Not command line, but something like this will open and read a file.

1.   string[] myfile
2.   File f = new File(dataPath("myfile.txt"));
3.   if (f.exists()) {
4.     myfile = loadStrings(f);
5.     if (myfile.length > 0) {
6.       for(int p = 0; p < myfile.length; p++) {
7.           println(myfile[p].trim());
8.       }
9.     }
10.   }

There is also the open() command.. which can run something.
You also have the " Runtime.getRuntime().exec()" command.