config: add modes for reading, writing and unsetting individual entries

Includes special config keys `lua_dir` and `lua_version`,
which are essentially persistent versions of --lua-dir
and --lua-version:

* `lua_dir` writes a number of LUA_* variables and
  cfg.lua_interpreter in the current scope's config file.
* `lua_version` writes default-lua-version.lua to the
  given scope.

Also deprecates the "flag" versions for various getters:

* `--lua-incdir`:    use `luarocks config variables.LUA_INCDIR`
* `--lua-libdir`:    use `luarocks config variables.LUA_LIBDIR`
* `--lua-ver`:       use `luarocks config lua_version`
* `--system-config`: use `luarocks config config_files.system.file`
* `--user-config`:   use `luarocks config config_files.user.file`
* `--rock-trees`:    use `luarocks config rocks_trees`

11 files changed, 534 insertions, 198 deletions

diff --git a/.gitignore b/.gitignore
index 628eab64..99850056 100644
--- a/.gitignore
+++ b/.gitignore
@@ -14,3 +14,4 @@
 /wiki
 /lua
 /lua_modules
+/luarocks
diff --git a/spec/config_spec.lua b/spec/config_spec.lua
index defc1d2f..2e76ae0c 100644
--- a/spec/config_spec.lua
+++ b/spec/config_spec.lua
@@ -10,28 +10,58 @@ test_env.unload_luarocks()
 
 describe("LuaRocks config tests #integration", function()
    
-   before_each(function()
+   setup(function()
       test_env.setup_specs()
       test_env.unload_luarocks() -- need to be required here, because hardcoded is created after first loading of specs
       hardcoded = require("luarocks.core.hardcoded")
    end)
 
-   describe("LuaRocks config - basic tests", function()
-      it("LuaRocks config with no flags/arguments", function()
-         assert.match("rocks_servers", run.luarocks("config"))
+   describe("full configuration query", function()
+      it("no flags/arguments", function()
+         assert.match("rocks_servers = {", run.luarocks("config"))
       end)
-      
-      it("LuaRocks config include dir returns a subdir of LUA_DIR", function()
+
+      it("--json", function()
+         assert.is_true(run.luarocks_nocov("install dkjson"))
+         finally(function()
+            assert.is_true(run.luarocks_nocov("remove dkjson"))
+         end)
+         assert.match('"rocks_servers":[', run.luarocks("config --json"), 1, true)
+      end)
+
+      it("--json fails without a json library", function()
+         assert.falsy(run.luarocks_bool("config --json"))
+      end)
+
+      it("with --tree respects custom config", function()
+         write_file("my_config.lua", [[
+            rocks_trees = {
+               {
+                  name = "system",
+                  root = "/example/tree",
+                  lua_dir = "/example/luadir",
+               },
+            }
+         ]], finally)
+         local output = run.luarocks("config", {LUAROCKS_CONFIG = "my_config.lua"})
+         assert.match([[deploy_lua_dir = "/example/luadir"]], output)
+         output = run.luarocks("config --tree=system", {LUAROCKS_CONFIG = "my_config.lua"})
+         assert.match([[deploy_lua_dir = "/example/luadir"]], output)
+      end)
+   end)
+
+   describe("query flags", function()
+      it("--lua-incdir returns a subdir of LUA_DIR", function()
          local output = run.luarocks("config --lua-incdir")
          assert.match(hardcoded.LUA_DIR, output, 1, true)
       end)
       
-      it("LuaRocks config library dir returns a subdir of LUA_DIR", function()
+      it("--lua-libdir returns a subdir of LUA_DIR", function()
          local output = run.luarocks("config --lua-libdir")
          assert.match(hardcoded.LUA_DIR, output, 1, true)
       end)
       
-      it("LuaRocks config lua version", function()
+      it("--lua-ver returns the Lua version", function()
          local output = run.luarocks("config --lua-ver")
          local lua_version = _VERSION:gsub("Lua ", "")
          if test_env.LUAJIT_V then
@@ -40,77 +70,158 @@ describe("LuaRocks config tests #integration", function()
          assert.are.same(lua_version, output)
       end)
       
-      it("LuaRocks config rock trees", function()
+      it("--rock-trees lists rock trees", function()
          assert.is_true(run.luarocks_bool("config --rock-trees"))
       end)
       
-      it("LuaRocks config user config", function()
-         local user_config_path = run.luarocks("config --user-config")
-         assert.is.truthy(lfs.attributes(user_config_path))
-      end)
+      describe("--user-config", function()
+         it("returns user config dir", function()
+            local user_config_path = run.luarocks("config --user-config")
+            assert.is.truthy(lfs.attributes(user_config_path))
+         end)
       
