How to install chrony in Linux

To install Chrony on CentOS, you can use the package manager yum. Here are the steps to install Chrony:

  1. Open a terminal on your CentOS system.
  2. Make sure your system is updated by running the following command:bashCopy codesudo yum update
  3. Install Chrony using the following command:bashCopy codesudo yum install chrony
  4. Once the installation is complete, you can start and enable the Chrony service to ensure it starts automatically on boot:bashCopy codesudo systemctl start chronyd sudo systemctl enable chronyd
  5. Check the status of the Chrony service to ensure it is running:bashCopy codesudo systemctl status chronyd If everything is set up correctly, you should see output indicating that the service is active and running.

Now, Chrony is installed and running on your CentOS system. You can configure Chrony by editing its configuration file located at /etc/chrony.conf. After making any changes to the configuration file, remember to restart the Chrony service:

bashCopy code

sudo systemctl restart chronyd

Make sure to adjust any firewall settings if necessary to allow NTP (Network Time Protocol) traffic if you want your system to synchronize its time with remote NTP servers.

How to Add Date & Time with History in Linux


The command history allows the use of words from previous command lines at the command prompt type. This simplifies spelling corrections and the repetition of complicated commands or arguments.

History Command

# history
  850  useradd admin
  851  passwd admin
  852  cd /home/admin/
  853  ls -l
  854  ls -la
  855  top

Edit the file /etc/bashrc add the entry at the end of the file /etc/bashrc

# vi /etc/bashrc
export HISTTIMEFORMAT="%h/%d - %H:%M:%S "

You can also directly execute the following command:

# export HISTTIMEFORMAT="%h/%d - %H:%M:%S "

Now again check the “history” command:

# history
  850  31/08/15 07:22:21 useradd admin
  851  31/08/15 07:22:21 passwd admin
  852  31/08/15 07:22:21 cd /home/admin/
  853  31/08/15 07:22:21 ls -l
  854  31/08/15 07:22:21 ls -la
  855  31/08/15 07:22:21 top

How to configure NTP Server in RedHat / CentOS / Fedora


Network Time Protocol (NTP) is a networking protocol for clock synchronization between computer systems over packet-switched, variable-latency data networks. NTP is a protocol designed to synchronize the clocks of computers over a network.

By Default Port # 123

Step 1: Install ntp Package

To configure NTP Server, firstly we need to install the ntp package:

# yum -y install ntp
Loaded plugins: amazon-id, rhui-lb
Resolving Dependencies
--> Running transaction check
---> Package ntp.x86_64 0:4.2.6p5-19.el7_0 will be installed
--> Processing Dependency: ntpdate = 4.2.6p5-19.el7_0 for package: ntp-4.2.6p5-19.el7_0.x86_64
--> Processing Dependency: for package: ntp-4.2.6p5-19.el7_0.x86_64
--> Running transaction check
---> Package autogen-libopts.x86_64 0:5.18-5.el7 will be installed
---> Package ntpdate.x86_64 0:4.2.6p5-19.el7_0 will be installed
--> Finished Dependency Resolution

Dependencies Resolved

 Package                 Arch           Version                  Repository                                Size
 ntp                     x86_64         4.2.6p5-19.el7_0         rhui-REGION-rhel-server-releases         540 k
Installing for dependencies:
 autogen-libopts         x86_64         5.18-5.el7               rhui-REGION-rhel-server-releases          66 k
 ntpdate                 x86_64         4.2.6p5-19.el7_0         rhui-REGION-rhel-server-releases          82 k

Transaction Summary
Install  1 Package (+2 Dependent packages)

Total download size: 689 k
Installed size: 1.6 M
Downloading packages:
(1/3): autogen-libopts-5.18-5.el7.x86_64.rpm                                             |  66 kB  00:00:00
(2/3): ntp-4.2.6p5-19.el7_0.x86_64.rpm                                                   | 540 kB  00:00:00
(3/3): ntpdate-4.2.6p5-19.el7_0.x86_64.rpm                                               |  82 kB  00:00:00
Total                                                                           1.3 MB/s | 689 kB  00:00:00
Running transaction check
Running transaction test
Transaction test succeeded
Running transaction
  Installing : ntpdate-4.2.6p5-19.el7_0.x86_64                                                              1/3
  Installing : autogen-libopts-5.18-5.el7.x86_64                                                            2/3
  Installing : ntp-4.2.6p5-19.el7_0.x86_64                                                                  3/3
  Verifying  : autogen-libopts-5.18-5.el7.x86_64                                                            1/3
  Verifying  : ntp-4.2.6p5-19.el7_0.x86_64                                                                  2/3
  Verifying  : ntpdate-4.2.6p5-19.el7_0.x86_64                                                              3/3

  ntp.x86_64 0:4.2.6p5-19.el7_0

Dependency Installed:
  autogen-libopts.x86_64 0:5.18-5.el7                     ntpdate.x86_64 0:4.2.6p5-19.el7_0

