Exploring Expect

Log And Diagnostic State

The log and diagnostic states can be obtained by calling the original commands with the -info flag. The previous parameters are returned. This is commonly used to establish a temporary state.

For example, a subroutine that must be sure that output to the user is disabled should save and restore the state of log_user.

proc sub {} {
    set state [log_user -info]      ;# save old state
    log_user 0                      ;# set new state
    # your code goes here
    eval log_user $state            ;# restore old state
}

Notice the final call to restore the old state. It uses eval in order to support the possibility of a multi-element state. The list of elements will be split in such a way that the command sees them as separate arguments.

The log_user command is not currently defined to take multiple parameters; however, future expansion is likely so it is a good idea to handle it this way. The log_file and exp_internal commands take multiple arguments already.