`df` - report filesystem disk space usage

df [-h] [-i] [-T] [path]

Options you will use

-h, --human-readable: Sizes in powers of 1024.
-i, --inodes: Report inode usage instead of blocks. Check this when df says there is space but writes fail.
-T: Print the filesystem type.
-x, --exclude-type TYPE: Leave out file systems of this type: -x tmpfs -x overlay.

Gotchas

Three reasons writes fail while df -h shows free space: (1) inodes exhausted - df -i; (2) a deleted file still held open - lsof +L1; (3) the 5% root-reserved blocks - tune2fs -l.

Taught in

Try df in a real terminal Free, in your browser - a real Ubuntu terminal to try it in, with missions that check your work.