Difference between EXT2, EXT3,EXT4

Features	EXT2 Second extended file system	EXT3 Third extended filesystem	EXT4 Fourth extended file system
Developer(s)	Rémy Card	Stephen Tweedie	Mingming Cao, Andreas Dilger, Alex Zhuravlev, others
File allocation	bitmap (free space), table (metadata)	bitmap (free space), table (metadata)	Extents/Bitmap
Max. volume size	2–32 TiB	4 TiB – 32 TiB	1 EiB
Max. file size	16 GiB – 2 TiB	16 GiB – 2 TiB	16 TiB (for 4k block filesystem)
Max. number of files	1018	Variable, allocated at creation time	4 billion (specified at filesystem creation time)
Max. file name length	255 bytes	255 bytes	255 bytes
Allowed characters in file names	All bytes except NUL ('\0') and '/'	All bytes except NUL ('\0') and '/'	All bytes except NUL ('\0') and '/' and the special file names "." and ".."
Date range	December 14, 1901 - January 18, 2038	December 14, 1901 – January 18, 2038	14 December 1901 - 25 April 2514
Supported operating systems	Linux, BSD, Windows (through an IFS), Mac OS X	Linux, BSD, Windows (through an IFS)	Linux FreeBSD (read-only with ext4fuse) Mac OS X (read-only with ext4fuse, full with ExtFS) Windows (Read/Write without journaling with ext2fsd)

Difference between LVM1 and LVM2

Features	LVM1	LVM2
RHEL 3 support	YES	NO
RHEL 4 support	NO	YES
Transactional metadata for fast recovery	NO	YES
Shared volume mounts with GFS	NO	YES
Cluster Suite failover supported	YES	YES
Striped volume expansion	NO	YES
Max number PVs	256 PVs	2**32 PVs (4294967296)
Max number LVs	256 LVs	2**32 PVs    (4294967296)
Max device size	2 Terabytes	8 Exabytes (64-bit CPUs)
Volume mirroring support	NO	YES

AWK Commands for Linux

1: Field/text processor
2: Default Field-delimiter is white-space.
3: Stores fields (columns) into tokens, which then become accessible during processing
4: Will accept input from: file or STDIN or pipe


Commands:

1: To print each line in its entirety.

# awk '{ print $0 }' test.txt

2: To print column #1 from each line.

# awk '{print $1}' test.txt

3: To print column #2 from each line.

# awk '{print $2}' test.txt

4: To print column #1 then #2.

# awk'{print $2,$1}' test.txt

5: To print all columns where line includes 'Red'.

# awk'/Red/ {print $0}' test.txt

6: To print column #1,#2,#3 where line include 'Red'.

# awk '/Red/ {print $1, "-",$2,"-",$3}' test.txt

7: To print all columns of records containing '2012' in the second column.

# awk '{if ($2 ~ /2012/) print $0}' test.txt

8: To print lines ending in : 2012.

# awk '/2012$/ {print $0}' test.txt

9: Accept a pipe using above point 8 commands.

# grep 2012 /var/log/messages | awk '/2012/ {print $0}'

10: To print columns $3-$6 where column 2 = '9'.

# awk '{if ($2 ~ /9/) print $3,$4,$5,$6}' /var/log/message

Most useful find commands for Linux SystemAdmin.

1 : Search the file aaa from current directory downwards and print it.

# find . -name aaa - print

2 : Find all files which begin with 'a' or 'b' from current directory.

# find . -name [ab]* -print

3 : Search directories called backup from /usr directory.

# find /usr -type d -name backup -print

4 : Search normal files called backup from /usr directory.

# find /usr -type f -name backup -print

5 : Search character special files called backup from /usr directory.

# find /usr -type c -name backup -print

6 : Search block special files called backup from /usr directory

# find /usr -type b -name backup -print

7 : Search all directories from /usr whose inode number is 1235 and print them.

# find /usr -inum 1235 -print

8 : Search in root directory for all files which have less than 2 links.

# find / -links -2 -print

9 : Search in current directory for all files whose owner is abc1 and group is grp1.