-      it("LuaRocks config missing user config", function()
-         local output = run.luarocks("config --user-config", {LUAROCKS_CONFIG = "missing_file.lua"})
-         assert.truthy(output:match("Warning"))
+         it("handles a missing user config", function()
+            local output = run.luarocks("config --user-config", {LUAROCKS_CONFIG = "missing_file.lua"})
+            assert.match("Warning", output)
+         end)
       end)
 
-      it("LuaRocks config with --tree respects custom config", function()
-         write_file("my_config.lua", [[
-            rocks_trees = {
-               {
-                  name = "system",
-                  root = "/example/tree",
-                  lua_dir = "/example/luadir",
-               },
-            }
-         ]], finally)
-         local output = run.luarocks("config", {LUAROCKS_CONFIG = "my_config.lua"})
-         assert.match([[deploy_lua_dir = "/example/luadir"]], output)
-         output = run.luarocks("config --tree=system", {LUAROCKS_CONFIG = "my_config.lua"})
-         assert.match([[deploy_lua_dir = "/example/luadir"]], output)
+      describe("--system-config", function()
+         local scdir = testing_paths.testing_lrprefix .. "/etc/luarocks"
+         local configfile = scdir .. "/config-" .. env_variables.LUA_VERSION .. ".lua"
+   
+         it("fails if system config doesn't exist", function()
+            os.rename(configfile, configfile .. ".bak")
+            finally(function()
+               os.rename(configfile .. ".bak", configfile)
+            end)
+            assert.is_false(run.luarocks_bool("config --system-config"))
+         end)
+         
+         it("outputs the path of the system config", function()
+            lfs.mkdir(testing_paths.testing_lrprefix)
+            lfs.mkdir(testing_paths.testing_lrprefix .. "/etc/")
+            lfs.mkdir(scdir)
+   
+            local sysconfig = io.open(configfile, "w+")
+            sysconfig:write(" ")
+            sysconfig:close()
+            finally(function()
+               os.remove(configfile)
+            end)
+            
+            local output = run.luarocks("config --system-config")
+            assert.are.same(configfile, output)
+         end)
+         
+         it("fails if system config is invalid", function()
+            lfs.mkdir(testing_paths.testing_lrprefix)
+            lfs.mkdir(testing_paths.testing_lrprefix .. "/etc/")
+            lfs.mkdir(scdir)
+   
+            local sysconfig = io.open(configfile, "w+")
+            sysconfig:write("if if if")
+            sysconfig:close()
+            finally(function()
+               os.remove(configfile)
+            end)
+            assert.is_false(run.luarocks_bool("config --system-config"))
+         end)
       end)
    end)
 
-   describe("LuaRocks config - more complex tests", function()
-      local scdir = testing_paths.testing_lrprefix .. "/etc/luarocks"
-      local configfile = scdir .. "/config-" .. env_variables.LUA_VERSION .. ".lua"
+   describe("read config keys", function()
+      it("reads a simple config key", function()
+         local output = run.luarocks("config user_agent")
+         assert.match("LuaRocks/", output)
+      end)
 
