How can the mac time code “b” be “n/a”?

Question

There are 4 mac time codes for linux ext2:

• M -- modified
• A -- accessed
• C -- created
• B -- n/a

How can one be "n/a"? What does it mean?

Answer 1

This just means there is no equivalent for the b flag e.g. no way to translate it for the specific file system.

b probably stands for birth time recording the time the file was created.The Wikipedia article about MAC times mentions the difference between Windows and Unix and also introduces the term birth time:

• Unix systems maintain the historical interpretation of ctime as being the time when certain file metadata, not its contents, were last changed, such as the file's permissions or owner (e.g. 'This files metadata was changed on 05/05/02 12:15pm').

• Windows systems use ctime to mean 'creation time' (also called 'birth time') (e.g. 'This file was created on 05/05/02 12:15pm').

Answer 2

C does not mean created. Unix traditionally stores three timestamps for each file:

• atime: the time at which the file was last accessed, i.e. the last time is was read from;
• mtime: the time at which the file was last modified, i.e. the last time it was written to;
• ctime: the inode change time, i.e. the last time the file was moved or its metadata (permissions, times, etc.) was changed.

The ctime is not a creation time. In fact, the ctime is always at least as recent as the modification time. On most unix systems, it is impossible to set the ctime to an arbitrary value, except by bypassing the filesystem and modifying the bits on the disk directly; the ctime is only ever set to the current system time.

Some filesystems have a notion of file creation time. In unix circles, this is generally known as the birth time. A word other than “creation” was chosen because the initial c was already taken.

Ext2 and UFS have only traditional unix features; they do not store a birth time. FAT32 and NTFS are Windows filesystems; they have a birth time (called creation time), but nothing exactly like ctime (NTFS has something close).