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I understand that mount -a achieves the same result as auto, but what remains unclear is when is mount -a used? As far as I know, auto is generally specified in the /etc/fstab file.

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It doesn't "achieve the same result as" auto; it implements auto. It's what is run during boot to mount the auto filesystems. (Twice: once for local filesystems early, once for NFS filesystems after the network has been brought up.)

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Thanks. Do you mean that if I have an entry in the /etc/fstab file with the option auto, it would only mount the device automatically if it is run with the command mount -a? If so, there is no other instance of where you would use mount -a? – PeanutsMonkey Apr 13 '12 at 3:05
You can use -a with specific filesystem types, but you'd have to distinguish then between mount at boot and your possible use. Don't try to use -a for your own uses; consider it part of the OS scaffolding. – geekosaur Apr 13 '12 at 3:11
Thanks. So when you say specific filesystems, is this restricted to just at boot time? Secondly how do I know which filesystems? – PeanutsMonkey Apr 13 '12 at 4:04
The point is that mount -a is run at boot time, so whatever else you might want to do with it, you must expect that it will also happen at boot. This is why you should probably not try to use it yourself. And I said specific filesystem types, not specific filesystems: this means filesystems whose fstype field is nfs or ext2 or etc. – geekosaur Apr 13 '12 at 4:08
Thanks. Sorry for being a newbie but when you say it is run at boot time, does it mean that is the only time it can be run or is it the only time it should be run? For example, you wouldn't advise me running when the system has already booted. If so, what would happen if I did run it after the system had booted at I attempt to mount a device e.g. USB. Sorry the term filesystems is so loosely used, it can be confusing. – PeanutsMonkey Apr 13 '12 at 4:13
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The discussion seems a little confusing to me, and I'm not sure why, unless there's something I'm missing.

I've always used "mount -a" to test-run changes to my fstab file to make sure it works correctly without the need to keep rebooting. A handy feature for testing. I'm pretty sure it does the same thing that is done at startup.

The "auto/noauto" option has nothing to do with "mount -a" directly. I think auto is the default, so it doesn't even need to be used. But it simply means that the device will be mounted at startup (and, when you type "mount -a", since it's equivalent). I personally have never needed to use the "noauto" option, but I believe it's used when you want to specify mount options for a device, but you don't want to actually mount it, unless it's done explicitly by the user with the mount command.

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When you say you have run tests against changes to the /etc/fstab file what do you mean? Also what do you mean mount -a is its equivalent? In what way? – PeanutsMonkey Apr 13 '12 at 7:48
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For tests, I mean when I'm adding or editing my fstab file, I may have problems which would be hard to troubleshoot at startup - they may just not work, or not work correctly. So, using mount -a, I can get instant feedback on the command line, and I don't need to reboot to test. Once it works with mount -a, then I know it's correct. And, by "it's equivalent" (notice apostrophe, meaning "it is"), I mean mount -a is equivalent to startup. I forgot to mention: if you change an entry that is already mounted, you would need to unmount it first. – Marty Fried Apr 13 '12 at 16:15

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