Exploring Expect

Logging Internal Diagnostics

It is possible to achieve all sorts of effects by playing games with redirection. One of the more useful things is to log both the standard output and standard error to a file. Expect provides this same capability in a more flexible way than can be achieved with redirection. This is done using the exp_internal command with the -f argument. An output file name must follow the -f along with a 0 or 1. The 0 or 1 disables or enables the generation of diagnostics just as before. However, the standard output and standard error is still logged to the file even if a 0 is supplied to exp_internal.

To summarize:

exp_internal 0	no diagnostics
exp_internal 1	send pattern diagnostics to standard error
exp_internal -ffile0	copy standard output and pattern diagnostics to file
exp_internal -ffile1	copy standard output and pattern diagnostics to file and send pattern diagnostics to standard error

The form "exp_internal -ffile 0" is particularly useful because Expect appears to act just as if no diagnostics were being generated, even though it is actually writing the usual output and diagnostics to a file at the same time. You can put this particular command in production scripts and users will not be affected by the command. If a user then reports a bug, you can check the file and see exactly what went wrong.

As before, exp_internal commands can be placed around pieces of code to limit the amount of diagnostics they produce. Each exp_internal command closes any previous file ...