Why Does UNIX Not Store the Creation Time of a File?

UNIX traditionally does not store the creation time of a file, a decision driven by its design philosophy emphasizing simplicity and efficiency. This article delves into the historical context, the rationale behind this design choice, and how modern developments have influenced the storage of file creation times.

Historical Context

UNIX was conceived in the late 1960s and early 1970s, a time when computing resources were scarce and performance was paramount. The file system was designed to prioritize essential features and keep overhead to a minimum. This historical context sets the stage for understanding why creation time was not originally included as a timestamp.

Design Philosophy

UNIX's design philosophy places a strong emphasis on simplicity and efficiency. Prioritizing fundamental features like access, modification, and change times over creation time aligns with this philosophy. These three timestamps (atime, mtime, and ctime) are sufficient for most use cases, especially given the hardware limitations of the time.

Simplicity

Simple is better than complex. The original UNIX file system tracked only three timestamps: atime (last time the file was read), mtime (last time the file was modified), and ctime (last time the file's metadata was changed). Adding creation time would have complicated the system without providing significant benefits in the context of early computing.

Modern Developments

As technology advanced, file systems started to evolve to meet new demands. Modern UNIX-like file systems, such as ext4 and APFS, now support a creation time as an additional timestamp. This change reflects the increasing importance of tracking when files were created, especially in more advanced and complex systems.

These new file systems store the creation time, often referred to as crtime or birthtime, alongside the traditional atime, mtime, and ctime. However, this change is not universally adopted across all UNIX systems, and there is no set POSIX standard for how to handle creation times.

Compatibility

Many UNIX systems prioritize backward compatibility with legacy applications and tools that expect only the three traditional timestamps. Changing the format or adding new features can break these tools, hindering the widespread adoption of creation time support.

Conclusion

In summary, the lack of a file creation time in traditional UNIX systems is primarily due to historical design choices that emphasized simplicity and efficiency over additional features. While modern file systems have started to address this limitation, reflecting the evolving needs of users and applications, the transition is not yet universal.

Accessing Creation Time in Modern File Systems

Although creation time is not universally supported, it can be accessed in some modern file systems. For example, in the ext4 file system, the creation time (crtime) is stored but may not be accessible via standard commands without additional tools. Here is an example of how to access the creation time:

touch test.txt
stat test.txt
File: 'test.txt'
Size: 0         Blocks: 0          IO Block: 4096   regular empty file
Device: 801h/2049d  Inode: 397342      Links: 1
Access: (0664/-rw-rw-r--)  Uid:  1000/  sinner  Gid:  1000/  sinn
er
Access: 2014-07-13 03:59:25.005338270 -0700
Modify: 2014-07-13 03:59:25.005338270 -0700
Change: 2014-07-13 03:59:25.005338270 -0700
Birth: -

To access the creation time, you can use the ls -i command and the debugfs tool:

ls -i test.txt
397342 test.txt
sudo debugfs -R 'stat' '/dev/sda1/397342'
crtime: 53c2668d:0145d278 -- Sun Jul 13 03:59:25 2014
Size of extra inode fields: 28
EXTENTS:

This approach allows you to retrieve the creation time, even though there is no kernel API available for this purpose.

References

POSIX standard for file timestamps. Documentation for ext4 file system. Documentation for APFS file system.