaces SysV and Upstart used in previous releases of Red Hat Enterprise Linux. systemd is compatible with SysV and Linux Standard Base init scripts.	Upstart
Enable/Start Service	For RHEL 7, the systemctl command replaces service and chkconfig. - Start Service : "systemctl start nfs-server.service". - Enable Service : To enable the service (example: nfs service ) to start automatically on boot : "systemctl enable nfs-server.service". Although one can still use the service and chkconfig commands to start/stop and enable/disable services, respectively, they are not 100% compatible with the RHEL 7 systemctl command	Using "service" command and "chkconfig" commands. - Start Service : "service nfs start" OR "/etc/init.d/nfs start" - Enable Service : To start with specific runlevel : "chkconfig --level 3 5 nfs on"
Default Firewall	"Firewalld (Dynamic Firewall)" The built-in configuration is located under the "/usr/lib/firewalld" directory. The configuration that you can customize is under the "/etc/firewalld" directory. It is not possible to use Firewalld and Iptables at the same time. But it is still possible to disable Firewalld and use Iptables as before.	Iptables
Network Bonding	"Team Driver" -/etc/sysconfig/network-scripts/ifcfg-team0 - DEVICE=”team0” - DEVICETYPE=”Team”	"Bonding" -/etc/sysconfig/network-scripts/ifcfg-bond0 - DEVICE=”bond0”
Network Time Synchronization	Using Chrony suite (faster time sync compared with ntpd)	Using ntpd
NFS	NFS4.1 NFSv2 is no longer supported. Red Hat Enterprise Linux 7 supports NFSv3, NFSv4.0, and NVSv4.1 clients.	NFS4
Cluster Resource Manager	Pacemaker	Rgmanager
Load Balancer Technology	Keepalived and HAProxy	Piranha
Desktop/GUI Interface	GNOME3 and KDE 4.10	GNOME2
Default Database	MariaDB is the default implementation of MySQL in Red Hat Enterprise Linux 7	MySQL
Managing Temporary Files	RHEL 7 uses systemd-tmpfiles (more structured, and configurable, method to manage tmp files and directories).	Using "tmpwatch"
References :- What are the file and file system size limitations for Red Hat Enterprise Linux? All about RHEL 7 Red Hat Enterprise Linux technology capabilities and limits RHEL 7 Videos

Friday, September 21, 2012

awk

why awk?

awk is small, fast, and simple, unlike, say, perl. awk also has a clean comprehensible C-like input language, unlike, say, perl. And while it can’t do everything you can do in perl, it can do most things that are actually text processing, and it’s much easier to work with.

what do you do?

In its simplest usage awk is meant for processing column-oriented text data, such as tables, presented to it on standard input. The variables $1, $2, and so forth are the contents of the first, second, etc. column of the current input line. For example, to print the second column of a file, you might use the following simple awk script:

        awk < file '{ print $2 }'

This means “on every line, print the second field”.
To print the second and third columns, you might use

        awk < file '{ print $2, $3 }'

Input separator

By default awk splits input lines into fields based on whitespace, that is, spaces and tabs. You can change this by using the -F option to awk and supplying another character. For instance, to print the home directories of all users on the system, you might do

        awk < /etc/passwd -F: '{ print $6 }'

