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My automatically generated /etc/fstab file has most of it's entries like so

# /dev/sda9
UUID=8de1f4d1-4620-49f0-aeaf-a499f7cb6c21   /home       ext4        rw,relatime 0 2

It's missing the defaults option. My system works fine, but I am not sure if:

  1. Default values are implicitly set, if so, why is rw there? I can't see any notice that they are implicit in the man page.
  2. My system is mis-configured and just happens to be running ok (for months) and I should insert the defaults option (maybe for async, etc).

My system is Arch with Systemd, maybe that is picking up the slack.

markson edwardson
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  • I use Ubuntu and have in my old notes: Since Hardy (or Intrepid), Ubuntu adds the relatime option by default. relatime = relatime + defaults = relatime, rw, suid, dev, exec, auto, nouser and async But cannot now find any info showing that. I use relatime for HDD partitions and noatime for SSD partitions. Old kernel info: https://unix.stackexchange.com/questions/17844/when-was-relatime-made-the-default My relatime only setting in fstab, shows as rw,relatime in mount. – oldfred Aug 08 '21 at 16:16

1 Answers1

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  • This option is not implicit.
  • defaults (may) mean different options for different filesystems.
  • It's OK not to have it if you're not interested in the options it bestows.
  • There are default (implicit) mount options depending on your kernel/distro. For Ubuntu it's rw,relatime.

From man mount:

       defaults
              Use the default options: rw, suid, dev, exec, auto, nouser, and async.

              Note that the real set of all default mount options depends on the kernel and filesystem type.
              See the beginning of this section for more details.
Artem S. Tashkinov
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  • Is there a way to find what defaults were set by the kernel? Searching for "arch mount options" etc doesnt show any useful information. If the defaults are not implicit, how does my `/home` mount on boot (`auto`) or permit execution of binaries (`exec`)? `zcat /proc/config.gz` doesn't really show anything related to this, nor does `/proc/mount` (just mirrors `fstab`). – markson edwardson Aug 09 '21 at 06:54
  • Related I guess https://unix.stackexchange.com/questions/191405/do-you-need-to-specify-the-defaults-option-in-fstab seems to imply defaults are implicit. – markson edwardson Aug 09 '21 at 06:55