NAME
escript - Erlang scripting support
DESCRIPTION
escript provides support for running short Erlang programs without
having to compile them first and an easy way to retrieve the command
line arguments.
EXPORTS
script-name script-arg1 script-arg2...
escript escript-flags script-name script-arg1 script-arg2...
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
11N = list_to_integer(String),
11F = fac(N),
11io:format("factorial ~w = ~w", [N,F])
catch
11_:_ ->
11 usage()
end;
main(_) ->
usage().
usage() ->
io:format("usage: factorial integer"),
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(3erl) 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
OPTIONS ACCEPTED BY ESCRIPT
-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.