Linux File Systems: Ext2 vs Ext3 vs Ext4

ext2, ext3 and ext4 are all filesystems created for Linux. This article explains the following:
High level difference between these filesystems.
How to create these filesystems.
How to convert from one filesystem type to another.

 Ext2

Ext2 stands for second extended file system.
It was introduced in 1993. Developed by Rémy Card.
This was developed to overcome the limitation of the original ext file system.
Ext2 does not have journaling feature.
On flash drives, usb drives, ext2 is recommended, as it doesn’t need to do the over head of journaling.
Maximum individual file size can be from 16 GB to 2 TB
Overall ext2 file system size can be from 2 TB to 32 TB

Ext3
Ext3 stands for third extended file system.
It was introduced in 2001. Developed by Stephen Tweedie.
Starting from Linux Kernel 2.4.15 ext3 was available.
The main benefit of ext3 is that it allows journaling.
Journaling has a dedicated area in the file system, where all the changes are tracked. When the system crashes, the possibility of file system corruption is less because of journaling.
Maximum individual file size can be from 16 GB to 2 TB
Overall ext3 file system size can be from 2 TB to 32 TB
There are three types of journaling available in ext3 file system.
Journal – Metadata and content are saved in the journal.
Ordered – Only metadata is saved in the journal. Metadata are journaled only after writing the content to disk. This is the default.
Writeback – Only metadata is saved in the journal. Metadata might be journaled either before or after the content is written to the disk.
You can convert a ext2 file system to ext3 file system directly (without backup/restore).
Ext4
 
Ext4 stands for fourth extended file system.
It was introduced in 2008.
Starting from Linux Kernel 2.6.19 ext4 was available.
Supports huge individual file size and overall file system size.
Maximum individual file size can be from 16 GB to 16 TB
Overall maximum ext3 file system size is 1 EB (exabyte). 1 EB = 1024 PB (petabyte). 1 PB = 1024 TB (terabyte).
Directory can contain a maximum of 64,000 subdirectories (as opposed to 32,000 in ext3)
You can also mount an existing ext3 fs as ext4 fs (without having to upgrade it).
Several other new features are introduced in ext4: multiblock allocation, delayed allocation, journal checksum. fast fsck, etc. All you need to know is that these new features have improved the performance and reliability of the filesystem when compared to ext3.
In ext4, you also have the option of turning the journaling feature “off”.
Warning: Don’t execute any of the commands given below, if you don’t know what you are doing. You will lose your data!

Creating an ext2, or ext3, or ext4 filesystem
 
Once you’ve partitioned your hard disk using fdisk command, use mke2fs to create either ext2, ext3, or ext4 file system.
Create an ext2 file system:
mke2fs /dev/sda1
Create an ext3 file system:
mkfs.ext3 /dev/sda1

(or)

mke2fs –j /dev/sda1
Create an ext4 file system:
mkfs.ext4 /dev/sda1

(or)

mke2fs -t ext4 /dev/sda1
Converting ext2 to ext3
For example, if you are upgrading /dev/sda2 that is mounted as /home, from ext2 to ext3, do the following.
umount /dev/sda2

tune2fs -j /dev/sda2

mount /dev/sda2 /home
Note: You really don’t need to umount and mount it, as ext2 to ext3 conversion can happen on a live file system. But, I feel better doing the conversion offline.
Converting ext3 to ext4
If you are upgrading /dev/sda2 that is mounted as /home, from ext3 to ext4, do the following.
umount /dev/sda2

tune2fs -O extents,uninit_bg,dir_index /dev/sda2

e2fsck -pf /dev/sda2

mount /dev/sda2 /home
Again, try all of the above commands only on a test system, where you can afford to lose all your data.

