# init

### MODULE

init

### MODULE SUMMARY

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

kernel 2.11
Copyright © 1991-2006 Ericsson AB