since the password file has fields delimited by colons and the home directory is the 6th field.
Print column1, column5 and column7 of a data file or output of any columns list
$awk ‘{print $1, $5, $7}’ data_file
$cat file_name |awk ‘{print $1 $5 $7}’
$ls –al |awk ‘{print $1, $5, $7}’ — Prints file_permissions,size and date
Syntax of running an awk program
Awk ‘program’ input file(s)
List all files names whose file size greater than zero.
$ls –al |awk ‘$5 > 0 {print $9}’
List all files whose file size equal to 512bytes.
$ls –al |awk ‘$5 == 0 {print $9}’
print all lines
$awk ‘{print }’ file_name
$awk ‘{print 0}’ file_name
Number of lines in a file
$awk ‘ END {print NR}’ file_name
Number of columns in each row of a file
$awk ‘ {print NF’} file_name
Sort the output of file and eliminate duplicate rows
$awk ‘{print $1, $5, $7}’ |sort –u
List all file names whose file size is greater than 512bytes and owner is “oracle”
$ls –al |awk ‘$3 == “oracle” && $5 > 512 {print $9}’
List all file names whose owner could be either “oracle” or “root”
$ls –al |awk ‘$3 == “oracle” || $3 == “root” {print $9}’
list all the files whose owner is not “oracle
$ls –al |awk ‘$3 != “oracle” {print $9}’
List all lines which has atlease one or more characters
$awk ‘NF > 0 {print }’ file_name
List all lines longer that 50 characters
$awk ‘length($0) > 50 ‘{print }’ file_name
List first two columns
$awk ‘{print $1, $2}’ file_name
Swap first two columns of a file and print
$awk ‘{temp = $1; $1 = $2; $2 = temp; print }’ file_name
Replace first column as “ORACLE” in a data file
$awk ‘{$1 = “ORACLE”; print }’ data_file
Remove first column values in a data file
$awk ‘{$1 =”"; print }’ data_file
Calculate total size of a directory in Mb
$ls –al |awk ‘{total +=$5};END {print “Total size: ” total/1024/1024 ” Mb”}’
Calculate total size of a directory including sub directories in Mb
$ls –lR |awk ‘{total +=$5};END {print “Total size: ” total/1024/1024 ” Mb”}’
Find largest file in a directory including sub directories
$ls –lR |awk ‘{print $5 “\t” $9}’ |sort –n |tail -1

SWAP PARTITION

SWAP PARTITION :

UNIX / Linux: 2 Ways to Add Swap Space Using dd, mkswap and swapon

by RAMESH NATARAJAN on AUGUST 18, 2010

Question: I would like to add more swap space to my Linux system. Can you explain with clear examples on how to increase the swap space?

Answer: You can either use a dedicated hard drive partition to add new swap space, or create a swap file on an existing filesystem and use it as swap space.

How much swap space is currently used by the system?

Free command displays the swap space. free -k shows the output in KB.

# free -k
             total       used       free     shared    buffers     cached
Mem:       3082356    2043700    1038656          0      50976    1646268
-/+ buffers/cache:     346456    2735900
Swap:      4192956          0    4192956

Swapon command with option -s, displays the current swap space in KB.

# swapon -s
Filename                        Type            Size    Used    Priority
/dev/sda2                       partition       4192956 0       -1

Swapon -s, is same as the following.

# cat /proc/swaps
Filename                        Type            Size    Used    Priority
/dev/sda2                       partition       4192956 0       -1

Method 1: Use a Hard Drive Partition for Additional Swap Space

If you have an additional hard disk, (or space available in an existing disk), create a partition using fdisk command. Let us assume that this partition is called /dev/sdc1

Now setup this newly created partition as swap area using the mkswap command as shown below.

# mkswap /dev/sdc1

Enable the swap partition for usage using swapon command as shown below.

# swapon /dev/sdc1

To make this swap space partition available even after the reboot, add the following line to the /etc/fstab file.

# cat /etc/fstab
/dev/sdc1               swap                    swap    defaults        0 0

Verify whether the newly created swap area is available for your use.

# swapon -s
Filename                        Type            Size    Used    Priority
/dev/sda2                       partition       4192956 0       -1
/dev/sdc1                       partition       1048568 0       -2

# free -k
             total       used       free     shared    buffers     cached
Mem:       3082356    3022364      59992          0      52056    2646472
-/+ buffers/cache:     323836    2758520
Swap:      5241524          0    5241524

Note: In the output of swapon -s command, the Type column will say “partition” if the swap space is created from a disk partition.

Method 2: Use a File for Additional Swap Space

If you don’t have any additional disks, you can create a file somewhere on your filesystem, and use that file for swap space.

The following dd command example creates a swap file with the name “myswapfile” under /root directory with a size of 1024MB (1GB).

# dd if=/dev/zero of=/root/myswapfile bs=1M count=1024
1024+0 records in
1024+0 records out

# ls -l /root/myswapfile
-rw-r--r--    1 root     root     1073741824 Aug 14 23:47 /root/myswapfile

Change the permission of the swap file so that only root can access it.

