Improve rename.lua's argument validation

src/main/resources/assets/computercraft/lua/rom/programs/rename.lua

@@ -7,8 +7,15 @@ end
 local sSource = shell.resolve( tArgs[1] )
 local sDest = shell.resolve( tArgs[2] )
 
-if fs.exists( sDest ) then
+if not fs.exists( sSource ) then
+    printError( "No matching files" )
+    return
+elseif fs.exists( sDest ) then
     printError( "Destination exists" )
+    return
+elseif fs.isReadOnly( sDest ) then
+    printError( "Destination is read-only" )
+    return
 end
 
 fs.move( sSource, sDest )

src/test/resources/test-rom/spec/programs/rename_spec.lua

@@ -0,0 +1,40 @@
+local capture = require "test_helpers".capture_program
+
+describe("The rename program", function()
+    local function touch(file)
+        io.open(file, "w"):close()
+    end
+
+    it("can rename a file", function()
+        touch("/test-files/rename/a.txt")
+
+        shell.run("rename /test-files/rename/a.txt /test-files/rename/b.txt")
+
+        expect(fs.exists("/test-files/rename/a.txt")):eq(false)
+        expect(fs.exists("/test-files/rename/b.txt")):eq(true)
+    end)
+
+    it("fails when renaming a file which doesn't exist", function()
+        expect(capture(stub, "rename nothing destination"))
+            :matches { ok = true, output = "", error = "No matching files\n" }
+    end)
+
+    it("fails when overwriting an existing file", function()
+        touch("/test-files/rename/c.txt")
+
+        expect(capture(stub, "rename /test-files/rename/c.txt /test-files/rename/c.txt"))
+            :matches { ok = true, output = "", error = "Destination exists\n" }
+    end)
+
+    it("fails when copying to read-only locations", function()
+        touch("/test-files/rename/d.txt")
+
+        expect(capture(stub, "rename /test-files/rename/d.txt /rom/test.txt"))
+            :matches { ok = true, output = "", error = "Destination is read-only\n" }
+    end)
+
+    it("displays the usage when given no arguments", function()
+        expect(capture(stub, "rename"))
+            :matches { ok = true, output = "Usage: rename <source> <destination>\n", error = "" }
+    end)
+end)