Check if the given path is in a btrfs filesystem

btrfs_checker.rb

require 'yast'
require 'yast2/execute'
require 'pathname'

# Returns if the given path is placed in a btrfs
#
# @param path [String, Pathname]
#
# @return [Boolean]
def btrfs_available_for?(path)
  mounted_btrfs = Yast::Execute.locally!.stdout(
    ["df", "-t", "btrfs", "--output=target"],
    ["tail", "-n", "+2"]
  ).split.uniq

  not_mounted_btrfs = Yast::Execute.locally!.stdout(
    ["df", "-x", "btrfs", "--output=target"],
    ["tail", "-n", "+2"]
  ).split.uniq

  candidate_paths = Pathname.new(path).descend.map(&:to_s)

  return false if mounted_btrfs.empty?
  return false if (not_mounted_btrfs & candidate_paths).any?

  (mounted_btrfs & candidate_paths).any?
end


# Simply for testing (manually, through irb) the scenario
# > for a system with "/" in btrfs and a separate "/home" with ext4
#
def fake_btrfs_available_for?(path)
  candidate_paths = Pathname.new(path).descend.map(&:to_s)

  return false if btrfs_targets.empty?
  return false if (not_btrfs_targets & candidate_paths).any?

  (btrfs_targets & candidate_paths).any?
end

def btrfs_targets
  ["/"]
end

def not_btrfs_targets
  ["/home"]
end

In a system with below df output

ytm@linux-9x6v:~/suse/src> df -t btrfs
Filesystem     1K-blocks    Used Available Use% Mounted on
/dev/vda2       10476524 6597212   3644836  65% /
/dev/vda2       10476524 6597212   3644836  65% /tmp
/dev/vda2       10476524 6597212   3644836  65% /srv
/dev/vda2       10476524 6597212   3644836  65% /var
/dev/vda2       10476524 6597212   3644836  65% /opt
/dev/vda2       10476524 6597212   3644836  65% /usr/local
/dev/vda2       10476524 6597212   3644836  65% /home
/dev/vda2       10476524 6597212   3644836  65% /boot/grub2/i386-pc
/dev/vda2       10476524 6597212   3644836  65% /boot/grub2/x86_64-efi
/dev/vda2       10476524 6597212   3644836  65% /root

it will returns true

irb(main):001:0> require_relative './btrfs_check_poc.rb'
=> true
irb(main):002:0> btrfs_available_for?('/home/ytm_tester')
=> true
irb(main):003:0>

In a system with below df output

❯ df -t btrfs
df: no file systems processed

it will returns false

irb(main):001:0> require_relative './btrfs_check_poc.rb'
=> true
irb(main):002:0> btrfs_available_for?('/home/ytm_tester')
=> false
irb(main):003:0> 

for a system with "/" in btrfs and a separate "/home" with ext4

irb(main):001:0> require_relative './btrfs_check_poc.rb'
=> true
irb(main):002:0> btrfs_available_for?('/home/ytm_tester')
=> false
irb(main):003:0> btrfs_available_for?('/alternative_home/ytm_tester')
=> true

I would have gone for something like df --output=fstype /home simply to avoid path operations and symlink complications.

What if /home is a symlink to a different file system?

Thank you @wfeldt

So,

def btrfs?(path)
  dirname = Pathname.new(path).dirname
  fstype = Yast::Execute.locally!.stdout(
    ["df", "--output=fstype", dirname],
    ["tail", "-n", "+2"]
  ).chomp

  fstype == "btrfs"
end

will be enough :)

Related to the recent security hardening, we should call /usr/bin/df (with full path).

But I found an easier way:

# /usr/bin/stat -f --format '%T' /home
xfs

No need for removing the df header from the output...

Ah, you might need to add the --dereference to follow symlinks...

@lslezak,

Pretty nice! Thank you!

BTW,

Related to the recent security hardening

Thank you for the reminder! :) I was using the Yast::Execute.locally! because of this, but I forgot to use the full path.