Permissions and umask

gistfile1.rb

def permissions_string(integer)
  triplet_to_string = lambda { |x, s| x.tr("01", "-#{s}") }

  integer.to_s(2).rjust(9, "0").chars.each_slice(3).map do |r, w, x|
    [
      triplet_to_string[r, "r"],
      triplet_to_string[w, "w"],
      triplet_to_string[x, "x"]
    ].join("")
  end.join("")
end

permissions_string 0777
#=> rwxrwxrwx

permissions_string 0205
#=> -w----r-x

permissions_string 0757
#=> rwxr-xrwx

permissions_string 0202
# -w-----w-
permissions_string 0777
# rwxrwxrwx

# use 0202 as the mask to substract permissions off 0777

permissions_string(0777 & ~0202)
# r-xrwxrwx
# =
# rwxrwxrwx masked with
# -w-----w-

# 0777 & ~mask is useful with File.umask to get the default
# permissions as if you'd used `touch`

permissions_string File.umask
# ----w--w-    (may differ in output for you)

permissions_string(0777 & ~File.umask)
# rwxr-xr-x

# also masking out executable bits, if creating a new file.
# (general executable permissions for files outside of your
# control is a bad idea)
permissions_string(0777 & ~0111 & ~File.umask)
# rwxr--r--

File.open("/tmp/foo", "w", 0777 & ~0111 & ~File.umask) {}

# interested only in the last 3 triples
file_mode = File.stat("/tmp/foo").mode & 0777
permissions_string(file_mode)
# rw-r--r--