git config --global user.name <username>

This command is used to set the global username for git.

By using the --global flag, this command sets the configuration globally for all repositories on the current system.

It is recommended to set a username before making any commits. Other flags that can be used include --local, which sets the configuration only for the current repository, and --unset, which removes the configured username.

The --replace-all flag would be necessary if you have multiple entries of the same key in your git configuration file and you want to replace all of them with a new value. Without this flag, only the first entry of the key would be replaced.