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I'd like to have a program executed before a mount attempt is made for a particular device/share/mount. For example, I'd like for autofs/amd to control /data/{1..10}, and when a process opens /data/4 (and /data/4 is not currently mounted), a script is invoked, such as '/usr/local/bin/preparedata 4' (4 being the mount point name within the autofs controlled directory), prior to the attempt to mount. For example, I could dynamically attach an iSCSI LUN (which would be referenced in the autofs map), or startup a remote system/VM which has an NFS export (which is specified in the map).

I'd be glad to add details if missing.

Update: I've noticed that systemd appears to be intercepting open() calls, is there some way to do this particularly in systemd?

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  • Any luck with this? I have a similar need. See unix.stackexchange.com/questions/189404/… Commented Mar 12, 2015 at 18:58
  • nope.. only thing I can imagine is modifying some underlying code... Commented Mar 30, 2015 at 3:19

2 Answers 2

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Autofs itself can run a custom script or program to dynamically provide "the map", i.e. the mount options and arguments autofs uses for mounting.

As an example, to automount home directories from an NFS-server one may prefer to use a pattern like "/home/user12/user123456" for the homedir paths to limit the number of sub-directories on a server when there are many users.

To dynamically mount such home directories, you could put this in your /etc/auto.master:

/home program:/usr/local/sbin/autofs-home-mapper.sh

The script /usr/local/sbin/autofs-home-mapper.sh could look like this:

#!/bin/bash echo "-fstype=nfs4,relatime nfs.example.com:/exported/${1%????}/${1}"

When the local directory /home/johndoe is accessed, autofs will run the script with one argument: johndoe

Output of this script will then be:

-fstype=nfs4,relatime nfs.example.com:/exported/joh/johndoe

...which is then used by autofs to mount /home/johndoe

Don't forget to set eXecute permission on the script, as it can be difficult to track down a bug like that.

More information in man 5 auto.master (look under "map-type") and man 5 autofs.

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ahh, I saw that when originally researching, but thought it was run once to generate the map, not run each time a keyword is looked up in the map. If the latter is the case, then this solves the problem quite nicely!
It'll be run every time it needs to get mounted.
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Take a look at x-systemd.requires=, and x-systemd.after= for hooks executed before mounting.

These options can be specified inside /etc/fstab.

Related question: Hook before an .automount unmounts.

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OP asked about autofs not about fstab
OP also asked about systemd and systemd can be used for automount in fstab. Drawback is that "requires" expects an unit and not simply a script like shown in Hkoof's answer.

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