Zero-byte file

A zero-byte file or zero-length file is a computer file containing no data. As the size (a.k.a. length) of a file is often measured in bytes, such a file has length of zero bytes.^[1]

Although there are many ways to create a zero-byte file, one common way to intentionally create one is via the touch command. The command line touch abc creates a zero-byte file named abc.

The creation of any file involves creating it with zero length, but usually data is written to a new file almost immediately after creation such that users don't generally notice files in a zero-byte state. The presence of a zero-byte file may indicate a failure of a program. One common scenario involves caching data in memory and then flushing to storage at a later time. A program that does not flush to storage (i.e. due to abnormal termination) may erroneously leave a zero-byte file. Sometimes, the time between file creation and writing data to it is long enough that a user does notice that the file has zero size before a program writes any content.

Even a zero-byte file requires storage space since every file has file system metadata. The file system may allocate a minimum size of space to every file. Such file system parameters may be discovered via an operating system utility such as stat.^[2][3] Often this size is 4096 bytes.

Some file formats include metadata (separate from file system metadata). In this case, a file that contains no content may have non-zero size. For example, a word processor document with no text, an image with zero-by-zero dimensions, or an audio sample of zero duration may contain metadata which makes the size of the file non-zero. On the other hand, some file formats have no metadata. For example, an ASCII text file has no metadata such that a file with no content has zero size. But, some text file formats do include a byte order mark header that describes the encoding – making a file with no content non-zero in size.

Uses

A zero-byte file is sometimes used for intentional purposes. For example, a user might find a zero-byte file named documents-moved-to-partition-D in an otherwise empty directory and then know to look in partition D for the files that used to be in that directory. Sometimes, a zero-byte file is added to a directory to ensure that it is not empty since some tools (such as backup and revision control software) ignore an empty directory.

The presence or absence of a file with a particular name can be used to direct program control flow. For example:

IF EXIST C:\NOTHING.TXT EXIT
IF NOT EXIST C:\NOTHING.TXT GOTO START
:START
REM Create the zero-length file
C:\>type null>nothing.txt
:EXIT