7 – Lua Standalone

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

lua [options] [script [args]]

The options are:

• -e stat: executes string stat;
• -l mod: "requires" mod;
• -i: enters interactive mode after running script;
• -v: prints version information;
• -E: ignores environment variables;
• --: stops handling options;
• -: executes stdin as a file and stops handling options.

After handling its options, lua runs the given script. When called without arguments, lua behaves as lua -v -i when the standard input (stdin) is a terminal, and as lua - otherwise.

When called without option -E, the interpreter checks for an environment variable LUA_INIT_5_3 (or LUA_INIT if the versioned name is not defined) before running any argument. If the variable content has the format @filename, then lua executes the file. Otherwise, lua executes the string itself.

When called with option -E, besides ignoring LUA_INIT, Lua also ignores the values of LUA_PATH and LUA_CPATH, setting the values of package.path and package.cpath with the default paths defined in luaconf.h.

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

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

will first set a to 1, then print the value of a, and finally run the file script.lua with no arguments. (Here $ is the shell prompt. Your prompt may be different.)

Before running any code, lua collects all command-line arguments in a global table called arg. The script name goes to index 0, the first argument after the script name goes to index 1, and so on. Any arguments before the script name (that is, the interpreter name plus its options) go to negative indices. For instance, in the call

$ lua -la b.lua t1 t2

the table is like this:

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

If there is no script in the call, the interpreter name goes to index 0, followed by the other arguments. For instance, the call

$ lua -e "print(arg[1])"

will print "-e". If there is a script, the script is called with parameters arg[1], ···, arg[#arg]. (Like all chunks in Lua, the script is compiled as a vararg function.)

In interactive mode, Lua repeatedly prompts and waits for a line. After reading a line, Lua first try to interpret the line as an expression. If it succeeds, it prints its value. Otherwise, it interprets the line as a statement. If you write an incomplete statement, the interpreter waits for its completion by issuing a different prompt.

If the global variable _PROMPT contains a string, then its value is used as the prompt. Similarly, if the global variable _PROMPT2 contains a string, its value is used as the secondary prompt (issued during incomplete statements).

In case of unprotected errors in the script, the interpreter reports the error to the standard error stream. If the error object is not a string but has a metamethod __tostring, the interpreter calls this metamethod to produce the final message. Otherwise, the interpreter converts the error object to a string and adds a stack traceback to it.

When finishing normally, the interpreter closes its main Lua state (see lua_close). The script can avoid this step by calling os.exit to terminate.

To allow the use of Lua as a script interpreter in Unix systems, the standalone interpreter skips the first line of a chunk if it starts with #. Therefore, Lua scripts can be made into executable programs by using chmod +x and the #! form, as in

#!/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

#!/usr/bin/env lua

is a more portable solution.)