Identity

Property	Value
Binary	`df`
Config	No persistent config — invoked directly
Logs	No persistent logs — output to terminal
Type	CLI tool (part of coreutils)
Install	`apt install coreutils` / `dnf install coreutils` (pre-installed on all Linux systems)

Key Operations

Task	Command
Human-readable sizes (KB/MB/GB)	`df -h`
Show all filesystems including zero-size	`df -a`
Show inode usage instead of block usage	`df -i`
Exclude tmpfs and devtmpfs from output	`df -h -x tmpfs -x devtmpfs`
Usage for a specific path or mountpoint	`df -h /var/log`
POSIX-compliant output (512-byte blocks)	`df -P`
Add a total summary line	`df -h --total`
Show filesystem type column	`df -hT`
Show filesystem type, exclude pseudo-fs	`df -hT -x tmpfs -x devtmpfs -x squashfs`
Sort by use percentage (highest first)	`df -h
Sync before reading (force flush to disk)	`df --sync`
Show sizes in 1K blocks (scriptable)	`df -k`

Symptom	Cause	Fix
Filesystem shows 100% but `du` total is less	Deleted files held open by a running process	`lsof +L1` to find processes holding deleted files; restart or kill the process to release space
"No space left on device" with disk not full	Inode exhaustion — all inodes consumed	`df -i` to confirm; remove many small files or recreate the filesystem with more inodes
`df -h` shows tmpfs/devtmpfs inflating totals	Virtual filesystems included in output	`df -h -x tmpfs -x devtmpfs` to suppress them
Disk usage jumps after Docker pulls	overlayfs layers counted per-snapshot	`docker system df` and `docker system prune` for Docker-specific accounting
btrfs reports less usage than expected	btrfs shows compressed/deduplicated allocation	Use `btrfs filesystem df /mount` and `btrfs filesystem usage /mount` for accurate btrfs stats
`/` partition full but large files are in `/home`	`/home` is a separate filesystem	Each mountpoint reports independently; check the correct mountpoint

Filesystem-level only: df reports per-filesystem totals. To find which directory or file consumes space within a filesystem, use du or ncdu.
Deleted-but-open files: When a process holds a file descriptor open, the file's blocks are not reclaimed even after rm. df shows the space as used; du shows it as gone. lsof +L1 reveals the offending process.
Inode exhaustion is silent: A filesystem with 0 inodes free returns "No space left on device" identically to a full-blocks condition. Always check df -i alongside df -h when diagnosing space errors.
tmpfs and overlay inflation: Without -x tmpfs -x devtmpfs, pseudo-filesystems appear in the output and the --total line includes them. On systems with many container overlayfs mounts the list can be very long.
btrfs accounting is non-obvious: btrfs reports allocation, not raw consumption, because it uses copy-on-write. Reflinks, snapshots, and compression mean df output and du output can diverge significantly. Use btrfs filesystem usage for authoritative btrfs stats.

See references/ for: