2.2 Block size

Some GNU programs (at least df, du, and ls) display file sizes in "blocks". You can adjust the block size to make file sizes easier to read. The block size used for display is independent of any filesystem block size.

Normally, disk usage sizes are rounded up, disk free space sizes are rounded down, and other sizes are rounded to the nearest value with ties rounding to an even value.

The default block size is chosen by examining the following environment variables in turn; the first one that is set determines the block size.

DF_BLOCK_SIZE

This specifies the default block size for the df command. Similarly, DU_BLOCK_SIZE specifies the default for du and LS_BLOCK_SIZE for ls.

BLOCK_SIZE

This specifies the default block size for all three commands, if the above command-specific environment variables are not set.

POSIXLY_CORRECT

If neither the command_BLOCK_SIZE nor the BLOCK_SIZE variables are set, but this variable is set, the block size defaults to 512.

If none of the above environment variables are set, the block size currently defaults to 1024 bytes, but this number may change in the future.

A block size specification can be a positive integer specifying the number of bytes per block, or it can be human-readable or si to select a human-readable format.

With human-readable formats, output sizes are followed by a size letter such as `M' for megabytes. BLOCK_SIZE=human-readable uses powers of 1024; `M' stands for 1,048,576 bytes. BLOCK_SIZE=si is similar, but uses powers of 1000; `M' stands for 1,000,000 bytes. (SI, the International System of Units, defines these power-of-1000 prefixes.)

An integer block size can be followed by a size letter to specify a multiple of that size. When this notation is used, the size letters normally stand for powers of 1024, and can be followed by an optional `B' for "byte"; but if followed by `D' (for "decimal byte"), they stand for powers of 1000. For example, BLOCK_SIZE=4MB is equivalent to BLOCK_SIZE=4194304, and BLOCK_SIZE=4MD is equivalent to BLOCK_SIZE=4000000.

The following size letters are defined. Large sizes like 1Y may be rejected by your computer due to limitations of its arithmetic.

`k'

kilo: 2^10 = 1024 for human-readable, or 10^3 = 1000 for si.

`M'

Mega: 2^20 = 1,048,576 or 10^6 = 1,000,000.

`G'

Giga: 2^30 = 1,073,741,824 or 10^9 = 1,000,000,000.

`T'

Tera: 2^40 = 1,099,511,627,776 or 10^12 = 1,000,000,000,000.

`P'

Peta: 2^50 = 1,125,899,906,842,624 or 10^15 = 1,000,000,000,000,000.

`E'

Exa: 2^60 = 1,152,921,504,606,846,976 or 10^18 = 1,000,000,000,000,000,000.

`Z'

Zetta: 2^70 = 1,180,591,620,717,411,303,424 or 10^21 = 1,000,000,000,000,000,000,000.

`Y'

Yotta: 2^80 = 1,208,925,819,614,629,174,706,176 or 10^24 = 1,000,000,000,000,000,000,000,000.

Block size defaults can be overridden by an explicit `--block-size=size' option. The `-k' or `--kilobytes' option is equivalent to `--block-size=1k', which is the default unless the POSIXLY_CORRECT environment variable is set. The `-h' or `--human-readable' option is equivalent to `--block-size=human-readable'. The `--si' option is equivalent to `--block-size=si'.