The following macros are available in the "Command:" entry 

Macros:
%f = Selected filenames in Active Directory
%F = Full pathnames of files selected in Inactive Directory
%d = Active Directory
%D = Inactive Directory
%{Prompt message} = Prompt user for input

Prefixing a command with 'x' will open an xterm to execute the command.
Prefixing a command with 'su' will open an xterm and su to root to execute the
command.
Appending an '&' to an action will cause emelFM to discard the output of the
command. (By default, emelFM will capture the output and print it to the output
window)

Examples:

  x rpm -qlip %f | less

This opens an xterm and executes an rpm query command and pipes the output
to 'less'. If the output were not piped to 'less', the xterm would exit after
the query finished and the user would not be able to see the results for very
long.

  su rpm -Uvh %f

This opens an xterm and su's to root, prompting for the root password. Then it
executes the RPM install command which would probably fail if done as a normal
user.

  tar xzvf %f -C %D &

This unpacks a tarball from the active directory into the inactive directory.
Because of the '&' at the end, emelFM will *not* capture the output.

  diff -c %f %F > %{Filename for patch:}

This runs the diff command to create a patch between the selected files,
prompting for the filename for the patch.