SED: How to comment or nullify N number of starting lines of a file using SED

Lets take an example of simple nagios host config file.

Example file name : localhost.cfg
Lets take an example of simple nagios host config file.

Example file name : localhost.cfg

# Define a host for the local machine
define host{
        use                     linux-server                    
        host_name               localhost
        alias                   localhost
        address                 127.0.0.1
        contacts                nagiosadmin
        }
        
define host{
        use                     windows-server                    
        host_name               windows
        alias                   windows-srv
        address                 10.0.0.1
        contacts                nagiosadmin
        }

Here I want to comment starting 8 lines of my file to make it invalid

to nullify the string values my localhost.cfg should  look like below.

## Define a host for the local machine
#define host{
#        use                     linux-server                    
#        host_name               localhost
#        alias                   localhost
#        address                 127.0.0.1
#        contacts                nagiosadmin
#        }
        
define host{
        use                     windows-server                    
        host_name               windows
        alias                   windows-srv
        address                 10.0.0.1
        contacts                nagiosadmin

 Here we go, we will use SED to add   #  at the beginning of line for starting 8 lines

Using sed to add string on seventh line.

cybeerkeeda@Linux-Maniac:~ sed -i '1,8 s:^:#:' localhost.cfg

Change number 8 with your desired line number and # with your desired word or string

Note :

sed  -i  : Here -i    parameter will directly write the changes into the file.

I suggest to take a automatically take up backup during this comand.

cybeerkeeda@Linux-Maniac:~ sed -i.bak '1,8 s:^:#:' localhost.cfg