Is there a Linux CLI tool for determining a partition's “used” space? (like gparted)

Question

I'm shrinking a Linux SDCard partition using AOakley's awesome guide. I'm trying to automate it, but there appears to be no command-line tool for determining the number of bytes in a partition that are used.

For example, gparted tells me the used space is 1.88GB out of 6.99GB (note, I'm using /dev/loop0 which is mounted to the second partition of /dev/sdb):

df is wrong:

% df /dev/loop0
Filesystem  ... Use%
udev        ... 0%

...and fdisk and parted don't print used space.

What can I use from the command-line to determine the used space in a partition the way gparted does?

ANSWER: Summary from below: The partition is ext4 which allows me to use dumpe2fs and extract: free blocks, block count (total), and block size which can be combined to exactly match gparted. Turns out that resize2fs handles fragmentation during the shrink. See the manpage for ext4 for more info (although the manpage editor is a tad bitter about GiB vs. GB! :)

Answer 1

df is almost the right tool but you should query the mountpoint (or any file below) that belongs to the filesystem you want to examine, not the device node. The node you used, /dev/loop0, is a special file that belongs to the udev virtual filesystem; hence you got udev.

Mount /dev/loop0 (or /dev/sdb2, I'm not sure why the need for /dev/loop0) somewhere (it can be ro mount) and invoke df /the/mountpoint. Then df will tell you what the filesystem reports about itself.

I said "df is almost the right tool" because there is no guarantee you can shrink the filesystem (and later the partition) to the size reported by the tool. What df prints can be your first (or rather zeroth) estimate.

You need different tools to resize different fielsystems. In a perfect world such tool (or some accompanying tool) is able to tell you the minimum possible size, or at least the minimum size for which resizing will succeed for sure. In reality you can encounter this:

Known Bugs
The minimum size of the filesystem as estimated by resize2fs may be incorrect, especially for filesystems with 1k and 2k blocksizes.

^(source)

gparted uses filesystem-specific tools. I did run it with strace and saw no evidence it uses df. I saw btrfs filesystem show …, dumpe2fs …, …

Summary:

• You used df in the wrong way.
• It's better to use a tool specific to the filesystem anyway; gparted does. I cannot tell you what tool you should use because you haven't specified the filesystem in question.