-      it("LuaRocks fail system config", function()
-         os.rename(configfile, configfile .. ".bak")
-         finally(function()
-            os.rename(configfile .. ".bak", configfile)
-         end)
-         assert.is_false(run.luarocks_bool("config --system-config"))
+      it("reads an array config key", function()
+         local output = run.luarocks("config rocks_trees[2]")
+         assert.match("{%s*name", output)
       end)
-      
-      it("LuaRocks system config", function()
-         lfs.mkdir(testing_paths.testing_lrprefix)
-         lfs.mkdir(testing_paths.testing_lrprefix .. "/etc/")
-         lfs.mkdir(scdir)
-
-         local sysconfig = io.open(configfile, "w+")
-         sysconfig:write(" ")
-         sysconfig:close()
+
+      it("can read as JSON", function()
+         assert.is_true(run.luarocks_nocov("install dkjson"))
          finally(function()
-            os.remove(configfile)
+            assert.is_true(run.luarocks_nocov("remove dkjson"))
          end)
-         
-         local output = run.luarocks("config --system-config")
-         assert.are.same(configfile, output)
+         local output = run.luarocks("config rocks_trees --json")
+         assert.match('^%["', output)
       end)
-      
-      it("LuaRocks fail system config invalid", function()
-         lfs.mkdir(testing_paths.testing_lrprefix)
-         lfs.mkdir(testing_paths.testing_lrprefix .. "/etc/")
-         lfs.mkdir(scdir)
-
-         local sysconfig = io.open(configfile, "w+")
-         sysconfig:write("if if if")
-         sysconfig:close()
-         finally(function()
-            os.remove(configfile)
-         end)
-         assert.is_false(run.luarocks_bool("config --system-config"))
+
+      it("--json does not work without a json library", function()
+         assert.is_false(run.luarocks_bool("config rocks_trees --json"))
+      end)
+
+      it("reads an array -> hash config key", function()
+         local output = run.luarocks("config rocks_trees[2].name")
+         assert.match("[a-z]+", output)
       end)
+
+      it("reads a hash config key", function()
+         local output = run.luarocks("config variables.ICACLS")
+         assert.same("icacls", output)
+      end)
+
+      it("fails on invalid config key", function()
+         local output = run.luarocks("config xyz")
+         assert.match("Error: Unknown entry xyz", output)
+      end)
+   end)
+
+   describe("write config keys", function()
+      it("rejects invalid --scope", function()
+         assert.is_false(run.luarocks_bool("config web_browser foo --scope=foo"))
+      end)
+
+      it("reads an array config key", function()
+         local output = run.luarocks("config rocks_trees[2]")
+         assert.match("{%s*name", output)
+      end)
+
+      it("writes a simple config key", function()
+         test_env.run_in_tmp(function(tmpdir)
+            local myproject = tmpdir .. "/myproject"
+            lfs.mkdir(myproject)
+            lfs.chdir(myproject)
+            
+            assert(run.luarocks("init"))
+            assert.truthy(run.luarocks_bool("config web_browser foo --scope=project"))
+
+            local output = run.luarocks("config web_browser")
+            assert.match("foo", output)
+         end, finally)
+      end)
+
+      it("writes a hash config key", function()
+         test_env.run_in_tmp(function(tmpdir)
+            local myproject = tmpdir .. "/myproject"
+            lfs.mkdir(myproject)
+            lfs.chdir(myproject)
+            
+            assert(run.luarocks("init"))
+            assert.truthy(run.luarocks_bool("config variables.FOO_DIR /foo/bar --scope=project"))
+
+            local output = run.luarocks("config variables.FOO_DIR")
+            assert.match("/foo/bar", output)
+         end, finally)
+      end)
+
+      it("writes an array config key", function()
+         test_env.run_in_tmp(function(tmpdir)
+            local myproject = tmpdir .. "/myproject"
+            lfs.mkdir(myproject)
+            lfs.chdir(myproject)
+            
+            assert(run.luarocks("init"))
+            assert.truthy(run.luarocks_bool("config external_deps_patterns.lib[1] testtest --scope=project"))
+
+            local output = run.luarocks("config external_deps_patterns.lib[1]")
+            assert.match("testtest", output)
+         end, finally)
+      end)
+
    end)
+
 end)
diff --git a/src/luarocks/cmd.lua b/src/luarocks/cmd.lua
index eb120c0b..d038852b 100644
--- a/src/luarocks/cmd.lua
+++ b/src/luarocks/cmd.lua
@@ -152,6 +152,33 @@ local function process_server_flags(flags)
    return true
 end
 
+local function get_lua_version(flags)
+   if flags["lua-version"] then
+      return flags["lua-version"] 
+   end
+   local dirs = {
+      cfg.home_tree,
+      cfg.sysconfdir,
+   }
+   if flags["project-tree"] then
+      table.insert(dirs, 1, dir.path(flags["project-tree"], ".."))
+   end
+   for _, d in ipairs(dirs) do
+      local f = dir.path(d, ".luarocks", "default-lua-version.lua")
+      local mod, err = loadfile(f, "t")
+      if mod then
+         local pok, ver = pcall(mod)
+         if pok and type(ver) == "string" and ver:match("%d+.%d+") then
+            if flags["verbose"] then
+               util.printout("Defaulting to Lua " .. ver .. " based on " .. f .. " ...")
+            end
+            return ver
+         end
+      end
+   end
+   return nil
+end
+
 --- Main command-line processor.
 -- Parses input arguments and calls the appropriate driver function
 -- to execute the action requested on the command-line, forwarding
@@ -249,6 +276,16 @@ function cmd.run_command(description, commands, external_namespace, ...)
    end
    command = command:gsub("-", "_")
 
