Exploring Expect

Sending Slowly

It is often useful to send large quantities of information. A common scenario in which this arises is when sending mail via a commercial mail system such as GEnie or CompuServe. To reduce costs, you create and edit mail messages offline. Once the messages are prepared, you can then log in, start up the mailer, and feed the messages to it. This allows you to use your own editor as well as lower your costs for connect time.

The fragment of the script that sends the message from your system to the remote system can be very simple. For example, if the message is already stored in the variable message, one send command is sufficient.

send $message

The size of things that can be sent this way is limited only by the amount of memory that can be dynamically allocated on your system. On modern computers, this limit is well above a megabyte, and a 32-bit computer should have no problem with 50Mb or even 100Mb.

Before sending very large files, it is wise to ensure that the remote side is not echoing (as described earlier). If the echoing cannot be disabled, the process output must be consumed, such as by an expect command. Otherwise, your operating system has to store all these characters in memory. Most systems limit unread characters to 10K or even less. If more characters arrive, the operating system temporarily stops the spawned process until the Expect process can read them into its own buffers. Expect’s own buffers are limited only by the amount of virtual memory on the computer. ...