Exploring Expect

Programs That Ignore Eof

There is an exception to the scenario that I just described. Some interactive programs are rather cavalier when they encounter an eof and do not handle it correctly. However, if you are prepared for this situation, you can work around it easily enough. There are two kinds of common misbehavior:

• Some programs ignore eof.

• Some programs ignore data just before eof.

I will discuss the two cases separately.

Some programs ignore eof. Even if you close the connection (by calling close, exiting the script, or pressing ^C), they ignore the eof and continue waiting for more characters to arrive. This is characteristic of the ubiquitous telnet implementation and many other programs that run in raw mode. Raw mode means that no special interpretations are applied to input characters. For instance, ^C no longer serves as an interrupt, and ^D no longer acts as an eof. Since users cannot send an eof, these programs have no reason to expect it and thus do not look for it. The problem is, an eof is exactly what they get when the connection is closed.

Avoid explicitly closing programs like these before they are ready. Instead, force them to close the connection in the way they would when using them manually. For instance, telnet will close the connection on its own once you log out of the remote host. If you do not gracefully log out, thereby letting telnet shut down the connection, you will be left with a telnet process on your system talking to no one. Such a process must then ...