+   if command == "config" then
+      if nonflags[1] == "lua_version" and nonflags[2] then
+         flags["lua-version"] = nonflags[2]
+      elseif nonflags[1] == "lua_dir" and nonflags[2] then
+         flags["lua-dir"] = nonflags[2]
+      end
+   end
+
+   local lua_version = get_lua_version(flags)
+
    if flags["deps-mode"] and not deps.check_deps_mode_flag(flags["deps-mode"]) then
       die("Invalid entry for --deps-mode.")
    end
@@ -256,47 +293,44 @@ function cmd.run_command(description, commands, external_namespace, ...)
    local detected
    if flags["lua-dir"] then
       local err
-      detected, err = util.find_lua(flags["lua-dir"], flags["lua-version"])
+      detected, err = util.find_lua(flags["lua-dir"], lua_version)
       if not detected then
          die(err)
       end
       assert(detected.lua_version)
       assert(detected.lua_dir)
-   elseif flags["lua-version"] then
+   elseif lua_version then
       local path_sep = (package.config:sub(1, 1) == "\\" and ";" or ":")
       for bindir in os.getenv("PATH"):gmatch("[^"..path_sep.."]+") do
          local parentdir = bindir:gsub("[\\/][^\\/]+[\\/]?$", "")
-         detected = util.find_lua(dir.path(parentdir), flags["lua-version"])
+         detected = util.find_lua(dir.path(parentdir), lua_version)
          if detected then
             break
          end
-         detected = util.find_lua(bindir, flags["lua-version"])
+         detected = util.find_lua(bindir, lua_version)
          if detected then
             break
          end
       end
       if not detected then
          util.warning("Could not find a Lua interpreter for version " ..
-                      flags["lua-version"] .. " in your PATH. " ..
+                      lua_version .. " in your PATH. " ..
                       "Modules may not install with the correct configurations. " ..
                       "You may want to specify to the path prefix to your build " ..
-                      "of Lua " .. flags["lua-version"] .. " using --lua-dir")
+                      "of Lua " .. lua_version .. " using --lua-dir")
          detected = {
-            lua_version = flags["lua-version"],
+            lua_version = lua_version,
          }
       end
    end
 
    if flags["project-tree"] then
       local project_tree = flags["project-tree"]:gsub("[/\\][^/\\]+$", "")
-      detected = detected or {
-         project_dir = project_tree
-      }
+      detected = detected or {}
+      detected.project_dir = project_tree
       local d = check_if_config_is_present(detected, project_tree)
       if d then
          detected = d
-      else
-         detected.project_dir = nil
       end
    else
       detected = detected or {}
diff --git a/src/luarocks/cmd/config.lua b/src/luarocks/cmd/config.lua
index 794cc989..5c7cc6c9 100644
--- a/src/luarocks/cmd/config.lua
+++ b/src/luarocks/cmd/config.lua
@@ -2,26 +2,61 @@
 -- Queries information about the LuaRocks configuration.
 local config_cmd = {}
 
+local persist = require("luarocks.persist")
 local cfg = require("luarocks.core.cfg")
 local util = require("luarocks.util")
 local deps = require("luarocks.deps")
 local dir = require("luarocks.dir")
-local fun = require("luarocks.fun")
+local fs = require("luarocks.fs")
 
 config_cmd.help_summary = "Query information about the LuaRocks configuration."
-config_cmd.help_arguments = "<flag>"
+config_cmd.help_arguments = "(<key> | <key> <value> --scope=<scope> | <key> --unset --scope=<scope> | )"
 config_cmd.help = [[
---lua-incdir     Path to Lua header files.
+* When given a configuration key, it prints the value of that key
+  according to the currently active configuration (taking into account
+  all config files and any command-line flags passed)
 
---lua-libdir     Path to Lua library files.
+  Examples:
+     luarocks config lua_interpreter
+     luarocks config variables.LUA_INCDIR
+     luarocks config lua_version
 
---lua-ver        Lua version (in major.minor format). e.g. 5.1
+* When given a configuration key and a value,
+  it overwrites the config file (see the --scope option below to determine which)
+  and replaces the value of the given key with the given value.
 
---system-config  Location of the system config file.
+  * `lua_dir` is a special key as it checks for a valid Lua installation
+    (equivalent to --lua-dir) and sets several keys at once.
+  * `lua_version` is a special key as it changes the default Lua version
+    used by LuaRocks commands (eqivalent to passing --lua-version). 
 
---user-config    Location of the user config file.
+  Examples:
+     luarocks config variables.OPENSSL_DIR /usr/local/openssl
+     luarocks config lua_dir /usr/local
+     luarocks config lua_version 5.3
 
---rock-trees     Rocks trees in use. First the user tree, then the system tree.
+* When given a configuration key and --unset,
+  it overwrites the config file (see the --scope option below to determine which)
+  and deletes that key from the file.
+
+  Example: luarocks config variables.OPENSSL_DIR --unset
+
+* When given no arguments, it prints the entire currently active
+  configuration, resulting from reading the config files from
+  all scopes.
+
+  Example: luarocks config
+
+OPTIONS
+--scope=<scope>   The scope indicates which config file should be rewritten.
+                  Accepted values are "system", "user" or "project".
+                  * Using a wrapper created with `luarocks init`,
+                    the default is "project".
+                  * Using --local (or when `local_by_default` is `true`),
+                    the default is "user".
+                  * Otherwise, the default is "system".
+
+--json           Output as JSON
 ]]
 config_cmd.help_see_also = [[
         https://github.com/luarocks/luarocks/wiki/Config-file-format
@@ -30,35 +65,13 @@ config_cmd.help_see_also = [[
 
 local function config_file(conf)
    print(dir.normalize(conf.file))
-   if conf.ok then
+   if conf.found then
       return true
    else
       return nil, "file not found"
    end
 end
 
