Similar thing is available within radare2 [1] too, it is available as a small library for different languages and called r2pipe [2].

[1] https://github.com/radareorg/radare2

[2] https://radare.gitbooks.io/radare2book/content/

I didn't understand the example in the README. There is no "main" in those lines, so why did grep output them?

  (gdb) shell-pipe disas | grep mov

nice and easy ! I remember that the piping functionality was discussed in GDB mailing list long ago, but I guess it never got accepted!

it would have allowed writing

    (gdb) disas | grep mov

instead of

    (gdb) shell-pipe disas | grep mov