The `—rsh` command line option, also known as `—remote-shell`, is an indispensable feature of the Unix-based data synchronization tool RSYNC. It is used to specify an alternative remote shell program to be used for communication between the local and remote copies of RSYNC. This command allows the user to choose an alternative method for communication other than the default shell, which is typically SSH on modern systems.
As per the ‘rsync man page’, under the section ‘USAGE’, the description of this option is as follows:
“—rsh=COMMAND: This option allows the replacement of rsh and ssh. It can also contain additional options that rsync does not supply to COMMAND…”
It means that instead of using the default shell for transference, you can specify a shell of your own like rsh, ssh, or any other customized shell.
When a command such as “`rsync -avz —rsh=/usr/bin/ssh file.txt user@192.168.x.x:/home/user/`” is issued, RSYNC will use the `/usr/bin/ssh` command specified in the `—rsh` option instead of its default shell to transfer the `file.txt` to the remote machine.
There are practical reasons you might need to use the `—rsh` option in different scenarios:
- To specify alternative authentication procedures: For instance, the system administrator could require added security layers that are not typically associated with the default shell. By using the `—rsh` option, the admin could specify a shell or specific command that includes those security protocols.
- If the default shell is not supported on the local or the destination server: In some cases, the remote machine may not have ssh support. The `—rsh` would then be handy to specify a shell that is supported on both machines.
- To pass extra options to the shell command: It can be used to specify additional features or flags that you’d like the shell command to implement.
After specifying the `—rsh` command, anything after it till the next space is treated as a command, and anything after the space is treated as a command option till the next space, and so forth.
As per the RSYNC’s man page and numerous well-known technical websites like “nixCraft” and “Computer Hope”, these are the widely accepted definitions and applications of the `—rsh` option in the RSYNC command. Therefore, whenever remote shell customization is needed in a file synchronization task, the `—rsh` option is the way to go about it.
Source: ‘rsync(1) – Linux man page’.
