When its necessary to add backslash before dot when using sed We need to replace the server_`mn.ftg.com` with localhost in file `ambari-agent.ini` as the following echo $machine server_mn echo $domain ftg.com more ambari-agent.ini [server] hostname=localhost is it necessary to add backslash before the dot as the following: sed "s/hostname=localhost/hostname=$machine\.$domain/g" ambari-agent.ini or it safe to do just ( without back slash ) sed "s/hostname=localhost/hostname=$machine.$domain/g" ambari-agent.ini

You only ever have to escape the dot if it is part of a regular expression and should match a literal dot. To match a literal dot with a regular expression, you may use either `\.` or `[.]`.

The replacement part of an `s` expression in `sed` is however _not_ a regular expression.

The only characters that need to be escaped there (with backslash) are `\`, `&`, newline, and the `s` command delimiter (`/` if using `s/pattern/replacement/`).

This means that your command


sed "s/hostname=localhost/hostname=$machine.$domain/g" ambari-agent.ini


would work, as long as the values of the variables involved adhere to the above-mentioned rules. I would personally additionally anchor the expression to the start of the line and drop the `g` flag from the `s` command.