Exploring Expect

Hung Ptys

Historically, UNIX systems have provided a fixed number of ptys, pre-allocating filenames in the file system for each one. Most versions of UNIX no longer do this, but there are still some that do. With a static set of ptys, it is necessary to search through the list of files. Expect performs several tests on each pty before using it. These tests ensure that no other process is still using the pty.

Usually these tests are very quick, but programs that have misbehaved and are sloppy in their pty allocation and deallocation can force Expect to take up to ten seconds, waiting for a response from a pty that is still in use.^[51] Normally, Expect goes on and continues trying other ptys until it finds one that can be allocated; however, such ptys can cause problems for most other programs. For example, programs that use ptys, such as xterm and Emacs, simply give up when encountering such a pty. If you see this happening, you can try spawning a process with Expect’s diagnostic mode enabled. Expect will then report the ptys it is ignoring and you can verify that each one is in use by a functioning program. In some cases, the program may have exited but left the pty in a bizarre state. Expect’s thorough pty-initialization procedure will reset the pty so that other processes can use it.

You can take advantage of Expect’s ability to fix ptys with the following script called ptyfix.

#!/usr/local/bin/expect--
spawn cat

Or even simpler, just put the following shell command in an alias or ...