-local function printf(fmt, ...)
-   print((fmt):format(...))
-end
-
-local cfg_maps = {
-   external_deps_patterns = true,
-   external_deps_subdirs = true,
-   rocks_provided = true,
-   rocks_provided_3_0 = true,
-   runtime_external_deps_patterns = true,
-   runtime_external_deps_subdirs = true,
-   upload = true,
-   variables = true,
-}
-
-local cfg_arrays = {
-   disabled_servers = true,
-   external_deps_dirs = true,
-   rocks_trees = true,
-   rocks_servers = true,
-}
-
 local cfg_skip = {
    errorcodes = true,
    flags = true,
@@ -67,60 +80,163 @@ local cfg_skip = {
    upload_servers = true,
 }
 
-local function print_config(cfg)
-   for k, v in util.sortedpairs(cfg) do
-      k = tostring(k)
-      if type(v) == "string" or type(v) == "number" then
-         printf("%s = %q", k, v)
-      elseif type(v) == "boolean" then
-         printf("%s = %s", k, tostring(v))
-      elseif type(v) == "function" or cfg_skip[k] then
-         -- skip
-      elseif cfg_maps[k] then
-         printf("%s = {", k)
-         for kk, vv in util.sortedpairs(v) do
-            local keyfmt = kk:match("^[a-zA-Z_][a-zA-Z0-9_]*$") and "%s" or "[%q]"
-            if type(vv) == "table" then
-               local qvs = fun.map(vv, function(e) return string.format("%q", e) end)
-               printf("   "..keyfmt.." = {%s},", kk, table.concat(qvs, ", "))
+local function should_skip(k, v)
+   return type(v) == "function" or cfg_skip[k]
+end
+
+local function cleanup(tbl)
+   local copy = {}
+   for k, v in pairs(tbl) do
+      if not should_skip(k, v) then
+         copy[k] = v
+      end
+   end
+   return copy
+end
+
+local function traverse_varstring(var, tbl, fn, missing_parent)
+   local k, r = var:match("^%[([0-9]+)%]%.(.*)$")
+   if k then
+      k = tonumber(k)
+   else
+      k, r = var:match("^([^.[]+)%.(.*)$")
+      if not k then
+         k, r = var:match("^([^[]+)(%[.*)$")
+      end
+   end
+   
+   if k then
+      if not tbl[k] and missing_parent then
+         missing_parent(tbl, k)
+      end
+
+      if tbl[k] then
+         return traverse_varstring(r, tbl[k], fn, missing_parent)
+      else
+         return nil, "Unknown entry " .. k
+      end
+   end
+
+   local i = var:match("^%[([0-9]+)%]$")
+   if i then
+      var = tonumber(i)
+   end
+   
+   return fn(tbl, var)
+end
+
+local function print_json(value)
+   local json_ok, json = util.require_json()
+   if not json_ok then
+      return nil, "A JSON library is required for this command. "..json
+   end
+
+   print(json.encode(value))
+   return true
+end
+
+local function print_entry(var, tbl, is_json)
+   return traverse_varstring(var, tbl, function(t, k)
+      if not t[k] then
+         return nil, "Unknown entry " .. k
+      end
+      local val = t[k]
+
+      if not should_skip(var, val) then
+         if is_json then
+            return print_json(val)
+         elseif type(val) == "string" then
+            print(val)
+         else
+            persist.write_value(io.stdout, val)
+         end
+      end
+      return true
+   end)
+end
+
+local function infer_type(var)
+   local typ
+   traverse_varstring(var, cfg, function(t, k)
+      if t[k] ~= nil then
+         typ = type(t[k])
+      end
+   end)
+   return typ
+end
+
+local function write_entries(keys, scope, do_unset)
+   if scope == "project" and not cfg.config_files.project then
+      return nil, "Current directory is not part of a project. You may want to run `luarocks init`."
+   end
+   
+   local tbl, err = persist.load_config_file_if_basic(cfg.config_files[scope].file, cfg)
+   if not tbl then
+      return nil, err
+   end
+   
+   for var, val in util.sortedpairs(keys) do
+      traverse_varstring(var, tbl, function(t, k)
+         if do_unset then
+            t[k] = nil
+         else
+            local typ = infer_type(var)
+            local v
+            if typ == "number" and tonumber(val) then
+               v = tonumber(val)
+            elseif typ == "boolean" and val == "true" then
+               v = true
+            elseif typ == "boolean" and val == "false" then
+               v = false
             else
-               printf("   "..keyfmt.." = %q,", kk, vv)
+               v = val
             end
+            t[k] = v
+            keys[var] = v
          end
-         printf("}")
-      elseif cfg_arrays[k] then
-         if #v == 0 then
-            printf("%s = {}", k)
+         return true
+      end, function(p, k)
+         p[k] = {}
+      end)
+   end
+
+   local ok, err = persist.save_from_table(cfg.config_files[scope].file, tbl)
+   if ok then
+      print(do_unset and "Removed" or "Wrote")
+      for var, val in util.sortedpairs(keys) do
+         if do_unset then
+            print(("\t%s"):format(var))
          else
-            printf("%s = {", k)
-            for _, vv in ipairs(v) do
-               if type(vv) == "string" then
-                  printf("   %q,", vv)
-               elseif type(vv) == "table" then
-                  printf("   {")
-                  if next(vv) == 1 then
-                     for _, v3 in ipairs(vv) do
-                        printf("      %q,", v3)
-                     end
-                  else
-                     for k3, v3 in util.sortedpairs(vv) do
-                        local keyfmt = tostring(k3):match("^[a-zA-Z_][a-zA-Z0-9_]*$") and "%s" or "[%q]"
-                        printf("      "..keyfmt.." = %q,", k3, v3)
-                     end
-                  end
-                  printf("   },")
-               end
-            end
-            printf("}")
+            print(("\t%s = %q"):format(var, val))
          end
       end
+      print(do_unset and "from" or "to")
+      print("\t" .. cfg.config_files[scope].file)
+      return true
+   else
+      return nil, err
+   end
+end
+
+local function check_scope(flags)
+   local scope = flags["scope"]
+                 or (flags["local"] and "user")
+                 or (flags["project-tree"] and "project")
+                 or (cfg.local_by_default and "user")
+                 or "system"
+   if scope ~= "system" and scope ~= "user" and scope ~= "project" then
+      return nil, "Valid values for scope are: system, user, project"
    end
+
+   return scope
 end
 
