df

Display all filesystems and their disk usage (using 512-byte units)

$ df

$ df [path/to/file_or_directory]

$ df -k

$ df -P

df [OPTION]... [FILE]...

-a, --all
    include pseudo, duplicate, inaccessible filesystems

-B, --block-size=SIZE
    use SIZE-byte blocks (e.g., 1K, 1M)

-h, --human-readable
    print sizes in powers of 1024 (1K, 1M, etc.)

-H, --si
    like -h, but use powers of 1000

-i, --inodes
    list inode usage instead of blocks

-k
    like --block-size=1K

-l, --local
    limit to local filesystems

--output[=FIELD_LIST]
    specify output fields (e.g., used,avail)

--sync
    invoke sync before reporting

--total
    elide small entries; produce grand total

-t, --type=TYPE
    limit to filesystems of type TYPE (e.g., ext4)

-T, --print-type
    print filesystem type

-x, --exclude-type=TYPE
    exclude filesystems of type TYPE

--help
    display help and exit

--version
    output version information and exit

The df command displays disk space usage for mounted filesystems, showing total size, used space, available space, usage percentage, and mount points. It queries kernel statistics via system calls like statvfs(2), providing a quick overview of storage status.

Default output includes all filesystems:
Filesystem      Size  Used Avail Use% Mounted on
/dev/sda1       50G  20G  28G  42% /

Essential for monitoring disk health, preventing outages from full drives. Options customize output: human-readable sizes (-h), inode counts (-i), or totals (--total). Specify files/directories to report their filesystems. Unlike du, it shows filesystem-level data, not subtree usage. Accurate for local mounts; remote filesystems (NFS) depend on connectivity. Run as root for all details; non-root skips some pseudo-filesystems.

Reports filesystem totals, not directory subtrees (use du). Reserved blocks (e.g., 5% for root) reduce 'Avail'. Remote filesystems may lag or fail. Non-root users see limited pseudo-filesystems. Sizes may not sum exactly due to metadata.

Filesystem (device), Size (total), Used, Avail (free), Use% (percentage), Mounted on (path)

df -h (human-readable all FS)
df -h /home (just /home FS)
df -i -t ext4 (inodes on ext4 FS)

Originated in Version 7 Unix (1979). GNU coreutils version since 1987, maintained by Free Software Foundation. Evolved with filesystem support (ext4, btrfs, NFS). Man page standardized in POSIX.1-2008.

du(1), mount(8), free(1), lsblk(8), statvfs(2)