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NAME
escript - Run a script written in Erlang
SYNOPSIS
script-name [arg1 arg2...]
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. escripts are created by either writing them by hand or
using escript:create/2.
escripts are run by directly invoking them (does not work on Windows):
script-name [arg1 arg2...]
or by calling the escript program (works on all platforms):
escript [escript-flags] script-name.escript [arg1 arg2...]
For example:
$ chmod u+x factorial
$ cat factorial
#!/usr/bin/env escript
%% -*- erlang -*-
%%! -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 as follows, the contents of the first line do not matter, but it
cannot contain Erlang code as it will be ignored:
$ escript factorial 5
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.
If a comment selecting the encoding exists, it can be located on the second line.
NOTE: The encoding specified by the above mentioned comment applies to the script itself. The encoding of
the I/O-server, however, must be set explicitly as follows:
io:setopts([{encoding, latin1}])
The default encoding of the I/O-server for standard_io is unicode if its supported. (see section Summary
of Options) in the STDLIB User's Guide.
On the third line (or second line depending on the presence of the Emacs directive), arguments can be
specified to the emulator, for example:
%%! -sname factorial -mnesia debug verbose
Such an argument line must start with %%! and the remaining line is 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 example:
#!/usr/local/bin/escript
As any other type of scripts, Erlang scripts do not work on Unix platforms if the execution bit for the
script file is not set. (To turn on the execution bit, use chmod +x script-name.)
The remaining Erlang script file can either contain Erlang source code, an inlined beam file, or an
inlined archive file.
An Erlang script file must always contain the main/1 function. When the script is run, the main/1
function is called with a list of strings representing the arguments specified 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 is 0. If an
exception is generated during execution, a short message is printed and the script terminates with exit
status 127.
To return your own non-zero exit code, call halt(ExitCode), for example:
halt(1).
To retrieve the pathname of the script, call escript:script_name/0 from your script (the pathname is
usually, but not always, absolute).
If the file contains source code (as in the example above), it is processed by the epp preprocessor. This
means that you, for example, can use predefined macros (such as ?MODULE) and include directives like the
-include_lib directive. For example, use
-include_lib("kernel/include/file.hrl").
to include the record definitions for the records used by function file:read_link_info/1. You can also
select encoding by including an encoding comment here, but if a valid encoding comment exists on the
second line, it takes precedence.
The script is checked for syntactic and semantic correctness before it is run. If there are warnings
(such as unused variables), they are printed and the script will still be run. If there are errors, they
are printed and the script will not be run and its exit status is 127.
Both the module declaration and the export declaration of the main/1 function are optional.
By default, the script will be compiled by the Erlang compiler.
It is possible to force it to be interpreted by including the following line somewhere in the script
file:
-mode(interpret).
Execution of interpreted code is slower than compiled code, and some language constructs will not work,
but there is no requirement for the Erlang compiler application to be available.
CHANGE: Before Erlang/OTP 27 the script would be interpreted by default.
Precompiled escripts
A script can also contains precompiled beam code. To create a precompiled escript it is recommended that
you use escript:create/2. In a precompiled script, the interpretation of the script header is the same as
in a script containing source code. This 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 main/1
function must be exported.
Another option is to have an entire Erlang archive in the script. In an archive script, the
interpretation of the script header is the same as in a script containing source code. This 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 main/1 function must be exported. By default the main/1
function in the module with the same name as the basename of the escript file is invoked. This behavior
can be overridden by setting flag -escript main Module as one of the emulator flags. Module must be the
name of a module that has an exported main/1 function. For more information about archives and code
loading, see escript and code.
It is often very convenient to have a header in the escript, especially on Unix platforms. However, the
header is optional, so 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 as follows:
$ escript factorial.erl 5
factorial 5 = 120
$ escript factorial.beam 5
factorial 5 = 120
$ escript factorial.zip 5
factorial 5 = 120
Bundling escripts
It is possible to bundle escript(s) with an Erlang runtime system to make it self-sufficient and
relocatable. In such a standalone system, the escript(s) should be located in the top bin directory of
the standalone system and given .escript as file extension. Further the (built-in) escript program should
be copied to the same directory and given the script's original name (without the .escript extension).
This will enable use of the bundled Erlang runtime system.
The (built-in) escript program first determines which Erlang runtime system to use and then starts it to
execute your script. Usually the runtime system is located in the same Erlang installation as the escript
program itself. But for standalone systems with one or more escripts it may be the case that the escript
program in your path actually starts the runtime system bundled with the escript. This is intentional,
and typically happens when the standalone system bin directory is not in the execution path (as it may
cause its erl program to override the desired one) and the escript(s) are referred to via symbolic links
from a bin directory in the path.
Options Accepted By escript
-c
Compiles the escript regardless of the value of the mode attribute.
-d
Debugs 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 option debug_info.
-i
Interprets the escript regardless of the value of the mode attribute.
-s
Performs 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 is 0 if any errors are found,
otherwise 127.
NOTE: The configuration of the Erlang emulator invoked by escript can also be controlled using the
environment variables understood by erl.