escript

escript provides support for running short Erlang programs without having to compile them first and an easy way to retrieve the command line arguments.

escript runs a script written in Erlang.

Here follows an example.

$ cat factorial        
#!/usr/bin/env escript
%% -*- erlang -*-
%%! -smp enable -sname factorial -mnesia debug verbose
main([String]) ->
    try
        N = list_to_integer(String),
        F = fac(N),
        io:format("factorial ~w = ~w\n", [N,F])
    catch
        _:_ ->
            usage()
    end;
main(_) ->
    usage().
        
usage() ->
    io:format("usage: factorial integer\n"),
    halt(1).
        
fac(0) -> 1;
fac(N) -> N * fac(N-1).
$ factorial 5
factorial 5 = 120
$ factorial
usage: factorial integer
$ factorial five
usage: factorial integer        

The header of the Erlang script in the example differs from a normal Erlang module. The first line is intended to be the interpreter line, which invokes escript. However if you invoke the escript like this

$ escript factorial 5        

the contents of the first line does not matter, but it cannot contain Erlang code as it will be ignored.

The second line in the example, contains an optional directive to the Emacs editor which causes it to enter the major mode for editing Erlang source files. If the directive is present it must be located on the second line.

On the third line (or second line depending on the presence of the Emacs directive), it is possible to give arguments to the emulator, such as

%%! -smp enable -sname factorial -mnesia debug verbose

Such an argument line must start with %%! and the rest of the line will interpreted as arguments to the emulator.

If you know the location of the escript executable, the first line can directly give the path to escript. For instance:

#!/usr/local/bin/escript        

As any other kind of scripts, Erlang scripts will not work on Unix platforms if the execution bit for the script file is not set. (Use chmod +x script-name to turn on the execution bit.)

The rest of the Erlang script file may either contain Erlang source code, an inlined beam file or an inlined archive file.

An Erlang script file must always contain the function main/1. When the script is run, the main/1 function will be called with a list of strings representing the arguments given to the script (not changed or interpreted in any way).

If the main/1 function in the script returns successfully, the exit status for the script will be 0. If an exception is generated during execution, a short message will be printed and the script terminated with exit status 127.

To return your own non-zero exit code, call halt(ExitCode); for instance:

halt(1).

Call escript:script_name/0 from your to script to retrieve the pathname of the script (the pathname is usually, but not always, absolute).

If the file contains source code (as in the example above), it will be processed by the preprocessor epp. This means that you for example may use pre-defined macros (such as ?MODULE) as well as include directives like the -include_lib directive. For instance, use

-include_lib("kernel/include/file.hrl").        

to include the record definitions for the records used by the file:read_link_info/1 function.

The script will be checked for syntactic and semantic correctness before being run. If there are warnings (such as unused variables), they will be printed and the script will still be run. If there are errors, they will be printed and the script will not be run and its exit status will be 127.

Both the module declaration and the export declaration of the main/1 function are optional.

By default, the script will be interpreted. You can force it to be compiled by including the following line somewhere in the script file:

-mode(compile).

Execution of interpreted code is slower than compiled code. If much of the execution takes place in interpreted code it may be worthwhile to compile it, even though the compilation itself will take a little while.

As mentioned earlier, it is possible to have a script which contains precompiled beam code. In a precompiled script, the interpretation of the script header is exactly the same as in a script containing source code. That means that you can make a beam file executable by prepending the file with the lines starting with #! and %%! mentioned above. In a precompiled script, the function main/1 must be exported.

As yet another option it is possible to have an entire Erlang archive in the script. In a archive script, the interpretation of the script header is exactly the same as in a script containing source code. That means that you can make an archive file executable by prepending the file with the lines starting with #! and %%! mentioned above. In an archive script, the function main/1 must be exported. By default the main/1 function in the module with the same name as the basename of the escript file will be invoked. This behavior can be overridden by setting the flag -escript main Module as one of the emulator flags. The Module must be the name of a module which has an exported main/1 function. See code(3) for more information about archives and code loading.

In many cases it is very convenient to have a header in the escript, especially on Unix platforms. But the header is in fact optional. This means that you directly can "execute" an Erlang module, beam file or archive file without adding any header to them. But then you have to invoke the script like this:

$ escript factorial.erl 5
factorial 5 = 120
$ escript factorial.beam 5
factorial 5 = 120
$ escript factorial.zip 5
factorial 5 = 120

-c

Compile the escript regardless of the value of the mode attribute.

-d

Debug the escript. Starts the debugger, loads the module containing the main/1 function into the debugger, sets a breakpoint in main/1 and invokes main/1. If the module is precompiled, it must be explicitly compiled with the debug_info option.

-i

Interpret the escript regardless of the value of the mode attribute.

-s

Only perform a syntactic and semantic check of the script file. Warnings and errors (if any) are written to the standard output, but the script will not be run. The exit status will be 0 if there were no errors, and 127 otherwise.