how to edit next line after matching pattern and append the line if doesn't exist

Question

I am looking for changing a file content which is repeated twice with that replacement I want to add extra line to second content in the same file

Example file

User YOURNAME
IdentityFile ~/.ssh/YOURKEY

.
.
.
User YOURNAME
Installing
Installing

Example Output after running script

User adminuser
IdentityFile ~/.ssh/id_rsa

.
.
.
User adminuser
IdentityFile ~/.ssh/id_rsa
Installing
Installing

I am able to change the user and YOURKEY with the below sed command

`sed- i s/"YOURNAME/adminuser"/g /root/.ssh/config`
`sed -i 's/YOURKEY/id_rsa/g' ff1`

But I'm unable to insert IdentityFile ~/.ssh/id_rsa in the next line.

edited

additional info ****User adminuser is having space at the beginning of the line. These files are getting synced on daily bases, So cant remove IdentityFile line. After sync it will get replaced

Final edit which works as required

perl -i -ne 'next if /IdentityFile/; 
            s#YOURNAME#adminuser\n    IdentityFile ~/.ssh/id_rsa#; 
            print' filename

Answer 1

Just remove all cases of IdentityFile and then add them again explicitly:

$ perl -i -ne 'next if /IdentityFile/; 
            s#YOURNAME#adminuser\nIdentityFile ~/.ssh/id_rsa#; 
            print' file
$ cat file
User adminuser
IdentityFile ~/.ssh/id_rsa

.
.
.
User adminuser
IdentityFile ~/.ssh/id_rsa
Installing
Installing

The next if /IdentityFile/ skips any lines matching IdentityFile. The s#YOURNAME#adminuser\nIdentityFile ~/.ssh/id_rsa# will replace any instance of YOURNAME with adminuser, a newline and the IdentityFile line. The final print prints all lines.

Answer 2

Part of the problem is that your template is inconsistent: the first occurrence has an IdentityFile line, while the second does not. You can make it consistent by first deleting the existing IdentityFile lines, and then by adding lines which you want.

To delete lines:

sed -i '/^IdentityFile /d' filename

To add lines, you can do this in sed by matching the User line, and appending a line, e.g.,

sed -e '/^User /'a'\
IdentityFile ~/.ssh/id_rsa' filename