POSTIN

<lua>

nl        = '\n'
sh        = '/bin/sh'..nl
bash      = '/bin/bash'..nl
f = io.open('/etc/shells', 'a+')
if f then
  local shells = nl..f:read('*all')..nl
  if not shells:find(nl..sh) then f:write(sh) end
  if not shells:find(nl..bash) then f:write(bash) end
  f:close()
end

POSTUN

<lua>

-- Run it only if we are uninstalling
if arg[2] == "0"
then
  t={}
  for line in io.lines("/etc/shells")
  do
    if line ~= "/bin/bash" and line ~= "/bin/sh"
    then
      table.insert(t,line)
    end
  end

  f = io.open("/etc/shells", "w+")
  for n,line in pairs(t)
  do
    f:write(line.."\n")
  end
  f:close()
end