 --- Driver function for "config" command.
 -- @return boolean: True if succeeded, nil on errors.
-function config_cmd.command(flags)
+function config_cmd.command(flags, var, val)
    deps.check_lua(cfg.variables)
+   
+   -- deprecated flags
    if flags["lua-incdir"] then
       print(cfg.variables.LUA_INCDIR)
       return true
@@ -133,12 +249,11 @@ function config_cmd.command(flags)
       print(cfg.lua_version)
       return true
    end
-   local conf = cfg.which_config()
    if flags["system-config"] then
-      return config_file(conf.system)
+      return config_file(cfg.config_files.system)
    end
    if flags["user-config"] then
-      return config_file(conf.user)
+      return config_file(cfg.config_files.user)
    end
    if flags["rock-trees"] then
       for _, tree in ipairs(cfg.rocks_trees) do
@@ -151,9 +266,68 @@ function config_cmd.command(flags)
       end
       return true
    end
+
+   if var == "lua_version" and val then
+      local scope, err = check_scope(flags)
+      if not scope then
+         return nil, err
+      end
+
+      if scope == "project" and not cfg.config_files.project then
+         return nil, "Current directory is not part of a project. You may want to run `luarocks init`."
+      end
+
+      local prefix = dir.dir_name(cfg.config_files[scope].file)
+      local ok, err = fs.make_dir(prefix)
+      if not ok then
+         return nil, "could not set default Lua version: " .. err
+      end
+      local fd, err = io.open(dir.path(prefix, "default-lua-version.lua"), "w")
+      if not fd then
+         return nil, "could not set default Lua version: " .. err
+      end
+      
+      fd:write('return "' .. val .. '"\n')
+      fd:close()
+      print("Lua version will default to " .. val .. " in " .. prefix)
+   end
    
-   print_config(cfg)
-   return true
+   if var == "lua_dir" and val then
+      local scope, err = check_scope(flags)
+      if not scope then
+         return nil, err
+      end
+      local keys = {
+         ["variables.LUA_DIR"] = cfg.variables.LUA_DIR,
+         ["variables.LUA_BINDIR"] = cfg.variables.LUA_BINDIR,
+         ["variables.LUA_INCDIR"] = cfg.variables.LUA_INCDIR,
+         ["variables.LUA_LIBDIR"] = cfg.variables.LUA_LIBDIR,
+         ["lua_interpreter"] = cfg.lua_interpreter,
+      }
+      return write_entries(keys, scope, flags["unset"])
+   end
+
+   if var then
+      if val or flags["unset"] then
+         local scope, err = check_scope(flags)
+         if not scope then
+            return nil, err
+         end
+   
+         return write_entries({ [var] = val }, scope, flags["unset"])
+      else
+         return print_entry(var, cfg, flags["json"])
+      end
+   end
+
+   local cleancfg = cleanup(cfg)
+
+   if flags["json"] then
+      return print_json(cleancfg)
+   else
+      print(persist.save_from_table_to_string(cleancfg))
+      return true
+   end
 end
 