# find . \(-user abc1 -a -group grp1 \) -print

10 : Search in current directory for all files whose owner is abc1 or whose name is myfile1.

# find . \(-user abc1 -o -name myfile1 \) -print

11 : Search in current directory for all files which have permissions 777.

# find . -perm 777 -print

12 : Search in current directory for all files chose size is 10 blocks.

# find . -size 10 -print

13 : Search in current directory for all files whose size  10 bytes(characters).

# find . -size 10c -print

14 : Search in current directory for all files whose size is greater than 10 byets.

# find . -size +10c -print

15 : Search in current directory for all files which were accessed exactly 7 days back.

# find . -atime 7 -print

16 : Search in current directory for all files which have not been accessed since last 7 days (or in other words which were accessed more than 7 days ago.)

# find . -atime +7 -print

17 : Search in current directory for all files which have not been modified since last 7 days (or in other words which have been modified more than 7 days ago.)

# find . -mtime +7 -print

18 : Search in current directory for all files whose status has changed (on creation or modification ) more than 7 days ago.

# find . -ctime +7 -print

19 : Search in current directory for all files whose name is cog and instead of printing their names executes a command rm on the searched files.

# find . -name cog -exec rm {} \;

Here, the {} indicate that the searched files would become arguments for rm.The semicolon is necessary and it has to be preceded by a \ to take away its special meaning.

20 : Same as above except that this time it should ask for confirmation before executing rm command.

# find . -name cog -ok rm {} \;

How to configure audit to monitor file deletion in Red Hat Enterprise Linux?

Issue:
What rules can be created to monitor file deletion operation.

Solution:
Add these two entry in  /etc/audit/audit.rules .


# vi /etc/audit/audit.rules

-a always,exit -F arch=b32 -S unlink -S unlinkat -S rename -S renameat -k delete
-a always,exit -F arch=b64 -S unlink -S unlinkat -S rename -S renameat -k delete


After writing the rules, restart the service auditd and make it on in to retain across reboot.

# /etc/init.d/auditd restart
# chkconfig auditd on

Now for testing create a file and delete it same time.

# touc testing
# rm -rf testing

Trace the log from below path.

# tail -f /var/log/audit/audit.log

How to restore a deleted volume group ?

Issue:

1. Unable to find the volume group, even though all devices are present.
2. LVM metadata has been deleted.

Solution:

Always there is a backup of metadata before and after running any LVM on a PV/VG/LV. which is allways taken by LVM.

Backup location is /etc/lvm/archive/ (the copies taken before executing a command) and in /etc/lvm/backup (copies taken after executing a command).
Also you can find the backup file using vgcfgrestore command.
 
# vgcfgrestore --list   <Volume-Group-Name>

# vgcfgrestore --list geo-vg

Ouput:

# data-vg_00_00000-16736.vg

Using the vgcfgrestore command metadata can be written back to the devices belonging to that volume Group which found from backup.

# vgcfgrestore -f /etc/lvm/archive/<file-name> <Voulme-Group-Name>

Example:

# vgcfgrestore -f /etc/lvm/archive/geo-vg_00_00000-16736.vg  geo-vg

Output:

# Restored volume group geo-vg

How to remove user list from GUI login in RHEL 6.3

Issue

Disable user list from login screen.

Solution

Run below command in terminal login as root.

# gconftool-2 –direct –config-source xml:readwrite:/etc/gconf/gconf.xml.defaults –type bool –set /apps/gdm/simple-greeter/disable_user_list true

Login Error - Unable to perform switch user from GUI RHEL 6

Issue

Switch user option in the GUI interface of locked screen does not work and following error message appears:

Solution

1: Reboot the system in runlevel 5
2: Manually start the gdm using below command.

