@schabrolles I know of Linux systems where the /boot file system is not always mounted. Then the variable would be empty.
What happens in such cases? You would get an invalid grub entry.
Perhaps, foresee a sanity check here?
Gratien

@gdha. df /boot | awk 'END {print $1}' returns the file system on which /boot stands : either /boot (if it is a file system) or / (if it is only a directory).
So, if /boot is not mounted, the UUID return by the command will be / UUID (and not /boot).

grub-mkconfig has the same behavior. it uses grub2-probe --target=device /boot to get the name of the boot device. If /boot is not mounted, it returns / device instead and grub.cfg generation won't be good.
It is not a good idea to not mount /boot, grub-mkconfig can be automatically run after some packages upgrade (kernel for example); if /bootis not mounted, grub.cfg won't be updated.

Anyway, your point is correct and we should may be test if a directory like /boot/grub* exist.
If not, we can exist and avoid to generate a bad grub.cfg

#Test if /boot is mounted 
if ls -ld /boot/grub* > /dev/null 2>&1 ; then
    grub_boot_uuid=$(df /boot | awk 'END {print $1}' | xargs blkid -s UUID -o value)
    ....
else
    echo "Error: /boot is empty, or may be not mounted..."
    exit 1     
fi

Your feedback is welcomed

@schabrolles You could make the test easier (and we use this a lot in rear):

ls -ld /boot/grub* > /dev/null 2>&1
StopIfError "/boot is empty, or may be not mounted..."

Thank you for reviewing the code once more.

@gdha

I check again the code, there is several test before to check /boot/grub/grub.cfg, verify that this one is writable etc ...
example line 66 67

[[ -w "$grub_conf" ]]
StopIfError "GRUB2 configuration cannot be modified."

Normally, if we pass this test, this mean that /boot/grub* exists (/boot is mounted or directory not empty). It is may be useless to add another test. What do you think ?

Another question : What is the difference between StopIfErrorand BugIfError ?

I agree with you - testing once is enough.
BugIfError function is mainly used when we think there might be a bug report needed against rear. StopIfError is meant to force an exit out of rear when there is a condition not met (missing executable or file or something else which is not caused by a bug)