%filter does not call the shell, as system() does in C.  Thus "sh -c
'...'" must be called to get shell functionalities (stderr
redirection, pipe, etc.).

However, mgp does not take ' into account when word-splitting %filter
command-line.  Hence the following ends with sh complaining about a
syntax error:

%filter "sh -c 'ls -l /'"
%endfilter