Capturing "command not found" errors from Ruby's backticks?

Is there a way to capture a "command not found" error in a Ruby script? For instance, given:

output = `foo`

How do I trap the situation where foo isn't installed? I expected that I could rescue an exception, but this doesn't seem to work on 1.8.7. Is there a different way of calling the subprocess that will do what I want? Or is there a different approach?

Update

My apologies, I forgot to mention a hidden requirement: I would prefer that the interpreter doesn't leak the command line to the user (it can contain sensitive data), hence wh开发者_如何学运维y the exception catching method is preferred. Apologies again for leaving this out the first time.

Use the return code!

irb(main):001:0> `date`
=> "Mo 24. Jan 16:07:15 CET 2011\n"
irb(main):002:0> $?
=> #<Process::Status: pid=11556,exited(0)>
irb(main):003:0> $?.to_i
=> 0
irb(main):004:0> `foo`
(irb):4: command not found: foo
=> ""
irb(main):005:0> $?.to_i
=> 32512

http://corelib.rubyonrails.org/classes/Process/Status.html

Redirecting STDERR to STDOUT will give you the output as return value instead of bloating it just out:

irb(main):010:0> `foo 2>&1`
=> "sh: foo: not found\n"
irb(main):011:0> $?.to_i
=> 32512