Perform SSH and SCP Without Entering Password on openSSH

In this article, I’ll explain how to perform ssh and scp without entering the password using the SSH Public Key authentication with SSH Agent on openSSH
There are two levels of security in the SSH key based authentication. In order for you to login, you need both the private key and the passphrase. Even if one of them is compromised, attacker still cannot login to your account, as both of them are needed to login. This is far better than typical password based authentication, where if the password is compromised, attacker can gain access to the system.
There are two ways to perform ssh and scp without entering the password:
No passphrase. While creating key pair, leave the passphrase empty. Use this option for the automated batch processing. for e.g. if you are running a cron job to copy files between machines this is suitable option.
Use passphrase and SSH Agent. If you are using ssh and scp interactively from the command-line and you don’t want to use the password everytime you perform ssh or scp, I don’t recommend the previous option (no passphrase), as you’ve eliminated one level of security in the ssh key based authentication. Instead, use the passphrase while creating the key pair and use SSH Agent to perform ssh and scp without having to enter the password everytime as explained in the steps below.
Following 8 steps explains how to perform SSH and SCP from local-host to a remote-host without entering the password on openSSH system

1. Verify that local-host and remote-host is running openSSH

 [local-host]$ ssh -V
OpenSSH_4.3p2, OpenSSL 0.9.8b 04 May 2006

[remote-host]$ ssh -V
OpenSSH_4.3p2, OpenSSL 0.9.8b 04 May 2006

2. Generate key-pair on the local-host using ssh-keygen

[local-host]$ ssh-keygen
Generating public/private rsa key pair.
Enter file in which to save the key (/home/jsmith/.ssh/id_rsa):<Hit enter>
Enter passphrase (empty for no passphrase): <Enter your passphrase here>
Enter same passphrase again:<Enter your passphrase again>
Your identification has been saved in /home/jsmith/.ssh/id_rsa.
Your public key has been saved in /home/jsmith/.ssh/id_rsa.pub.
The key fingerprint is:
31:3a:5d:dc:bc:81:81:71:be:31:2b:11:b8:e8:39:a0 jsmith@local-host

 The public key and private key are typically stored in .ssh folder under your home directory. In this example, it is under /home/jsmith/.sshd. You should not share the private key with anybody.
By default the ssh-keygen on openSSH generates RSA key pair. You can also generate DSA key pair using: ssh-keygen -t dsa command.

3. Install public key on the remote-host

Copy the content of the public key from the local-host and paste it to the /home/jsmith/.ssh/authorized_keys on the remote-host. If the /home/jsmith/.ssh/authorized_keys already has some other public key, you can append this to the end of it. If the .ssh directory under your home directory on remote-host doesn’t exist, please create it.

[remote-host]$ vi ~/.ssh/authorized_keys
ssh-rsa ABIwAAAQEAzRPh9rWfjZ1+7Q369zsBEa7wS1RxzWR jsmith@local-host

In simple words, copy the local-host:/home/jsmith/.ssh/id_rsa.pub to remote-host:/home/jsmith/.ssh/authorized_keys

4. Give appropriate permission to the .ssh directory on the remote-
 host.
 [remote-host]$ chmod 755 ~/.ssh
[remote-host]$ chmod 644 ~/.ssh/authorized_keys

 5. Login from the local-host to remote-host using the SSH key authentication to verify whether it works properly.

[local-host]$ <You are on local-host here>

[local-host]$ ssh -l jsmith remote-host
Enter passphrase for key '/home/jsmith/.ssh/id_rsa': <Enter your passphrase here>
Last login: Sat Jun 07 2008 23:03:04 -0700 from 192.168.1.102
No mail.

[remote-host]$ <You are on remote-host here>

6. Start the SSH Agent on local-host to perform ssh and scp without having to enter the passphrase several times

Verify whether SSH agent is already running, if not start it as shown below.
[local-host]$ ps -ef | grep ssh-agent
511 9789 9425 0 00:05 pts/1 00:00:00 grep ssh-agent

[local-host]$ ssh-agent $SHELL

[local-host]$ ps -ef | grep ssh-agent
511 9791 9790 0 00:05 ? 00:00:00 ssh-agent /bin/bash
511 9793 9790 0 00:05 pts/1 00:00:00 grep ssh-agent

7. Load the private key to the SSH agent on the local-host

[local-host]$ ssh-addEnter passphrase for /home/jsmith/.ssh/id_rsa: <Enter your passphrase here>
Identity added: /home/jsmith/.ssh/id_rsa (/home/jsmith/.ssh/id_rsa)
Following are the different options available in the ssh-add:
ssh-add <key-file-name>: Load a specific key file.
ssh-add -l: List all the key loaded in the ssh agent.
ssh-add -d <key-file-name>: Delete a specificy key from the ssh agent
ssh-add -D: Delete all key

8. Perform SSH or SCP to remote-home from local-host without entering the password

local-host]$<You are on local-host here>
[local-host]$ ssh -l jsmith remote-host
Last login: Sat Jun 07 2008 23:03:04 -0700 from 192.168.1.102
No mail.
<ssh did not ask for passphrase this time>
[remote-host]$ <You are on remote-host here>

Linux disk recovery

After any power problem with the linux machine, the filesystem could be corrupted and you may not be able to login. When this happens, you will need e2fsck, the tool to recover ext2 or ext3 partitions. (Do not ever use it with a partition that has AFS or another format).

