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Exploring Expect by Don Libes

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The send_error Command

A task related to printing information is that of printing error messages. It is a good idea to print error messages in such a way that they can be easily separated from the normal output of the program. In particular, error messages go to the standard error stream while normal output goes to the standard output stream.

The benefit of sending errors to a separate stream is that, once separated, the two streams can be easily sent to different places. Typically, the standard output is redirected to a file or piped to another process while the standard error remains undirected so that it can be seen at the terminal.[33] For example, the command "cat foo | wc" (to count the words in foo) will print "foo: no such file" if foo does not exist. You see the error message because the standard error is not redirected. If the diagnostic was sent along with the normal output of cat, wc would report that foo had four words only because that is how many are in the error message!

Expect scripts should be written the same way. Normal output that you might wish to redirect or pipe should be sent to the standard output. Error messages should be sent to the standard error. There are two ways of doing this: send_error and "puts stderr“. send_error is similar to send_user and send whereas "puts stderr" is similar to puts.

I will show more differences later. To summarize the differences so far between variations on puts and the variations on send:

puts

automatically appends newline ...

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