Exploring Expect

Handling Errors

When typing commands interactively, errors cause the interpreter to give up on the current command and reprompt for a new command. All well and good. However, you do not want this to happen while running a script.

While many errors are just the result of typing goofs, some errors are more difficult to avoid and it is easier to react to them “after the fact”. For example, if you write a procedure that does several divisions, code before each division can check that the denominator is not zero. A much easier alternative is to check that the whole procedure did not fail. This is done using the catch command. catch evaluates its argument as another command and returns 1 if there was an error or 0 if the procedure returned normally.

Assuming your procedure is named divalot, you can call it this way:

if [catch divalot] {
    puts "got an error in divalot!"
    exit
}

The argument to catch is a list of the command and arguments to be evaluated. If your procedure takes arguments, then they must be grouped together. For example:

catch {puts "Hello world"}
catch {divalot some args}

If your procedure returns a value itself, this can be saved by providing a variable name as the second argument to catch. For example, suppose divalot normally returns a value of 17 or 18.

tclsh> catch {divalot some args} result
0
tclsh> set result
17

Here, catch returned 0 indicating divalot succeeded. The variable result is set to the value returned by divalot.

This same mechanism can be used to get the messages ...