We can always set time to the commands showed in history command. Why we require to set time for this?

This is totally a security measure and used for troubleshooting/fingerprinting a security threat.

history command sample output before setting this variable.

 466  df -h
 467  df 
 468  exit
 469  nfsstat 
 470  find / -iname *.ppt
 471  ftp2
 472  man ping
 473  ping -R google.com

So how to set that?

Use inbuilt variable HISTTIMEFORMAT to set the values as shown below

#export HISTTIMEFORMAT='%F %T '

%F for setting year/month/day

%T for setting time

now try to execute history command to see the difference

#history

sample output

500  2011-03-16 17:06:09 exit
 501  2011-03-16 17:06:14 apt-get install tree
 502  2011-03-16 17:06:36 tree 
 503  2011-03-16 17:06:54 man tree
 504  2011-03-16 17:07:02 tree -d
 505  2011-03-16 17:07:07 tree 

If you restart/logout from machine will this setting available to you?

A simple answer for this is no. Then how can we make it permanent?

Just append this variable to .bash_profile for every user.

echo "export HISTTIMEFORMAT='%F %T '" >> ~/.bash_profile

How about setting this value to the new user which are going to be created on the machine?

export it to /etc/profile file

echo "export HISTTIMEFORMAT='%F %T '" >> /etc/profile

Please feel free to comment your thoughts on this.