6 – Lua Stand-alone

Although Lua has been designed as an extension language, to be embedded in a host C program, it is also frequently used as a stand-alone language. An interpreter for Lua as a stand-alone language, called simply lua, is provided with the standard distribution. The stand-alone interpreter includes all standard libraries plus the reflexive debug interface. Its usage is:

  1.       lua [options] [script [args]]

The options are:

  • - executes stdin as a file;
  • -e stat executes string stat;
  • -l file “requires” file;
  • -i enters interactive mode after running script;
  • -v prints version information;
  • -- stop handling options.

After handling its options, lua runs the given script, passing to it the given args. When called without arguments, lua behaves as lua -v -i when stdin is a terminal, and as lua - otherwise.

Before running any argument, the interpreter checks for an environment variable LUA_INIT. If its format is @filename, then lua executes the file. Otherwise, lua executes the string itself.

All options are handled in order, except -i. For instance, an invocation like

  1.        $ lua -e'a=1' -e 'print(a)' script.lua

will first set a to 1, then print a, and finally run the file script.lua. (Here, $ is the shell prompt. Your prompt may be different.)

Before starting to run the script, lua collects all arguments in the command line in a global table called arg. The script name is stored in index 0, the first argument after the script name goes to index 1, and so on. The field n gets the number of arguments after the script name. Any arguments before the script name (that is, the interpreter name plus the options) go to negative indices. For instance, in the call

  1.        $ lua -la.lua b.lua t1 t2

the interpreter first runs the file a.lua, then creates a table

  1.        arg = { [-2] = "lua", [-1] = "-la.lua", [0] = "b.lua",
  2.                [1] = "t1", [2] = "t2"; n = 2 }

and finally runs the file b.lua.

In interactive mode, if you write an incomplete statement, the interpreter waits for its completion.

If the global variable _PROMPT is defined as a string, then its value is used as the prompt. Therefore, the prompt can be changed directly on the command line:

  1.        $ lua -e"_PROMPT='myprompt> '" -i

(the outer pair of quotes is for the shell, the inner is for Lua), or in any Lua programs by assigning to _PROMPT. Note the use of -i to enter interactive mode; otherwise, the program would end just after the assignment to _PROMPT.

In Unix systems, Lua scripts can be made into executable programs by using chmod +x and the #! form, as in

  1. #!/usr/local/bin/lua

(Of course, the location of the Lua interpreter may be different in your machine. If lua is in your PATH, then

  1. #!/usr/bin/env lua

is a more portable solution.)