Exploring Expect

The —c Flag

The —c flag provides a way of executing commands specified on the command line rather than in a script. This is handy when writing shell scripts, and you have a really short task for Expect that does not justify a separate script. For instance, the following shell command is similar to the timed—read command in Chapter 3(p. 77).

expect -c 'expect "\n" {send $expect_out(buffer)}'

Since no timeout is specified, the command waits up to 10 seconds. If the user types a string and presses return within the allotted time, the user’s string is returned, otherwise the empty string is returned. Notice that the entire argument to -c is quoted using single quotes. This tells the shell not to perform any variable expansion.

The -c flag can also be used to execute commands before a script takes control. For example, you can set the variable debug to 1 by invoking Expect from the shell as:

% expect -c "set debug 1" script

Inside the script, you can check the value of this variable:

if [info exists debug] {
    puts "debugging mode: on"
} else {
    set debug 0
}

# imagine more commands here

if $debug {puts "value of x = $x"}

When the script is run, it checks if debug is defined by evaluating "info exists“, a Tcl command which returns 1 if the variable is defined or 0 if it is not. If it is defined, the script can then test it later to determine if it should print debugging information internal to the script. The else clause sets debug to 0 just so that later a simple "if $debug" test can be used. ...