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. Optionally, there can be a special FISH server program (called start_fish_server) on the server, which executes FISH commands instead of Unix shell and thus speeds up operations.

The protocol was designed by Pavel Machek in 1998 for Midnight Commander.

Protocol messages

#FISH_COMMAND arguments...

equivalent shell commands,

which may be multi-line

Fish commands are all defined, shell equivalents may vary. Fish commands always have priority: the server is expected to execute a fish command if it understands it. If it does not, however, it can try and execute a shell command. When there is no special server program, Unix shell ignores the fish command as a comment and executes the equivalent shell command(s).

Server replies are multi-line, but always end with

### xyz

line. ### is a prefix to mark this line, xyz is the return code. Return codes are superset to those used in []. The codes 000 and 001 are special, their meaning depends on presence of server output before the end line.

Session initiation

#FISH

echo; start_fish_server; echo '### 200'

#VER 0.0.2   <...>

echo '### 000'

VER 0.0.0  <...>

### 200