Bash – Have cron email output to MAILTO based on exit status

You're going to have to buffer the output somewhere no matter what, since you need to wait for the exit code to know what to do. Something like this is probably easiest:

$ output=`my_long_noisy_script.sh 2>&1` || echo $output

First of all, while they are functionally equivalent, $(…) is widely considered to be clearer than `…` — see this, this, and this.  Secondly, you don’t need to use $? to check whether a command succeeded or failed.  My attention was recently drawn to Section 2.9.1, Simple Commands of The Open Group Base Specifications for Shell & Utilities (Issue 7):

A "simple command" is a sequence of optional variable assignments and redirections, in any sequence, optionally followed by words and redirections, terminated by a control operator.

When a given simple command is required to be executed …
                          ⋮                                                                                (blah, blah, blah …)

If there is a command name, execution shall continue as described in Command Search and Execution.  If there is no command name, but the command contained a command substitution, the command shall complete with the exit status of the last command substitution performed.  …

For example,

• the exit status of the command

  ls -ld "module_$(uname).c"
  

  is the exit status from the ls, but

• the exit status of the command

  myfile="module_$(uname).c"
  

  is the exit status from the uname.

So ferada’s answer can be streamlined a bit:

if output=$(/etc/grub.d/30_os-prober)  &&  [ -z "$output" ]
    # i.e., if 30_os-prober successfully produced no output
then
    install_linux_only
else
    install_dual_boot
fi

Note that it is good practice to use all-upper-case names only for environment variables (or variables to be visible throughout the script).  Variables of limited scope are usually named in lower case (or, if you prefer, camelCase).