Programming in Lua : 1.4

This first edition was written for Lua 5.0. While still largely relevant for later versions, there are some differences.
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Programming in Lua

Part I. The Language Chapter 1. Getting Started

1.4 – The Stand-Alone Interpreter

The stand-alone interpreter (also called lua.c due to its source file, or simply lua due to its executable) is a small program that allows the direct use of Lua. This section presents its main options.

When the interpreter loads a file, it ignores its first line if that line starts with a number sign (`#´). That feature allows the use of Lua as a script interpreter in Unix systems. If you start your program with something like

    #!/usr/local/bin/lua

(assuming that the stand-alone interpreter is located at /usr/local/bin), or

    #!/usr/bin/env lua

then you can call the program directly, without explicitly calling the Lua interpreter.

The usage of lua is

    lua [options] [script [args]]

Everything is optional. As we have seen already, when we call lua without arguments the interpreter enters in interactive mode.

The -e option allows us to enter code directly into the command line. For instance,

    prompt> lua -e "print(math.sin(12))"   --> -0.53657291800043

(Unix needs the double quotes to stop the shell from interpreting the parentheses.) As we previously saw, -l loads a file and -i enters interactive mode after running the other arguments. So, for instance, the call

    prompt> lua -i -l a.lua -e "x = 10"

will load the file a.lua, then execute the assignment x = 10, and finally present a prompt for interaction.

Whenever the global variable _PROMPT is defined, lua uses its value as the prompt when interacting. So, you can change the prompt with a call like this:

    prompt> lua -i -e "_PROMPT=' lua> '"
     lua>

We are assuming that "prompt" is the system's prompt. In the example, the outer quotes stop the shell from interpreting the inner quotes, which are interpreted by Lua. More exactly, Lua receives the following command to run:

    _PROMPT=' lua> '

which assigns the string " lua> " to the global variable _PROMPT.

Before it starts running arguments, lua looks for an environment variable called LUA_INIT. If there is such a variable and its content is @filename, then lua loads the given file. If LUA_INIT is defined but does not start with `@´, then lua assumes that it contains Lua code and runs it. This variable gives you great power when configuring the stand-alone interpreter, because you have the full power of Lua in the configuration. You can pre-load packages, change the prompt and the path, define your own functions, rename or delete functions, and so on.

A main script can retrieve its arguments in the global variable arg. In a call like

    prompt> lua script a b c

lua creates the table arg with all the command-line arguments, before running the script. The script name goes into index 0; its first argument (a in the example), goes to index 1, and so on. Eventual options go to negative indices, as they appear before the script. For instance, in the call

    prompt> lua -e "sin=math.sin" script a b

lua collects the arguments as follows:

    arg[-3] = "lua"
    arg[-2] = "-e"
    arg[-1] = "sin=math.sin"
    arg[0] = "script"
    arg[1] = "a"
    arg[2] = "b"

More often than not, the script only uses the positive indices (arg[1] and arg[2], in the example).