Bash, double quotes and "reboot" command

Question

Assume you have those two statements in your bash script:

# No. 1
MSG="Automatic reboot now."
echo $MSG

# No. 2
MSG=""Automatic reboot now.""
echo $MSG

The output of statement number 1 is as expected (it is simply printed). If bash runs statement two, the machine is rebooted (any valid bash command will be executed).

But why?

Answer 1

That's because the meaning of MSG=""Automatic reboot now."" is the following:

Execute reboot now. with the env. var. MSG set to Automatic.

It's equivalent to:

MSG=Automatic reboot now.

Answer 2

A lesser known shell feature is the ability to set environment variables for the duration of a single command. This is done by prepending a command with one or more assignments, as in: var1=foo var2=bar command.

Here's a demonstration. Notice how the original value of $MSG is preserved.

$ export MSG=Hello
$ bash -c 'echo $MSG'
Hello
$ MSG=Goodbye bash -c 'echo $MSG'
Goodbye
$ bash -c 'echo $MSG'
Hello

Now on to your question:

MSG=""Automatic reboot now.""

The pairs of double quotes nullify each other, and might as well not be there. It's equivalent to:

MSG=Automatic reboot now.

which executes reboot with an argument of now. and the $MSG environment variable set to Automatic.