The advantage of FISH is that all it requires on the server-side is an SSH or RSH implementation, Unix shell, and a set of standard Unix utilities (like ls, cat or dd—unlike other methods of remote access to files via a remote shell, scp for example, which requires scp on the server side).
The protocol was designed by Czech Linux Kernel Hacker, Pavel Machek, in 1998 for the Midnight Commander software tool.
[1] Client sends text requests of the following form: Fish commands are all defined, shell equivalents may vary.
The codes 000 and 001 are special, their meaning depends on presence of server output before the end line.
The client initiates SSH or RSH connection with echo FISH:;/bin/sh as the command executed on remote machine.