make cd.lua nearly POSIX compliant

This submission omits support for -L and -P options for cd which specify whether to resolve sym links logically or physically, respectively
We rely on the openos filesystem for symlinks, which is only partially supported (e.g. not-persistent) and thus I feel we should omit explicit support
for these symlink related options to cd - and just allow shell.resolve and shell.setWorkingDirectory to handle the path resolution - which from testing appears
to follow all symlinks logically (which is logical from the user perspective - so a good default).

src/main/resources/assets/opencomputers/loot/OpenOS/bin/cd.lua

@@ -1,11 +1,105 @@
 local shell = require("shell")
+local fs = require("filesystem")
 
-local args = shell.parse(...)
+local shell_name = '-' .. fs.name(os.getenv("SHELL"))
+local cmd_name = "cd"
+local error_prefix = shell_name .. ": " .. cmd_name .. ": "
+
+local args, ops = shell.parse(...)
+local path = nil
+local verbose = false
+
+-- order of if blocks following POSIX specification on cd
+-- this is not a complete support of the POSIX spec - I am
+-- not supporting -L and -P options for differentiating between
+-- logically and physically following symlink paths
+-- but, we do support CDPATH
 if #args == 0 then
-  io.write("Usage: cd <dirname>")
+  local home = os.getenv("HOME")
+  if not home then
+    io.stderr:write(error_prefix .. "HOME not set\n")
+    return 1
+  end
+  path = home
+elseif args[1]:len() > 0 and args[1]:sub(1,1) == '/' then
+  path = args[1]
+elseif args[1]:len() > 0 and args[1]:sub(1,1) == "." then
+  path = args[1]
+elseif args[1] == '-' then
+  verbose = true
+  local oldpwd = os.getenv("OLDPWD");
+  if not oldpwd then
+    io.stderr:write(error_prefix .. "OLDPWD not set\n")
+    return 1
+  end
+  path = oldpwd
 else
-  local result, reason = shell.setWorkingDirectory(shell.resolve(args[1]))
-  if not result then
-    io.stderr:write(reason)
+  local CDPATH = os.getenv("CDPATH")
+  if CDPATH then
+    for cd_path in CDPATH:gmatch("[^:]*:?") do
+      -- [^:]+ would be simpler but CDPATH can define
+      -- empty paths if the user wants to check . before
+      -- the end of the cdpath list
+
+      if cd_path and cd_path:len() > 0 then
+        -- remove trailing : now if it was matched
+        if cd_path:sub(-1) == ':' then
+          cd_path = cd_path:sub(1, -2)
+        end
+
+        -- if cd_path is empty (POSIX says null) then check ./
+        -- there is a case where we don't hit this if block for
+        -- an empty cd_path, i.e. CDPATH="" which SHOULD be considered
+        -- null for this purpose - but, that scenario would also act the
+        -- same as no CDPATH and test ./ independently - so we get the
+        -- same results. I've tested this on gentoo linux and confirmed
+        -- curiously, empty checks are not verbose
+        local empty_check = false
+        if cd_path == "" then
+          cd_path = "."
+          empty_check = true
+        end
+
+        -- concat / on cd_path if it doesn't have one
+        if cd_path:sub(-1) ~= '/' then
+          cd_path = cd_path .. '/'
+        end
+
+        local test_path = cd_path .. args[1]
+
+        -- resolve here because fs.exists doesn't
+        -- understand . and ..
+        test_path = shell.resolve(test_path)
+        if fs.exists(test_path) then
+          path = test_path
+          verbose = not empty_check
+          break
+        end
+      end
+    end
+  end
+  if not path then
+    path = args[1]
   end
 end
+
+local resolved = shell.resolve(path)
+if not fs.exists(resolved) then
+  io.stderr:write(error_prefix .. path .. ": No such file or directory\n")
+  return 1
+end
+
+path = resolved
+
+local oldpwd = shell.getWorkingDirectory()
+local result, reason = shell.setWorkingDirectory(path);
+if not result then
+  io.stderr:write(error_prefix .. reason)
+  return 1
+else
+  os.setenv("OLDPWD", oldpwd)
+end
+
+if verbose then
+  os.execute("pwd")
+end