truncate

A tool for creating empty files

This tool uses the POSIX ftruncate function to set the length of a file.

• If the file exists and is longer than that, it will be truncated (bytes after that length cut off).
• If the file exists and is shorter than that, it will be extended and the new bytes zero-filled. (Whether this actually consists of writing zero-bytes to disk is dependent on your operating system's implementation.)
• If the file doesn't already exist, it will be created, then extended to the desired length.

This is a very handy way to instantly create raw disk images for emulators. You can do things like truncate 1G oneBEELLIONbytes.img to create a blank disk image that you can then format with either host tools (like newfs_hfs or newfs_msdos) or guest tools (like the built-in Initialize alert in classic Mac OS).

Sizes can be specified as either whole numbers of bytes or whole numbers followed by a suffix:

B (or no suffix)

Bytes

S

Sectors of 512 bytes each)

K

Kibibytes (KiB) of 1024 bytes each

M

Mebibytes (MiB): 1024**2 = 1048576 bytes

G

Gibibytes (GiB): 1024**3 = >1 billion bytes

T

Tebibytes (TiB): 1024**4 = >1 triillion bytes

P

Pebibytes (PiB): 1024**5 = 1024 tebibytes

E

Exbibytes (EiB): 1024**6 = 1048576 tebibytes

Z

Zebibytes (ZiB): 1024**7

Y

Yobibytes (YiB): 1024**8

Compatibility

This works on macOS. It should work on, or at least be portable to, Linux-based operating systems.