 return config_cmd
diff --git a/src/luarocks/cmd/help.lua b/src/luarocks/cmd/help.lua
index e5f1b799..dcc9e358 100644
--- a/src/luarocks/cmd/help.lua
+++ b/src/luarocks/cmd/help.lua
@@ -90,14 +90,14 @@ function help.command(description, commands, command)
       util.printout()
       util.printout("\tConfiguration files:")
       local conf = cfg.config_files
-      util.printout("\t\tSystem  : ".. dir.normalize(conf.system.file) .. " (" .. get_status(conf.system.ok) ..")")
+      util.printout("\t\tSystem  : ".. dir.normalize(conf.system.file) .. " (" .. get_status(conf.system.found) ..")")
       if conf.user.file then
-         util.printout("\t\tUser    : ".. dir.normalize(conf.user.file) .. " (" .. get_status(conf.user.ok) ..")")
+         util.printout("\t\tUser    : ".. dir.normalize(conf.user.file) .. " (" .. get_status(conf.user.found) ..")")
       else
          util.printout("\t\tUser    : disabled in this LuaRocks installation.")
       end
       if conf.project then
-         util.printout("\t\tProject : ".. dir.normalize(conf.project.file) .. " (" .. get_status(conf.project.ok) ..")")
+         util.printout("\t\tProject : ".. dir.normalize(conf.project.file) .. " (" .. get_status(conf.project.found) ..")")
       end
       util.printout()
       util.printout("\tRocks trees in use: ")
diff --git a/src/luarocks/cmd/init.lua b/src/luarocks/cmd/init.lua
index b1bb3b51..3b8c9fea 100644
--- a/src/luarocks/cmd/init.lua
+++ b/src/luarocks/cmd/init.lua
@@ -92,14 +92,7 @@ function init.command(flags, name, version)
 
    if flags["reset"] then
       fs.delete(lua_wrapper)
-      for v in util.lua_versions() do
-         local config_file = dir.path(".luarocks", "config-"..v..".lua")
-         if v ~= cfg.lua_version then
-            fs.move(config_file, config_file .. "~")
-         else
-            fs.delete(config_file)
-         end
-      end
+      fs.delete(dir.path(".luarocks", "default_lua_version.lua"))
    end
 
    util.printout("Adding entries to .gitignore ...")
diff --git a/src/luarocks/core/cfg.lua b/src/luarocks/core/cfg.lua
index 0c2e3f67..af4327c6 100644
--- a/src/luarocks/core/cfg.lua
+++ b/src/luarocks/core/cfg.lua
@@ -728,33 +728,31 @@ function cfg.init(detected, warning)
    cfg.variables.LUA = cfg.variables.LUA or (cfg.variables.LUA_BINDIR and (cfg.variables.LUA_BINDIR .. "/" .. cfg.lua_interpreter):gsub("//", "/"))
    cfg.user_agent = "LuaRocks/"..cfg.program_version.." "..cfg.arch
 
+   cfg.config_files = {
+      project = detected.project_dir and {
+         file = project_config_file,
+         found = not not project_config_ok,
+      },
+      system = {
+         file = sys_config_file,
+         found = not not sys_config_ok,
+      },
+      user = {
+         file = home_config_file,
+         found = not not home_config_ok,
+      },
+      nearest = project_config_ok
+                and project_config_file
+                or (home_config_ok
+                    and home_config_file
+                    or sys_config_file),
+   }
+
    ----------------------------------------
    -- Attributes of cfg are set.
    -- Let's add some methods.
    ----------------------------------------
 
-   function cfg.which_config()
-      return {
-         project = detected.project_dir and {
-            file = project_config_file,
-            ok = project_config_ok,
-         },
-         system = {
-            file = sys_config_file,
-            ok = sys_config_ok,
-         },
-         user = {
-            file = home_config_file,
-            ok = home_config_ok,
-         },
-         nearest = project_config_ok
-                   and project_config_file
-                   or (home_config_ok
-                       and home_config_file
-                       or sys_config_file),
-      }
-   end
-
    do
       local function make_paths_from_tree(tree)
          local lua_path, lib_path, bin_path
diff --git a/src/luarocks/fs/lua.lua b/src/luarocks/fs/lua.lua
index 0273cd53..71529cc0 100644
--- a/src/luarocks/fs/lua.lua
+++ b/src/luarocks/fs/lua.lua
@@ -114,7 +114,7 @@ function fs_lua.is_tool_available(tool_cmd, tool_name, arg)
    if not fs.execute_quiet(tool_cmd, arg) then
       local msg = "'%s' program not found. Make sure %s is installed and is available in your PATH " ..
                   "(or you may want to edit the 'variables.%s' value in file '%s')"
-      return nil, msg:format(tool_cmd, tool_name, tool_name:upper(), cfg.which_config().nearest)
+      return nil, msg:format(tool_cmd, tool_name, tool_name:upper(), cfg.config_files.nearest)
    else
       return true
    end
diff --git a/src/luarocks/persist.lua b/src/luarocks/persist.lua
index 73f92bea..b8060ea0 100644
--- a/src/luarocks/persist.lua
+++ b/src/luarocks/persist.lua
@@ -20,8 +20,9 @@ local write_table
 -- @param level number: the indentation level
 -- @param sub_order table: optional prioritization table
 -- @see write_table
-local function write_value(out, v, level, sub_order)
+function persist.write_value(out, v, level, sub_order)
    if type(v) == "table" then
+      level = level or 0
       write_table(out, v, level + 1, sub_order)
    elseif type(v) == "string" then
       if v:match("[\r\n]") then
@@ -80,7 +81,7 @@ local function write_table_key_assignment(out, k, level)
       out:write(k)
    else
       out:write("[")
-      write_value(out, k, level)
+      persist.write_value(out, k, level)
       out:write("]")
    end
    
