init(3)

MODULE
init - Coordination of System Startup

DESCRIPTION

The init module is pre-loaded and contains the code for the init system process which coordinates the start-up of the system. The first function evaluated at start-up is boot(BootArgs), where BootArgs is a list of command line arguments supplied to the Erlang runtime system from the local operating system. See erl(1).

init reads the boot script which contains instructions on how to initiate the system. See script(4) for more information about boot scripts.

init also contains functions to restart, reboot, and stop the system.

EXPORTS

boot(BootArgs) -> void()

    Types
        BootArgs = [binary()]

    Starts the Erlang runtime system. This function is called when the emulator is started and coordinates system start-up.

    BootArgs are all command line arguments except the emulator flags, that is, flags and plain arguments. See erl(1).

    init itself interprets some of the flags, see Command Line Flags below. The remaining flags ("user flags") and plain arguments are passed to the init loop and can be retrieved by calling get_arguments/0 and get_plain_arguments/0, respectively.

get_args() -> [Arg]

    Types
        Arg = atom()

    Returns any plain command line arguments as a list of atoms (possibly empty). It is recommended that get_plain_arguments/1 is used instead, because of the limited length of atoms.

get_argument(Flag) -> {ok, Arg} | error

    Types
        Flag = atom()
        Arg = [Values]
        Values = [string()]

    Returns all values associated with the command line user flag Flag. If Flag is provided several times, each Values is returned in preserved order.
    %erl -a b c -a d
    ...
    1>init:get_argument(a).
    {ok,[["b","c"],["d"]]}

    There are also a number of flags, which are defined automatically and can be retrieved using this function:

        root:
            The installation directory of Erlang/OTP, $ROOT.

            2>init:get_argument(root).
            {ok,[["/usr/local/otp/releases/otp_beam_solaris8_r10b_patched"]]}

        progname:
            The name of the program which started Erlang.

            3>init:get_argument(progname).
            {ok,[["erl"]]}

        home:
            The home directory.

            4>init:get_argument(home).
            {ok,[["/home/harry"]]}

    Returns error if there is no value associated with Flag.

get_arguments() -> Flags

    Types
        Flags = [{Flag, Values}]
        Flag = atom()
        Values = [string()]

    Returns all command line flags, as well as the system defined flags, see get_argument/1.

get_plain_arguments() -> [Arg]

    Types
        Arg = string()

    Returns any plain command line arguments as a list of strings (possibly empty).

get_status() -> {InternalStatus, ProvidedStatus}

    Types
        InternalStatus = starting | started | stopping
        ProvidedStatus = term()

    The current status of the init process can be inspected. During system startup (initialization), InternalStatus is starting, and ProvidedStatus indicates how far the boot script has been interpreted. Each {progress, Info} term interpreted in the boot script affects ProvidedStatus, that is, ProvidedStatus gets the value of Info.

reboot() -> void()

    All applications are taken down smoothly, all code is unloaded, and all ports are closed before the system terminates. If the -heart command line flag was given, the heart program will try to reboot the system. Refer to heart(3) for more information.

    To limit the shutdown time, the time init is allowed to spend taking down applications, the -shutdown_time command line flag should be used.

restart() -> void()

    The system is restarted inside the running Erlang node, which means that the emulator is not restarted. All applications are taken down smoothly, all code is unloaded, and all ports are closed before the system is booted again in the same way as initially started. The same BootArgs are used again.

    To limit the shutdown time, the time init is allowed to spend taking down applications, the -shutdown_time command line flag should be used.

script_id() -> Id

    Types
        Id = term()

    Get the identity of the boot script used to boot the system. Id can be any Erlang term. In the delivered boot scripts, Id is {Name, Vsn}. Name and Vsn are strings.

stop() -> void()

    All applications are taken down smoothly, all code is unloaded, and all ports are closed before the system terminates. If the -heart command line flag was given, the heart program is terminated before the Erlang node terminates. Refer to heart(3) for more information.

    To limit the shutdown time, the time init is allowed to spend taking down applications, the -shutdown_time command line flag should be used.

Command Line Flags

The init module interprets the following command line flags:

    --: Everything following -- up to the next flag is considered plain arguments and can be retrieved using get_plain_arguments/0.

    -eval Expr:
        Scans, parses and evaluates an arbitrary expression Expr during system initialization. If any of these steps fail (syntax error, parse error or exception during evaluation), Erlang stops with an error message. Here is an example that seeds the random number generator:

        %erl -eval '{X,Y,Z}' = now(), random:seed(X,Y,Z).'

        This example uses Erlang as a hexadecimal calculator:

        %erl -noshell -eval 'R = 16#1F+16#A0, io:format("~.16B~n", [R])' \
        -s erlang halt
        BF

        If multiple -eval expressions are specified, they are evaluated sequentially in the order specified. -eval expressions are evaluated sequentially with -s and -run function calls (this also in the order specified). As with -s and -run, an evaluation that does not terminate, blocks the system initialization process.

    -extra:
        Everything following -extra is considered plain arguments and can be retrieved using get_plain_arguments/0.

    -run Mod [Func [Arg1, Arg2, ...]]:
        Evaluates the specified function call during system initialization. Func defaults to start. If no arguments are provided, the function is assumed to be of arity 0. Otherwise it is assumed to be of arity 1, taking the list [Arg1, Arg2, ...] as argument. All arguments are passed as strings. If an exception is raised, Erlang stops with an error message.

        Example:

        %erl -run foo -run foo bar -run foo bar baz 1 2

        This starts the Erlang runtime system and evaluates the following functions:

        foo:start()
        foo:bar()
        foo:bar(["baz", "1", "2"]).

        The functions are executed sequentially in an initialization process, which then terminates normally and passes control to the user. This means that a -run call which does not return will block further processing; to avoid this, use some variant of spawn in such cases.

    -s Mod [Func [Arg1, Arg2, ...]]:
        Evaluates the specified function call during system initialization. Func defaults to start. If no arguments are provided, the function is assumed to be of arity 0. Otherwise it is assumed to be of arity 1, taking the list [Arg1, Arg2, ...] as argument. All arguments are passed as atoms. If an exception is raised, Erlang stops with an error message.

        Example:

        %erl -s foo -s foo bar -s foo bar baz 1 2

        This starts the Erlang runtime system and evaluates the following functions:

        foo:start()
        foo:bar()
        foo:bar([baz, '1', '2']).

        The functions are executed sequentially in an initialization process, which then terminates normally and passes control to the user. This means that a -s call which does not return will block further processing; to avoid this, use some variant of spawn in such cases.

        Due to the limited length of atoms, it is recommended that -run be used instead.

Example
%erl -- a b -children thomas claire -ages 7 3 -- x y
...
1>init:get_plain_arguments().
["a", "b", "x", "y"]
2>init:get_argument(children).
{ok, [["thomas", "claire"]]}
3>init:get_argument(ages).
{ok, [["7", "3"]]}
4>init:get_argument(silly).
error

SEE ALSO

erl_prim_loader(3), heart(3)

AUTHORS
Claes Wikström - support@erlang.ericsson.se
Magnus Fröberg - support@erlang.ericsson.se