Perform SSH Login Without Password

 Perform SSH Login Without Password Using only 3 Commands.

 $ssh-keygen
 $ssh-copy-id -i ~/.ssh/id_rsa.pub remote_host
 $ssh remote_host

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Common Linux log files name and usage

• /var/log/message: General message and system related stuff
• /var/log/auth.log: Authenication logs
• /var/log/kern.log: Kernel logs
• /var/log/cron.log: Crond logs (cron job)
• /var/log/maillog: Mail server logs
• /var/log/qmail/ : Qmail log directory (more files inside this directory)
• /var/log/httpd/: Apache access and error logs directory
• /var/log/lighttpd: Lighttpd access and error logs directory
• /var/log/boot.log : System boot log
• /var/log/mysqld.log: MySQL database server log file
• /var/log/secure: Authentication log
• /var/log/utmp or /var/log/wtmp : Login records files

Postfix Flush the Mail Queue commands

# postfix flush

To see mail queue, enter:
# mailq



To remove all mail from the queue, enter:
# postsuper -d ALL


To remove all mails in the deferred queue, enter:
# postsuper -d ALL deferred

Install and Configure Mail Server on Linux System

Mail Transfer Agent
A Mail Transfer Agent (MTA) is the program which receives and sends out the email from your server, and is therefore the key part. The default MTA in Ubuntu is Postfix, but exim4 is also fully supported and in the main repository.
Postfix - this guide explains how to set up Postfix.

In order to install Postfix with SMTP-AUTH and TLS, first install the postfix package from the Main repository using your favorite package manager. For example:
sudo aptitude install postfix


Configuration
From a terminal prompt:
sudo dpkg-reconfigure postfix
Insert the following details when asked (replacing server1.example.com with your domain name if you have one):
General type of mail configuration: Internet Site
NONE doesn't appear to be requested in current config
System mail name: server1.example.com
Root and postmaster mail recipient: <admin_user_name>
Other destinations for mail: server1.example.com, example.com, localhost.example.com, localhost
Force synchronous updates on mail queue?: No
Local networks: 127.0.0.0/8
Yes doesn't appear to be requested in current config
Mailbox size limit (bytes): 0 
Local address extension character: + 
Internet protocols to use: all

Using patch and diff commands

Creating patches with diff

Using diff is simple when you are working with single files. To create a patch for a single .
diff -u file1.txt  new.txt  > new_file.patch

For  the directory, use following command:
diff -rupN  directory1/   directory2/   >  dir.patch




Apply  patch  

To apply a patch to a single file, change to the directory where the file is located and call patch:

patch  < dir.patch

you can specify the file on the command line:

patch  file.txt < dir.patch

To remove a patch, use the -R flag, 

patch -R file.txt < file.patch