Exploring Expect

Procedures Introduce New Scopes

A procedure introduces a new scope. This normally hides variables unless the global command (or upvar or uplevel) is used. Because Expect depends so much on implicit variables (spawn_id, timeout, etc.), Expect commands have a special behavior when it comes to reading variables.

• When reading a variable, if a global command has declared the variable, the variable is looked up in the global scope. If undeclared, the variable is first looked up in the current scope, and if not found, it is then looked up in the global scope.

The italicized phrase emphasizes how Expect differs from the usual Tcl scoping mechanism. To say this a different way, while reading variables, Expect commands search the global scope for variables if they are not found in the local scope.

In the report procedure defined above, spawn_id was defined locally. By the rule just stated, spawn_id would be found in the local scope. Without the set command in report, spawn_id would be found in the global scope.

This rule can be used to simplify scripts. In the ftp example on page 237, each time a command was sent to ftp, it was immediately followed by an expect to check that the command succeeded.

send "get $file2\r";                            expect "220*ftp> "

You can wrap this sequence into a procedure so that each time a command is sent, the response is checked:

proc ftpcmd {cmd} {
    send "cmd\r"
    expect "220*ftp> "
}

In this procedure, again, spawn_id is not defined locally, nor is it mentioned in a global command. Thus, ...