Step 2: Configuration

Make changes in ntp.conf file

# vi /etc/ntp.conf

restrict mask nomodify notrap
server iburst
server iburst
server iburst
server iburst

Now start the ntpd service

# /etc/rc.d/init.d/ntpd start
Starting ntpd:            [ OK ]

Enable ntpd in multi-user levels.

# chkconfig ntpd on

To verify NTP peers synchronization status, delay, offset, jitter use the following command:

# ntpq -p
     remote           refid      st t when poll reach   delay   offset  jitter
*natasha.netwurx   2 u    -   64    1   67.464    6.336   0.038
 static-72-87-88     2 u    1   64    1   89.650   -3.463   0.000      2 u    -   64    1   56.794    0.794   0.000
 ns.tx.primate.n .INIT.          16 u    -   64    0    0.000    0.000   0.000

How to Change Time in Linux

Check Date and Time

To check the date and current time of server,use date command:

# date

Wed Jun 17 14:39:35 CEST 2015

Step 1: Modify /etc/localtime File

Edit /etc/localtime to change the timezone data file.

# file /etc/localtime
/etc/localtime: timezone data# file /usr/share/zoneinfo/Asia/Kuala_Lumpur
/usr/share/zoneinfo/Asia/Kuala_Lumpur: timezone data
# date
Wed Jun 17 14:39:35 CEST 2015
# cp /usr/share/zoneinfo/Asia/Kuala_Lumpur /etc/localtime
cp: overwrite `/etc/localtime'? y
# date
Wed Jun 17 14:39:35 CEST 2015

There is also a nice way in order to change the server time using tzselect

# tzselect
Please identify a location so that time zone rules can be set correctly.
Please select a continent or ocean.
1) Africa
2) Americas
3) Antarctica
4) Arctic Ocean
5) Asia
6) Atlantic Ocean
7) Australia
8) Europe
9) Indian Ocean
10) Pacific Ocean
11) none - I want to specify the time zone using the Posix TZ format.
#? 5
Please select a country.
1) Afghanistan 18) Israel 35) Palestine
2) Armenia 19) Japan 36) Philippines
3) Azerbaijan 20) Jordan 37) Qatar
4) Bahrain 21) Kazakhstan 38) Russia
5) Bangladesh 22) Korea (North) 39) Saudi Arabia
6) Bhutan 23) Korea (South) 40) Singapore
7) Brunei 24) Kuwait 41) Sri Lanka
8) Cambodia 25) Kyrgyzstan 42) Syria
9) China 26) Laos 43) Taiwan
10) Cyprus 27) Lebanon 44) Tajikistan
11) East Timor 28) Macau 45) Thailand
12) Georgia 29) Malaysia 46) Turkmenistan
13) Hong Kong 30) Mongolia 47) United Arab Emirates
14) India 31) Myanmar (Burma) 48) Uzbekistan
15) Indonesia 32) Nepal 49) Vietnam
16) Iran 33) Oman 50) Yemen
17) Iraq 34) Pakistan
#? 29
Please select one of the following time zone regions.
1) peninsular Malaysia
2) Sabah & Sarawak
#? 1

Is the above information OK?
1) Yes
2) No
#? 1

You can make this change permanent for yourself by appending the line
TZ='Asia/Kuala_Lumpur'; export TZ
to the file '.profile' in your home directory; then log out and log in again.

Here is that TZ value again, this time on standard output so that you
can use the /usr/bin/tzselect command in shell scripts:

TO set the Hardware Clock of System Time use the command:

# hwclock --systohc


# hwclock -w

How to Set a Cronjob in Linux


A crontab is a simple text file with a list of commands meant to be run at specified times. It is edited with a command-line utility. These commands (and their run times) are then controlled by the cron daemon, which executes them in the system background. Each user has a crontab file which specifies the actions and times at which they should be executed, these jobs will run regardless of whether the user is actually logged into the system. There is also a root crontab for tasks requiring administrative privileges. This system crontab allows scheduling of system wide tasks (such as log rotations and system database updates).


Minutes      Hours       Day-of-Month        Month-field         Day-of-Week        /path/to/Command

Set Cronjob

To set the cronjob that runs every 5 minutes

# crontab -e
*/5 * * * * /root/