Why Does ‘Touch -a’ Also Set the Ctime? – Understanding Coreutils

coreutilsfilesfilesystemstimestamps

Is it touch(1) from coreutils-8.4-37.el6.x86_64 or my brain which is broken?

$ touch abc
$ LANG=C stat abc
  File: `abc'
  Size: 0               Blocks: 0          IO Block: 4096   regular empty file
Device: fd04h/64772d    Inode: 10485773    Links: 1
Access: (0664/-rw-rw-r--)  Uid: (3060399/ nodakai)   Gid: (  418/   quant)
Access: 2016-10-14 18:42:06.189751847 +0800
Modify: 2016-10-14 18:42:06.189751847 +0800
Change: 2016-10-14 18:42:06.189751847 +0800
$ touch -a abc
$ LANG=C stat abc
  File: `abc'
  Size: 0               Blocks: 0          IO Block: 4096   regular empty file
Device: fd04h/64772d    Inode: 10485773    Links: 1
Access: (0664/-rw-rw-r--)  Uid: (3060399/ nodakai)   Gid: (  418/   quant)
Access: 2016-10-14 18:42:17.374235446 +0800
Modify: 2016-10-14 18:42:06.189751847 +0800
Change: 2016-10-14 18:42:17.374235446 +0800
$ touch --help | grep 'access time'
  -a                     change only the access time

As you see, it was not only atime but also ctime which was updated by touch -a !?!?

The filesystem is ext4 over LVM if that makes any differences.

Best Answer

touch is specified as changing file access and modification times; changing the change time is a side-effect of the change to the file's metadata, and touch doesn't have any control over that (see also the futimens() and utimensat() functions used by touch).

-a and -m are understood in this context: by default touch changes both access and modification times (and the system updates the change time); with -a, it only changes the access time, with -m, it only changes the modification time.

You can see the difference if you specify a time other than the current time: the access and/or modification times will be changed to the value you specify, but the change time will be updated to the current time.

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