# chmod 600 /root/myswapfile

Make this file as a swap file using mkswap command.

# mkswap /root/myswapfile
Setting up swapspace version 1, size = 1073737 kB

Enable the newly created swapfile.

# swapon /root/myswapfile

To make this swap file available as a swap area even after the reboot, add the following line to the /etc/fstab file.

# cat /etc/fstab
/root/myswapfile               swap                    swap    defaults        0 0

Verify whether the newly created swap area is available for your use.

# swapon -s
Filename                        Type            Size    Used    Priority
/dev/sda2                       partition       4192956 0       -1
/root/myswapfile                file            1048568 0       -2

# free -k
             total       used       free     shared    buffers     cached
Mem:       3082356    3022364      59992          0      52056    2646472
-/+ buffers/cache:     323836    2758520
Swap:      5241524          0    5241524

Note: In the output of swapon -s command, the Type column will say “file” if the swap space is created from a swap file.

If you don’t want to reboot to verify whether the system takes all the swap space mentioned in the /etc/fstab, you can do the following, which will disable and enable all the swap partition mentioned in the /etc/fstab

# swapoff -a

# swapon -a

Configuring NIS under Red Hat Linux
=========================================

The following describes a procedure to set up NIS network name service under Red Hat Linux. This is geared

toward a small intallation with only one domain. However, it should be fairly evident how to add more NIS

domains. The NIS domain name has nothing to do with any DNS naming convention being used.

-------------------------------------------------------
In these examples, the following conventions are used:
=======================================================

NIS domain: "internal"
Code or configuration file data: colored
Root prompt on NIS master server: master#
Root prompt on NIS client host: client#

==================================
Setting up a NIS master server:

Required packages: yp-tools, ypbind, ypserv, portmap
================================================

Set up "time" service to run via inetd/xinetd, or configure xntpd, or otherwise make sure the host's clock is

synchronized.
--------------------
Edit /etc/yp.conf:
domain internal server ip.of.nis.server
=======================
Edit /etc/ypserv.conf:
dns: no
files: 30
xfr_check_port: yes
* : * : shadow.byname : port
* : * : passwd.adjunct.byname : port
=======================

Edit /etc/sysconfig/network:

NISDOMAIN="internal"
Set NIS domain name:
master# domainname internal
master# ypdomainname internal

Create file /var/yp/securenets:

host 127.0.0.1
255.255.255.0 10.0.0.0

Make sure the "portmap" service is running:

master# service portmap start
master# chkconfig portmap on

Portmap will need a rule in /etc/hosts.allow to allow access from localhost and any hosts that need to access

NIS.
Start ypserv service:

master# service ypserv start

Check that it's listening:

master# rpcinfo -u localhost ypserv

You should see:
program 100004 version 1 ready and waiting
program 100004 version 2 ready and waiting
Initialize the NIS maps:

master# /usr/lib/yp/ypinit -m
=================
Specify local hostname, Ctrl-D, y, let finish.
Start up ypbind, yppasswdd, ypxfrd:
====================
master# service ypbind start
master# service yppasswdd start
master# service ypxfrd start

Set YP services to run on boot-up:
=====================
master# chkconfig ypserv on
master# chkconfig ypbind on
master# chkconfig yppasswdd on
master# chkconfig ypxfrd on

NIS client host setup

Required packages: yp-tools ypbind portmap

Edit /etc/sysconfig/network:
NISDOMAIN=internal

Edit /etc/yp.conf:
domain internal server ip.of.master.server

Edit /etc/hosts:
ip.of.master.server hostname.domain hostname
Set NIS domain-name:
client# domainname internal
client# ypdomainname internal
Edit /etc/nsswitch.conf:
passwd: files nis
shadow: files nis
group: files nis
Make sure the portmap service is running:
client# service portmap start
client# chkconfig portmap on
The /etc/hosts.allow file will need rules allowing access from localhost and the NIS master server.
Start ypbind service:
client# service ypbind start
client# chkconfig ypbind on
Test it out:
client# rpcinfo -u localhost ypbind
client# ypcat passwd