Boot with a live cd or the first cd of Redhat and write "linux rescue".
Do not let the rescue mode to detect the linux partitions, else it would mount them and you do not want to mount the partitions yet, the file system check (e2fsck) can cause severe problems on a mounted partition.

When you are at the rescue mode prompt, write:

# e2fsck -c -c -v -y /dev/hda1
It is important to user "-c -c" (twice) this means that badblocks will scan using a non-destructive read-write test.
"-v" to verbose and see the status of fsck.
"-y" says to answer "yes" to all the questinons from fsck.

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How to Add a new yum repository to install software under CentOS / Redhat Linux

CentOS / Fedora Core / RHEL 5 uses yum for software management. Yum allows you to add a new repository as a source to install binary software.

Understanding yum repository
 
yum repository configured using /etc/yum.conf file. Additional configuration files are also read from the directories set by the reposdir option (default is /etc/yum.repos.d and /etc/yum/repos.d.
RPMforge repository
Usually repository carries extra and useful packages. RPMforge is one of such repository. You can easily configure RPMforge repository for RHEL5 just by running following single RPM command:
# rpm -Uhv http://apt.sw.be/packages/rpmforge-release/rpmforge-release-0.3.6-1.el5.rf.i386.rpm
For 64 bit RHEL 5 Linux, enter:
# rpm -Uhv http://apt.sw.be/packages/rpmforge-release/rpmforge-release-0.3.6-1.el5.rf.x86_64.rpm
Now you can install software from RPMforge.

How do I install 3rd party repository manually?
 
Let us say you would like to install 3rd party repository from foo.nixcraft.com. Create a file called foo:
# cd /etc/yum.repos.d
# vi foo
Append following code:
[foo]
name=Foo for RHEL/ CentOS $releasever - $basearch
baseurl=http://foo.nixcraft.com/centos/$releasever/$basearch/
enabled=1
gpgcheck=1
gpgkey=http://foo.nixcraft.com/RPM-GPG-KEY.txt
Save and close the file.
Where,
[foo] : Repository name i.e. The [main] section must exist for yum to do anything.
name=Foo for RHEL/ CentOS $releasever - $basearch : A human readable string describing the repository name
baseurl=http://foo.nixcraft.com/centos/$releasever/$basearch/ : Must be a URL to the directory where the yum repository’s ‘repodata’ directory lives
enabled=1 : Enabled or disabled repo. To disable the repository temporarily, set the enabled to 0
gpgcheck=1 : Security feature, use GPG key
gpgkey=http://foo.nixcraft.com/RPM-GPG-KEY.txt : GPL file location
Also you need to import the gpg key for the repository as follows:
# rpm --import http://foo.nixcraft.com/RPM-GPG-KEY.txt
Now you are ready to install software from foo repository. For further information refer to yum.conf man page:
$ man yum.conf
$ man yum
Hope this tip will help you to configure repository as and when required

Linux: TMOUT To Automatically Log Users Out

How do I auto Logout my shell user in Linux after certain minutes of inactivity?

Linux bash shell allows you to define the TMOUT environment variable. Set TMOUT to automatically log users out after a period of inactivity. The value is defined in seconds. For example,

export TMOUT=120

The above command will implement a 2 minute idle time-out for the default /bin/bash shell. You can edit your ~/.bash_profile or /etc/profile file as follows to define a 5 minute idle time out:

# set a 5 min timeout policy for bash shell
TMOUT=300
readonly TMOUT
export TMOUT

Save and close the file. The readonly command is used to make variables and functions readonly i.e. you user cannot change the value of variable called TMOUT.
How Do I Disable TMOUT?

To disable auto-logout, just set the TMOUT to zero or unset it as follows:
$ export TMOUT=0
or
$ unset TMOUT
Please note that readonly variable can only be disabled by root in /etc/profile or ~/.bash_profile.