@@ -112,7 +113,7 @@ write_table = function(out, tbl, level, field_order)
          write_table_key_assignment(out, k, level)
       end
 
-      write_value(out, v, level, sub_order)
+      persist.write_value(out, v, level, sub_order)
       if type(v) == "number" then
          sep = ", "
          indent = false
@@ -139,13 +140,13 @@ local function write_table_as_assignments(out, tbl, field_order)
          return nil, "cannot store '"..tostring(k).."' as a plain key."
       end
       out:write(k.." = ")
-      write_value(out, v, 0, sub_order)
+      persist.write_value(out, v, 0, sub_order)
       out:write("\n")
    end
    return true
 end
 
---- Write a table as series of assignments to a writer object.
+--- Write a table using Lua table syntax to a writer object.
 -- @param out table or userdata: a writer object supporting :write() method.
 -- @param tbl table: the table to be written.
 local function write_table_as_table(out, tbl)
@@ -153,7 +154,7 @@ local function write_table_as_table(out, tbl)
    for k, v, sub_order in util.sortedpairs(tbl) do
       out:write("   ")
       write_table_key_assignment(out, k, 1)
-      write_value(out, v, 1, sub_order)
+      persist.write_value(out, v, 1, sub_order)
       out:write(",\n")
    end
    out:write("}\n")
@@ -216,4 +217,26 @@ function persist.save_as_module(filename, tbl)
    return true
 end
 
+function persist.load_config_file_if_basic(filename, cfg)
+   local env = {
+      home = cfg.home
+   }
+   local result, err, errcode = persist.load_into_table(filename, env)
+   if errcode == "load" or errcode == "run" then
+      -- bad config file or depends on env, so error out
+      return nil, "Could not read existing config file " .. filename
+   end
+
+   local tbl
+   if errcode == "open" then
+      -- could not open, maybe file does not exist
+      tbl = {}
+   else
+      tbl = result
+      tbl.home = nil
+   end
+
+   return tbl
+end
+
 return persist
diff --git a/src/luarocks/upload/api.lua b/src/luarocks/upload/api.lua
index bb2197ee..0e71432f 100644
--- a/src/luarocks/upload/api.lua
+++ b/src/luarocks/upload/api.lua
@@ -11,11 +11,10 @@ local multipart = require("luarocks.upload.multipart")
 local Api = {}
 
 local function upload_config_file()
-   local conf = cfg.which_config()
-   if not conf.user.file then
+   if not cfg.config_files.user.file then
       return nil
    end
-   return (conf.user.file:gsub("/[^/]+$", "/upload_config.lua"))
+   return (cfg.config_files.user.file:gsub("/[^/]+$", "/upload_config.lua"))
 end
 
 function Api:load_config()
diff --git a/src/luarocks/util.lua b/src/luarocks/util.lua
index aac52344..1714952e 100644
--- a/src/luarocks/util.lua
+++ b/src/luarocks/util.lua
@@ -106,6 +106,7 @@ local supported_flags = {
    ["homepage"] = "\"<url>\"",
    ["index"] = true,
    ["issues"] = true,
+   ["json"] = true,
    ["keep"] = true,
    ["labels"] = true,
    ["lib"] = "<library>",
@@ -149,6 +150,7 @@ local supported_flags = {
    ["rock-trees"] = true,
    ["rockspec"] = true,
    ["rockspec-format"] = "<ver>",
+   ["scope"] = "<system|user|project>",
    ["server"] = "<server>",
    ["sign"] = true,
    ["skip-pack"] = true,
@@ -161,6 +163,7 @@ local supported_flags = {
    ["timeout"] = "<seconds>",
    ["to"] = "<path>",
    ["tree"] = "<path>",
+   ["unset"] = true,
    ["user-config"] = true,
    ["verbose"] = true,
    ["verify"] = true,