append after a line using sed

The command sed -i '/Port 22/a Port 143' /etc/ssh/sshd_config will insert the line Port 143 after the line containing Port 22 in the file /etc/ssh/sshd_config. Here's a breakdown of the command:

• sed: Stream editor for filtering and transforming text.
• -i: In-place editing. It edits files in place instead of writing to standard output.
• '/Port 22/a Port 143': This part of the command uses a sed command to append (a) the line Port 143 after (/Port 22/) the line containing Port 22.
• /etc/ssh/sshd_config: The file path where the changes will be made, in this case, it's the SSH server configuration file.

By running this command, you are instructing sed to find the line with Port 22 in /etc/ssh/sshd_config and add a new line Port 143 after it. This can be useful for changing or adding configuration options in files without manually editing them.