I'm trying to look into the innards of FAT32, and towards that I'm trying to create a FAT32 image, mount it and do a few file operations on the command line. Per the question here, I know there's no way around using sudo to mount the image. I'm still wondering though why I end up needing sudo in order to do file operations within the mountpoint. A small bash script follows which demonstrates what works and what doesn't. Could someone show me how to do this without root?

rm -f $DISK

# create disk image
qemu-img create -f raw $DISK $SIZE
# format disk image
mkfs.fat -v -F 32 $DISK
# make mountpoint
mkdir -p $MOUNTPOINT

# can't be helped
sudo mount -o loop -t msdos $DISK $MOUNTPOINT
# should work but doesn't
mkdir -p $MOUNTPOINT/tmp/
# actually works
sudo mkdir -p $MOUNTPOINT/tmp/
# should work but doesn't
dd of=$MOUNTPOINT/tmp/ticket2.txt if=/dev/zero bs=512 count=9
# actually works
sudo dd of=$MOUNTPOINT/tmp/ticket2.txt if=/dev/zero bs=512 count=9

sudo umount $MOUNTPOINT
  • What matters are permissions and ownership inside the filesystem. Could the uid and gid options for fat filesystems help here? Try adding -o uid=n to the mount command, where n is your user id. – njsg Apr 16 '18 at 17:36
  • Thanks @njsg, the answer below expanded on your point and the advice worked for me. – airbornemihir Apr 16 '18 at 18:56

The MS-DOS filesystem variants do not support file permissions or owners (stored on disk). So instead, the kernel defaults them to the mounting user — in this case, root.

You can override this by passing the uid= and gid= options. E.g., sudo mount -o loop,uid=1000,gid=1000 -t msdos "$DISK" "$MOUNTPOINT". (I added quoting there, which is a good habit to get in to). You can check what your uid/gid is with id; it may well be something other than 1000, or alternatively do the following:

UID=`id -u`
GID=`id -g`
sudo mount -o loop,uid=$UID,gid=$GID -t msdos "$DISK" "$MOUNTPOINT"

These options are documented in man 8 mount, at least.

PS: There are several options for mounting w/o sudo mentioned in that question; e.g., udisks.

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