bash -i behavior and “why is my terminal hung?”

Question

This happens on every distro I've tried:

$ cat | bash -i
bash-3.2$ ls
foo bar
bash-3.2$

And now the session is hung. Ie, you get one command and then you pretty much have to close the terminal window. SIGTERM (aka ^C) is caught by bash but it won't get you back to a working shell.

I'm guessing it has something to do with the -i causing bash to fight with cat for control of the tty, but I can't find anything definitive. Can anyone explain what's going on? And how do I automate input to a program that's running execlp("/bin/bash", "bash", "-i") without hanging? Do I have to write an expect-like program that handles tty vagaries?

Answer 1

The short answer: yes, you're going to have to use Expect.

As for why it's failing: based on its behavior, bash -i turns on readline (even with --noediting), which plays rather badly with pipes. It also appears to be setting terminal modes (including non-blocking mode) via stdout instead of stdin, which means you lose the expected behavior of most control characters. (It is not, however, reading the tty directly.)

Side note: ^C sends SIGINT, not SIGTERM. Both are, however, trapped in interactive (-i) mode; SIGHUP works to kill it.