April 2019 – Linux Book Center

How to boot with an old kernel in RHEL4,5,6/CentOS

In this post, we are going to see How to boot with an old kernel in RHEL4,5,6/CentOS Operating systems.

RedHat Operating System uses GRUB boot loader as default one globally.

We can boot update the kernel using Yum/ RPM Package management like other package upgrades which we are doing.

Use the below command to know which boot loader installed on your OS.

#grubby -bootloader-probe

Changing kernel:

/boot/grub/grub.conf is the grub configuration file.

#cat /boot/grub/grub.conf 

default=0 timeout=5 password --encrypted $6$GXGrYVEnbKXAnQoT$p64OkyclNDt4qM2q47GMsgNxJxQaclNs79gvYYsl4h07ReDtJpt5P5kQn1KQ52u2eW8pKHTqcG50ffv0UlRcW0 splashimage=(hd0,0)/grub/splash.xpm.gz hiddenmenu title Red Hat Enterprise Linux 6.4 (2.6.32-358.el6.x86_64)      ===> kernel 0 root (hd0,0) kernel /vmlinuz-2.6.32-358.el6.x86_64 ro root=/dev/mapper/vg_geeklab-lv_root rd_NO_LUKS  KEYBOARDTYPE=pc KEYTABLE=us LANG=en_US.UTF-8 rd_NO_MD SYSFONT=latarcyrheb-sun16 crashkernel=auto rd_LVM_LV=vg_geeklab/lv_swap rd_LVM_LV=vg_geeklab/lv_root rd_NO_DM rhgb quiet initrd /initramfs-2.6.32-358.el6.x86_64.img title Red Hat Enterprise Linux 6.3 (2.6.32-279.el6.x86_64)      ===> kernel 1 root (hd0,0) kernel /vmlinuz-2.6.32-279.el6.x86_64 ro root=/dev/mapper/vg_geeklab-lv_root rd_NO_LUKS  KEYBOARDTYPE=pc KEYTABLE=us LANG=en_US.UTF-8 rd_NO_MD SYSFONT=latarcyrheb-sun16 crashkernel=auto rd_LVM_LV=vg_geeklab/lv_swap rd_LVM_LV=vg_geeklab/lv_root rd_NO_DM rhgb quiet initrd /initramfs-2.6.32-279.el6.x86_64.img

In this, we can see Default will be 0. From this, we can understand OS will read by the default top kernel.

Whenever we are upgrading the kernel that will be coming up and considered as 0 and old kernel will be marked as 1.

So, We should change the number from “0” to “1” on below line of /boot/grub/grub.conf file using vi editor and save it.

default=1

We done the needed configuration change to boot the OS with an old kernel.

On the next boot it will take effect.

Using below command reboot the system and then check whether its booting with old kernel or new one.

#shutdown -r now

Use the below command to check the kernel versions which is in use right now

#uname -r

awk command in linux

We are going to see how to use awk command in Linux in this post.

It’s a scripting language and it’s used to generate Reports and Data Manipulation.

Syntax:

#awk <option> 'criteria {action}' input_file > output_file

Awk command to print file content:

[root@localhost ~]# awk '{print}' testfile.txt
Abu 1234
Thahir 5678
Tharun 9101
Rishi 2345

Above example is only to print all the content of a file.

Awk command to print the lines which match with the given pattern:

[root@localhost ~]# awk '/Rishi/{print}' testfile.txt
Rishi 2345

awk command to split a line to fields:

$1 will be considered the first word as the first field in a line. accordingly $2,$3, etc…

[root@localhost ~]# awk '{print $2}' testfile.txt
1234
5678
9101
2345
[root@localhost ~]#

Built-in variables in awk:

NF: We can print the last field of the lines by using NF in awk command

Example:

[root@localhost ~]# awk '{print $NF}' testfile.txt
25000
30000
20000
15000

NR: Using NR built-in option, we can print the specific fields along with line numbers and can print all content of a file along with the line numbers. Also, we can print the range of lines using NR in awk command.

Examples:

1. Displaying specific row with a specific field in a file

[root@localhost ~]# awk 'NR==2 {print $1,$3}' testfile.txt
Thahir 30000

2.Displaying content of a range of lines(from 2 to 4th line)

[root@localhost ~]# awk 'NR==2, NR==4 {print $1,$3}' testfile.txt
Thahir 30000
Tharun 20000
Rishi 15000

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How to install Ansible on RHEL7/ CentOS7

We are going to see how to install Ansible on RHEL7/ CentOS7 in this post.

Control node needs to install Python 2.6 or latest version and windows doesn’t support for control node.

Since the ansible agentless tool, on Managed hosts no need to install any specific agent/client. And need to install python 2.4 or latest version on managed hosts.

Installing Ansible on RHEL7/ CentOS7:

To install the Ansible we should have Enabled EPEL repository on our server already

Once enable EPEL Repo, then we can start installing Ansible using yum.

[root@localhost ~]# yum install ansible -y

Post installation of ansible will check the version of Ansible by using below command

[root@localhost ~]# ansible --version
ansible 2.7.9
 config file = /etc/ansible/ansible.cfg
 configured module search path = [u'/root/.ansible/plugins/modules', u'/usr/share/ansible/plugins/modules']
 ansible python module location = /usr/lib/python2.7/site-packages/ansible
 executable location = /usr/bin/ansible
 python version = 2.7.5 (default, Aug 2 2016, 04:20:16) [GCC 4.8.5 20150623 (Red Hat 4.8.5-4)]
[root@localhost ~]#

Finally, we installed ansible over our machine which we are going to take it as a control node.

Hereafter if we want to deploy or manage any remote hosts(Managed Host) from the control node, SSH authentication is mandatory. So, We should copy and paste the SSH keys to the remote hosts to make the communication available between the control and managed node.

Reference: Ansible documented site