The script required the destination to be its own mount point, while the
documentation and its own usage text both told the user to pass
/media/IPOD/Music. The documented invocation was rejected.
A subdirectory is the better target, so the guard was what was wrong. --delete
is confined to it, and the device path budget is now derived from it -- the
part of the destination below its mount point -- rather than configured, so the
budget cannot disagree with where the files are actually going. The check that
matters is that the destination sits on a FAT filesystem, which is also what
catches an unmounted device: /media/IPOD/Music then resolves to the host's own
root filesystem, and emptying that is the outcome all of these guards exist to
prevent.
Three further faults found by testing the guards rather than reasoning about
them:
Stripping the trailing slash from "/" left an empty string, so the guard
refusing the host root never fired and the user got "destination is not a
directory" instead.
die() printed only its first argument, so the second half of the non-FAT
message -- the half saying to check whether the device is mounted -- was
silently dropped.
--help was not handled at all. Only -h reached the usage text, and it exited 2
to stderr, which is right for misuse and wrong for someone asking a question.
Help now goes to stdout and exits zero, and carries the guidance rather than
leaving it to the README, since the question it answers is asked at a terminal.
The test stage installs bash, rsync and findmnt, none of which are in the base
image, and the tests skip rather than fail where they are absent -- a machine
without rsync is not a machine that would run this script.
