poor feedback when misusing `losetup`

Benno Schulenberg <[email protected]> Thu, 9 Oct 2025 11:44:52 +0200

Newsgroups	org.kernel.vger.util-linux
Message-ID	<[email protected]>

I've attached an image of some file system to a loop device with:

   # losetup loop2 some.img

Then I wanted to list information about this loop device:

   # losetup loop2
   losetup: loop2: failed to use device: No such device

It results that when setting up a loop device, one can use the short
name, but when doing anything else, one needs to give the full path:

   # losetup /dev/loop2
   /dev/loop2: [66306]:5792049 (/home/ben/some.img)

Wouldn't it be nicer if one can always use the short name -- when
the relevant loop device is located in /dev/?  Because the man page
always says 'loopdev'; it doesn't distinguish between setting up a
device and doing anything else with it.


Then I detach the device, forget that I have detached it, and detach
it again later:

   # losetup -d /dev/loop2
   # losetup -d /dev/loop2
   losetup: /dev/loop2: detach failed: No such device or address

"No such device"?  So I request information about the device:

   # losetup /dev/loop2
   losetup: /dev/loop2: No such file or directory

But it exists alright:

   # ls -l /dev/loop2
   brw-rw---- 1 root disk 7, 2 okt  9 11:29 /dev/loop2

Wouldn't it be better if `losetup` reported "Nothing is attached"
when trying to detach or get info from an unassociated loop device?


Benno

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