File access time not updated

Question

I just have viewed /etc/resolv.conf file using a cat command (or opening it with mcedit). Now when i list file details using ls -ul the "access time" doesn't change/update - have the same, old value. It's strange since for any other files "access time" is updated after cat.

Answer 1

Invoke df /etc/resolv.conf and note under what mountpoint it exists. Example from my Kubuntu:

$ df /etc/resolv.conf
Filesystem     1K-blocks  Used Available Use% Mounted on
tmpfs             806600  9660    796940   2% /run

Surprisingly my resolv.conf is under /run. True:

$ ls -l /etc/resolv.conf
lrwxrwxrwx 1 root root 29 Nov 24  2016 /etc/resolv.conf -> ../run/resolvconf/resolv.conf

Then we need to know its mount options:

$ mount | grep " /run "
tmpfs on /run type tmpfs (rw,nosuid,noexec,relatime,size=806600k,mode=755)

Pay attention to atime-related option. In my case this is relatime. man 8 mount explains:

relatime
Update inode access times relative to modify or change time. Access time is only updated if the previous access time was earlier than the current modify or change time. (Similar to noatime, but doesn't break mutt or other applications that need to know if a file has been read since the last time it was modified.)

(Check the rest of *atime options to have a full picture.)

relatime is very common nowadays. It saves frequent writing yet still provides atime to programs that need it. In my Kubuntu only these mounts don't use relatime:

$ mount | grep -v relatime
tmpfs on /dev/shm type tmpfs (rw,nosuid,nodev)
tmpfs on /sys/fs/cgroup type tmpfs (ro,nosuid,nodev,noexec,mode=755)

Probably you broadly use relatime too.

It's strange since for any other files "access time" is updated after cat.

Maybe "the previous access time was earlier than the current modify or change time", so the atime was updated. Or "other files" are under a different mountpoint (e.g. with strictatime option). Note my resolv.conf is on a different filesystem than the whole /etc/ (/etc/resolv.conf is a symlink to elsewhere), so in general you shouldn't even assume that "files" in the same directory are on the same filesystem, under the same mountpoint with the same options.

Whatever your setup is, now you know how to investigate the issue.