Exploring Expect

The Expect Library

In Chapter 2 (p. 68), I mentioned that Tcl provides support for libraries of Tcl procedures. Expect follows the Tcl model. Two directories are provided in which users at your site can create and use publicly-accessible Expect procedures or other data. The directory names are stored in the global variables exp_library and exp_exec_library.

The exp_library directory contains files which are platform-independent. If you have the directory NFS-mounted to multiple computers of different types, all of them can share the same common procedures. In contrast, the exp_exec_library directory is specific to a particular machine (and if necessary, different releases of the operating system).

Although scripts are the most common use for these libraries, non-executable data can be stored there as well. For example, in Chapter 13 (p. 287) I mentioned that cat might need a -u flag. Unfortunately, on other systems, the -u flag imposes a performance penalty. So -u should only be used when it is required. One way to detect this is to spawn a cat process, send a character to it, and see if it echoes. Unfortunately, the failure to echo requires an expect command to timeout, and this can take a relatively long time in a script. To avoid this, Expect executes this test upon installation and leaves a marker behind in the form of a file for which scripts can test. Because the behavior is dependent upon a particular executable (namely, /bin/cat), the marker is stored in exp_exec_library