# gdm


Root Cause

1: System is running in runlevel 3.
2: Gui started using startx command, which does not initialize the display Manager

How to configure and analize Kdump in RHEL 6

Verify the kexec-tools package is installed:
# rpm -q kexec-tools
kexec-tools-2.0.0-245.el6.x86_64
If it is not installed, proceed to install it via yum:
# yum install kexec-tools

Specifying the Kdump Location

Kdump can be configured to dump directly to a device by using below mount points  in /etc/kdump.conf.
# vi /etc/kdump.conf


#raw /dev/sda5
#ext4 /dev/sda3
#ext4 LABEL=/boot
#ext4 UUID=03138356-5e61-4ab3-b58e-27507ac41937
#net my.server.com:/export/tmp
#net user@my.server.com
#path /var/crash
#core_collector makedumpfile -c --message-level 1 -d 31
#core_collector cp --sparse=always
#link_delay 60
#kdump_post /var/crash/scripts/kdump-post.sh
#extra_bins /usr/bin/lftp
#disk_timeout 30
#extra_modules gfs2
#options modulename options
#default shell

Here we have configured with local partition  that is /dev/sdc1.

# df -hT

Filesystem    Type    Size  Used Avail Use% Mounted on
/dev/mapper/VolGroup-lv_root

              ext4     13G  9.0G  2.8G  77% /

tmpfs        tmpfs    182M   23M  160M  13% /dev/shm

/dev/sda1     ext4    485M   37M  423M   8% /boot

/dev/sdc1     ext4    5.0G  138M  4.6G   3% /kdump

Below settings will help you to configure kdump in local mounted partition as this has been done above in /kdump mount point.

# vi /etc/kdump.conf

#raw /dev/sda5
ext4 /dev/sdc1
ext4 LABEL=/kdump
ext4 UUID=03138356-5e61-4ab3-b58e-27507ac41937
#net my.server.com:/export/tmp
#net user@my.server.com
#path /var/crash
#core_collector makedumpfile -c --message-level 1 -d 31
#core_collector cp --sparse=always
#link_delay 60
#kdump_post /var/crash/scripts/kdump-post.sh
#extra_bins /usr/bin/lftp
#disk_timeout 30
#extra_modules gfs2
#options modulename options
#default shell

After the changes done then save and exit and run below two commands.

# e2lable /dev/sdc1 /kdump
# service kdump restart

So now kdump location has been configured  

Testing the Configuration

Since there is no kernel panic or no issue we need to force the Linux kernel to crash, and the YYYY-MM-DD-HH:MM/vmcore file will be copied to the location you have selected in the configuration (that is, to /var/crash by default but here we have customize the path from kdump.conf).

# echo 1 > /proc/sys/kernel/sysrq
# echo c > /proc/sysrq-trigger

You need to wait for few minutes  as the system will reboot and then dump file will be created
After Rebooted then you can check the  YYYY-MM-DD-HH:MM/vmcore will be there.

Kdump Analyze

To analyze the vmcore dump file, you must have the crash and (kernel-debuginfo,kernel-debuginfo-common) packages installed. To install the crash package in your system, type the following at a shell prompt as root:

#yum install crash

Note that in order to use this command, you need to have access to the repository with debugging packages.
kernel-debuginfo,kernel-debuginfo-common packages only available in Redhat Repo.

Running the crash Utility

#crash /usr/lib/debug/lib/modules/`uname -r`/vmlinux     /kdump/YYYY-MM-DD-HH:MM/vmcore
From crash prompt you can type below command for more analysis.
Displaying the Message Buffer
crash> log
Displaying the kernel stack trace.
crash> bt
Displaying status of processes in the system.
crash> ps
Displaying virtual memory information of the current context
crash> vm
Displaying information about open files of the current context
crash> files
Exiting the crash utility
crash> exit

How to disable USB storage devices on Red Hat Enterprise Linux?

 Environment

• Red Hat Enterprise Linux(RHEL) 5
• Red Hat Enterprise Linux(RHEL) 6
• Red Hat Enterprise Linux(RHEL) 7

Solution:

Add the following line in the file /etc/modprobe.d/blacklist.conf

blacklist usb-storage

Note: blacklist.conf file exists in RHEL 5 and 6 environments, but will be created by this method in RHEL 7.