TIL: Directory modification times

Recently, I needed a way to check when the content of a folder had last been updated. If it hadn’t seen any changes within the last couple of days I wanted to remove it.

One solution for this would be to recursively check the modification time of every single file in that folder but I also got curious about what actually defines the modification time of the folder itself.

As this is something that can be different from file-system to file-system, let’s look at some of the more popular systems for macOS, Linux, and Windows. All of them are basically handling the mtime of a folder in pretty much the same way:

The mtime of a folder is only modified if a file directly below it is added, renamed, or removed. Changing anything about the content of any of the files or their attributes won’t change the mtime of the directory that contains them (Source).

To verify this I’ve created a little Python script and tried it on my local machine (macOS), a Linux box, and a Windows box.

There still seem to be some differences between the various file-systems, though, esp. regarding the amount of precision they provide.

For HFS+ (macOS) it was pretty easy to find some details in the specs that specify the basic behavior:

contentModDate: The date and time the folder’s contents were last changed. This is the time when a file or folder was created or deleted inside this folder, or when a file or folder was moved in or out of this folder. See HFS Plus Dates for a description of the format.

attributeModDate: The last date and time that any field in the folder’s catalog record was changed. An implementation may treat this field as reserved. In Mac OS X, the BSD APIs use this field as the folder’s change time (returned in the st_ctime field of struct stat). All versions of Mac OS 8 and 9 treat this field as reserved. See HFS Plus Dates for a description of the format.

[…]

HFS Plus Dates: HFS Plus stores dates in several data structures, including the volume header and catalog records. These dates are stored in unsigned 32-bit integers (UInt32) containing the number of seconds since midnight, January 1, 1904, GMT. This is slightly different from HFS, where the value represents local time.

Compared to more modern systems the mtime et al. are only exposed in seconds, while others are exposing nanoseconds. That also shows when you look at the st_mtime_ns value in Python (inspecting one of the source files of this blog):

>>> import pathlib
>>> pathlib.Path('32c3.md').stat().st_mtime_ns
1473583000000000000

ext4 (Linux) on the other hand offers nanosecond precision through some additional fields which are then merged on the API layer for Python et al. to use:

>>> import pathlib
>>> pathlib.Path("README.md").stat().st_mtime_ns
1457287154109435711

On Windows NTFS seems to handle modification times in a similar way to ext4 and HFS+ according to the Description of NTFS date and time stamps for files and folders document, but I couldn’t yet find anywhere in what precision that value is actually stored. It is exposed as nanoseconds according to File Times and also the little Python snippet returned a result similar to ext4.

While looking into all that I also learnt something about how the different file-systems handle things like creation time … a whole other can of worms, though (some details).

Personally, I’m just glad that mtime doesn’t seem to come close to the configurable nightmare that atime is.

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