Booting Process In Linux Step By Step

Step by step explanation of Linux boot sequence

In this topic we will discuss indepth of Linux Boot Sequence.How a linux system boots?This will help administrators in
troubleshooting some bootup problem.Before discussing about I will notedown the major component we need to know
who are responsible for the booting process.
        1.BIOS(Basic Input/Output System)
        2.MBR(Master Boot Record)
        3.LILO or GRUB
             LILO:-LInux LOader
             GRUB:-GRand Unified Bootloader
        4.Kernel
        5.init
        6.Run Levels

1.BIOS:
      i.When we power on BIOS performs a Power-On Self-Test (POST) for all of the different hardware components in the system to make sure everything is working properly
     ii.Also it checks for whether the computer is being started from an off position (cold boot) or from a restart (warm boot) is
stored at this location.
     iii.Retrieves information from CMOS (Complementary Metal-Oxide Semiconductor) a battery operated memory chip on the motherboard that stores time, date, and critical system information.
     iv.Once BIOS sees everything is fine it will begin searching for an operating system Boot Sector on a valid master boot sector
on all available drives like hard disks,CD-ROM drive etc.
     v.Once BIOS finds a valid MBR it will give the instructions to boot and executes the first 512-byte boot sector that is the first
sector (“Sector 0″) of a partitioned data storage device such as hard disk or CD-ROM etc .
2.MBR
     i. Normally we use multi-level boot loader.Here MBR means I am referencing to DOS MBR.
     ii.Afer BIOS executes a valid DOS MBR,the DOS MBR will search for a valid primary partition marked as bootable on the hard disk.
     iii.If MBR finds a valid bootable primary partition then it executes the first 512-bytes of that partition which is second level MBR.
     iv. In linux we have two types of the above mentioned second level MBR known as LILO and GRUB
3.LILO
     i.LILO is a linux boot loader which is too big to fit into single sector of 512-bytes.
     ii.So it is divided into two parts :an installer and a runtime module.
     iii.The installer module places the runtime module on MBR.The runtime module has the info about all operating systems installed.
     iv.When the runtime module is executed it selects the operating system to load and transfers the control to kernel.
     v.LILO does not understand filesystems and boot images to be loaded and treats them as raw disk offsets
GRUB
     i.GRUB MBR consists of 446 bytes of primary bootloader code and 64 bytes of the partition table.
     ii.GRUB locates all the operating systems installed and gives a GUI to select the operating system need to be loaded.
     iii.Once user selects the operating system GRUB will pass control to the karnel of that operating system.
see below what is the difference between LILO and GRUB
4.Kernel
     i.Once GRUB or LILO transfers the control to Kernel,the Kernels does the following tasks

Intitialises devices and loads initrd module
mounts root filesystem

5.Init
     i.The kernel, once it is loaded, finds init in sbin(/sbin/init) and executes it.
     ii.Hence the first process which is started in linux is init process.
     iii.This init process reads /etc/inittab file and sets the path, starts swapping, checks the file systems, and so on.
     iv.It runs all the boot scripts(/etc/rc.d/*,/etc/rc.boot/*)
     v.starts the system on specified run level in the file /etc/inittab

6.Runlevel
     i.There are 7 run levels in which the linux OS runs and different run levels serves for different purpose.The descriptions are
given below.

0 – halt
1 – Single user mode
2 – Multiuser, without NFS (The same as 3, if you don’t have networking)
3 – Full multiuser mode
4 – unused
5 – X11
6 – Reboot

ii.We can set in which runlevel we want to run our operating system by defining it on /etc/inittab file.
Now as per our setting in /etc/inittab the Operating System the operating system boots up and finishes the bootup process.
Below are given some few important differences about LILO and GRUB

LILO	GRUB
LILO has no interactive command interface	GRUB has interactive command interface
LILO does not support booting from a network	GRUB does support booting from a network
If you change your LILO config file, you have to rewrite the LILO stage one boot loader to the MBR	GRUB automatically detects any change in config file and auto loads the OS
LILO supports only linux operating system	GRUB supports large number of OS