Exploring Expect

Echoing

Normally, the interact command depends on the spawned process to echo characters. For example, when you have spawned a shell and are interacting with it through the interact command, all the printable characters that you type are echoed by the shell. Nonprintable characters are not usually echoed back but instead invoke special actions such as erasing previous characters or generating signals. As with echoing, this special processing of nonprintables is also handled by the shell. The interact command does no special processing. It just shuttles characters back and forth.

The interact command works the same way when matching patterns. That is, no special action is taken to echo characters. However, because the characters are being buffered and not sent on to the spawned process, you will not see them appear on your screen. This is similar to the way the tip program works. tip does not echo its escape character unless additional characters are entered that force the escape not to match any known sequence.

For patterns that match short character sequences, the lack of echoing is rarely a problem. In most cases, users do not need to see a single character being echoed before they enter the next one. But patterns that are long enough (however you choose to define this) can use some sort of feedback. A simple strategy is to echo the typed characters.

By preceeding a pattern with the -echo flag, interact echoes characters that match a pattern. Partial matches are also echoed. This ...