Linux – Touch Command Date Precision

dateext4filesystemslinux

On my linux system if I display the current date "t1", touch a file "f" then display the modification time "t2" of that "f" I would expect t1 < t2.

But that's not what I always get when I execute this on my system:

date +'%Y-%m-%d %H:%M:%S.%N'; \
touch f; \
stat -c %y f

Example output:

2017-09-18 21:47:48.855229801
2017-09-18 21:47:48.853831698 +0200

Notice the second timestamp (stat) is before the first one (date): 855229801 > 853831698

My fs is ext4, but I also tried with a file on tmpfs, same effect.
Why is it the case?

Thanks

Some info about the setup

% which date
/usr/bin/date

% which touch
/usr/bin/touch

% pacman -Qo /usr/bin/date /usr/bin/touch
/usr/bin/date is owned by coreutils 8.28-1
/usr/bin/touch is owned by coreutils 8.28-1

% uname -a
Linux machine 4.12.12-1-ARCH #1 SMP PREEMPT Sun Sep 10 09:41:14 CEST 2017 x86_64 GNU/Linux

% findmnt
TARGET                                SOURCE     FSTYPE    OPTIONS
/                                     /dev/sda1  ext4      rw,relatime,data=ordered
└─/tmp                                tmpfs      tmpfs     rw,nosuid,nodev

Best Answer

Per https://stackoverflow.com/questions/14392975/timestamp-accuracy-on-ext4-sub-millsecond :

ext4 filesystem code calls current_fs_time() which is the current cached kernel time truncated to the time granularity specified in the file system's superblock which for ext4 is 1ns.

The current time within the Linux kernel is cached, and generally only updated on a timer interrupt. So if your timer interrupt is running at 10 milliseconds, the cached time will only be updated once every 10 milliseconds. When an update does occur, the accuracy of the resulting time will depend on the clock source available on your hardware.

lsof

The only way I can think to gain access to a mount such as this would be to mount it as you did before using Nautilus or whatever file browser, and then using a tool such as lsof to see what files/devices are opened by Nautilus.

$ lsof -p $(pgrep nautilus)

But if you're having to connect to your phone via gphoto2 then you're likely not mounting the device a a mass storage device but rather a PTP - Picture Transfer Protocol. There's a Linux FUSE implementation for PTP too.

gvfs?

I would also look in your $HOME directory for a sub-directory called .gvfs. Usually when GNOME or Nautilus are doing the mounting this directory is created as a convenience.

In newer versions of GNOME (3+) this directory has moved and is now here, /run/user/$UID/gvfs.

Example

$ ls -l /run/user/$UID/gvfs/smb-share\:server\=tank\,share\=t 
total 2
drwx------. 1 saml saml    0 Oct 31 09:16 Development
drwx------. 1 saml saml    0 Nov 18 14:52 home
drwx------. 1 saml saml    0 May 14  2013 incoming
...

NOTE: That's a environment variable $UID that is often set in Bash on most modern systems. If it isn't set you can find your user's ID like so:

$ id
uid=1000(saml) gid=1000(saml) groups=1000(saml),10(wheel) context=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023

Linux – How to get relative date based on relative date by linux date command

If you have faketime and your date is dynamically linked:

faketime -f -10m date -d 'last monday' '+%F %T'

With ksh93 (only builtin commands):

printf '%(%F %T)T\n' "$(printf '%(%Y.%m.%d)T' '10 minutes ago')-0 last monday"

Here, if it was last Sunday instead of last Monday, you could do:

date -d "$(date -d '10 minutes ago' +"%F -%u day")"

Or if it was last Saturday:

date -d "$(date -d '10 minutes ago' +"%F -%w day -1 day")"

But for last Monday or any other day of the week, it involves a bit of arithmetic:

eval "date -d \"$(date -d '10 minutes ago' +'%F -$(((%u+5)%%7+1)) day')\""

Otherwise, you can always do:

d=$(date -d 'last monday'); sleep 600; printf '%s\n' "$d"

;-)

Best Answer

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lsof

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Linux – How to get relative date based on relative date by linux date command

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