Function: fileextern
Section: programming/specific
C-Name: gp_fileextern
Prototype: ls
Help: fileextern(str): execute shell command str and returns a file
 descriptor attached to the command output as if it were read from a file.
Doc: the string \var{str} is the name of an external command, i.e.~one you
 would type from your UNIX shell prompt. This command is immediately run and
 the function returns a file descriptor attached to the command output as if
 it were read from a file.
 \bprog
 ? n = fileextern("ls -l");
 ? while(l = filereadstr(n), print(l))
 ? fileclose(n)
 @eprog\noindent If the \kbd{secure} default is set, this function will raise
 en exception.
