Exploring Expect

More Actions That Affect Control Flow

Just as break was used in the rogue script, so can all of the other flow-control commands be used inside of expect commands. For example, a return command inside of an expect causes the current procedure to return:

proc foo {
    expect {
        "1" return
        "2"
    }
    someproc
}

The continue command causes control to resume at the beginning of the nearest enclosing while or for loop. continue, break, and return can be mixed in intuitive ways. In the following example, the patterns 1, 2, and 3 do not mean anything in particular. They are just placeholders. The actions are what is interesting.

proc foo {
    while 1 {
        expect {
            "1" {
                return         ;# return from foo
            }
            "2" {
                break          ;# break out of while
            }
            "3" {
                if {0==[func]} {
                    exit       ;# exit program
                } else {
                    continue   ;# restart while
                }
            }
        }
        someproc
    }
    some-other-proc
}

In Chapter 3 (p. 83), I showed a script that started an anonymous ftp session and let you interact after performing the login automatically. Using some of the things you have seen since, it is possible to write a more capable version of aftp. The one below retries the connection if the remote host refuses because it is down or there are too many users. A procedure called connect is defined and called repeatedly in a loop. Anonymous ftp administrators may not appreciate this approach, but it is sometimes the only way to get through to a site that is very popular. Once connected, the script sends the binary command to disable any data conversions. As with the earlier ...