Changing /etc/fstab with puppet and augeas

Changing /etc/fstab with puppet and augeas

Puppet Augeas documentation

My case: Add tmpfs

Trying with augtool, and breaking my head on how to add an entry at the end of the file, I realized eventually figured out, that you don't have to care at all with augeas, just add a 01 prefixed value and it will be added to the end.

So to add I use:

defvar fstab /files/etc/fstab
set $fstab/01/spec tmpfs
set $fstab/01/file /dev/shm
set $fstab/01/vfstype tmpfs
set $fstab/01/opt defaults
set $fstab/01/dump 0
set $fstab/01/passno 0
save

But what if the entry already exists?
Since in fstab the entries are just numbered, it took a while to figure out how to use the last() function.

This should work (e.g. change option)

set $fstab/*[spec = 'tmpfs'][file = '/dev/shm']/opt size
set $fstab/*[spec = 'tmpfs'][file = '/dev/shm']/opt/value 12G

This will only change/add the option size=12G, where spec equals 'tmpfs' AND file equals '/dev/shm'.

Then, all left to do was finding a rule for puppet to not add a new line on each run, that's done quite easily with 'match':

match *[spec='tmpfs'][file = '/dev/shm']

So in puppet it looks like this:

    augeas{"Add tmpfs for case ${caseusername}@${hostname}":
        context => "/files/etc/fstab",
        changes => [
            "set 01/spec tmpfs",
            "set 01/file /dev/shm",
            "set 01/vfstype tmpfs",
            "set 01/opt size",
            "set 01/opt value ${shmem}",
            "set 01/dump 0",
            "set 01/passno 0",
            ],
        onlyif => "match *[spec='tmpfs'][file = '/dev/shm'] size == 0",
    }

    augeas{"change tmpfs size for ${caseusername}@${hostname} to ${shmem}":
        context => "/files/etc/fstab",
        changes => [
            "set *[spec = 'tmpfs'][file = '/dev/shm']/opt size",
            "set *[spec = 'tmpfs'][file = '/dev/shm']/opt/value ${shmem}",
            ],
        require => Augeas["Add tmpfs for case ${caseusername}@${hostname}"],
    }