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Disk usage

No disk can hold an infinite amount of data. These commands report on how much disk storage is in use or available. (This has nothing much to do with how much main memory, i.e., RAM, a program is using when it runs; for that, you want ps or pstat or swap or some such command.)

df: Report filesystem disk space usage

df reports the amount of disk space used and available on filesystems. Synopsis: 

df [option]... [file]...

With no arguments, df reports the space used and available on all currently mounted filesystems (of all types). Otherwise, df reports on the filesystem containing each argument file.

Disk space is shown in 1024-byte blocks by default, unless the environment variable POSIXLY_CORRECT is set, in which case 512-byte blocks are used (unless the `-k' option is given).

If an argument file is a disk device file containing a mounted filesystem, df shows the space available on that filesystem rather than on the filesystem containing the device node (i.e., the root filesystem). GNU df does not attempt to determine the disk usage on unmounted filesystems, because on most kinds of systems doing so requires extremely nonportable intimate knowledge of filesystem structures.

The program accepts the following options. Also see section Common options.

`-a'
`--all'
Include in the listing filesystems that have 0 blocks, which are omitted by default. Such filesystems are typically special-purpose pseudo-filesystems, such as automounter entries. Filesystems of type "ignore" or "auto", supported by some operating systems, are only included in the listing if this option is specified.
`-i'
`--inodes'
List inode usage information instead of block usage. An inode (short for index node) is contains information about a file such as its owner, permissions, timestamps, and location on the disk.
`-k'
`--kilobytes'
Print sizes in 1024-byte blocks. This overrides the environment variable POSIXLY_CORRECT.
`--no-sync'
Do not invoke the sync system call before getting any usage data. This may make df run significantly faster on systems with many disks, but on some systems the results may be slightly out of date.
`-P'
`--portability'
Use the POSIX output format. This is like the default format except that the information about each filesystem is always printed on exactly one line; a mount device is never put on a line by itself. This means that if the mount device name is more than 20 characters long (e.g., for some network mounts), the columns are misaligned.
`--sync'
Invoke the sync system call before getting any usage data. On some systems, doing this yields more up to date results, but in general this option makes df much slower, especially when there are many or very busy filesystems.
`-t fstype'
`--type=fstype'
Limit the listing to filesystems of type fstype. Multiple filesystem types can be specified by giving multiple `-t' options. By default, nothing is omitted.
`-T'
`--print-type'
Print each filesystem's type. The types printed here are the same ones you can include or exclude with `-t' and `-x'. The particular types printed are whatever is supported by the system. Here are some of the common names (this list is certainly not exhaustive):
`nfs'
An NFS filesystem, i.e., one mounted over a network from another machine. This is the one type name which seems to be used uniformly by all systems.
`4.2, ufs, efs...'
A filesystem on a locally-mounted hard disk. (The system might even support more than one type here; Linux does.)
`hsfs, cdfs'
A filesystem on a CD-ROM drive. HP-UX uses `cdfs', most other systems use `hsfs' (`hs' for `High Sierra').
`pcfs'
An MS-DOS filesystem, usually on a diskette.
`-x fstype'
`--exclude-type=fstype'
Limit the listing to filesystems not of type fstype. Multiple filesystem types can be eliminated by giving multiple `-x' options. By default, no filesystem types are omitted.
`-v'
Ignored; for compatibility with System V versions of df.

du: Estimate file space usage

du reports the amount of disk space used by the specified files and for each subdirectory (of directory arguments). Synopsis: 

du [option]... [file]...

With no arguments, du reports the disk space for the current directory. The output is in 1024-byte units by default, unless the environment variable POSIXLY_CORRECT is set, in which case 512-byte blocks are used (unless `-k' is specified).

The program accepts the following options. Also see section Common options.

`-a'
`--all'
Show counts for all files, not just directories.
`-b'
`--bytes'
Print sizes in bytes, instead of kilobytes.
`-c'
`--total'
Print a grand total of all arguments after all arguments have been processed. This can be used to find out the total disk usage of a given set of files or directories.
`-D'
`--dereference-args'
Dereference symbolic links that are command line arguments. Does not affect other symbolic links. This is helpful for finding out the disk usage of directories, such as `/usr/tmp', which are often symbolic links.
`-k'
`--kilobytes'
Print sizes in kilobytes. This overrides the environment variable POSIXLY_CORRECT.
`-l'
`--count-links'
Count the size of all files, even if they have appeared already (as a hard link).
`-L'
`--dereference'
Dereference symbolic links (show the disk space used by the file or directory that the link points to instead of the space used by the link).
`-s'
`--summarize'
Display only a total for each argument.
`-S'
`--separate-dirs'
Report the size of each directory separately, not including the sizes of subdirectories.
`-x'
`--one-file-system'
Skip directories that are on different filesystems from the one that the argument being processed is on.

On BSD systems, du reports sizes that are half the correct values for files that are NFS-mounted from HP-UX systems. On HP-UX systems, it reports sizes that are twice the correct values for files that are NFS-mounted from BSD systems. This is due to a flaw in HP-UX; it also affects the HP-UX du program.

sync: Synchronize data on disk with memory

sync writes any data buffered in memory out to disk. This can include (but is not limited to) modified superblocks, modified inodes, and delayed reads and writes. This must be implemented by the kernel; The sync program does nothing but exercise the sync system call.

The kernel keeps data in memory to avoid doing (relatively slow) disk reads and writes. This improves performance, but if the computer crashes, data may be lost or the filesystem corrupted as a result. sync ensures everything in memory is written to disk.

Any arguments are ignored, except for a lone `--help' or `--version' (see section Common options).

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