W3cubDocs

/Erlang 20

init

Module

init

Module Summary

Coordination of system startup.

Description

This module is preloaded and contains the code for the init system process that coordinates the startup of the system. The first function evaluated at startup 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. For more information about boot scripts, see script(4).

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

Exports

boot(BootArgs) -> no_return()

Types

Starts the Erlang runtime system. This function is called when the emulator is started and coordinates system startup.

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

init interprets some of the flags, see section 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_argument(Flag) -> {ok, Arg} | error

Types

Returns all values associated with the command-line user flag Flag. If Flag is provided several times, each Values is returned in preserved order. Example:

% erl -a b c -a d
...
1> init:get_argument(a).
{ok,[["b","c"],["d"]]}

The following flags 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 no value is associated with Flag.

get_arguments() -> Flags

Types

Returns all command-line flags and the system-defined flags, see get_argument/1.

get_plain_arguments() -> [Arg]

Types

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

get_status() -> {InternalStatus, ProvidedStatus}

Types

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() -> ok

All applications are taken down smoothly, all code is unloaded, and all ports are closed before the system terminates. If command-line flag -heart was specified, the heart program tries to reboot the system. For more information, see heart(3).

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

restart() -> ok

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, command-line flag -shutdown_time is to be used.

script_id() -> Id

Types

Gets 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() -> ok

The same as stop(0).

stop(Status) -> ok

Types

All applications are taken down smoothly, all code is unloaded, and all ports are closed before the system terminates by calling halt(Status). If command-line flag -heart was specified, the heart program is terminated before the Erlang node terminates. For more information, see heart(3).

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

Command-Line Flags

Warning

The support for loading of code from archive files is experimental. The only purpose of releasing it before it is ready is to obtain early feedback. The file format, semantics, interfaces, and so on, can be changed in a future release. The -code_path_choice flag is also experimental.

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.

-code_path_choice Choice

Can be set to strict or relaxed. It controls how each directory in the code path is to be interpreted:

  • Strictly as it appears in the boot script, or

  • init is to be more relaxed and try to find a suitable directory if it can choose from a regular ebin directory and an ebin directory in an archive file.

This flag is particular useful when you want to elaborate with code loading from archives without editing the boot script. For more information about interpretation of boot scripts, see script(4). The flag has also a similar effect on how the code server works; see code(3).

-epmd_module Module

Specifies the module to use for registration and lookup of node names. Defaults to erl_epmd.

-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. In the following example Erlang is used 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 that does not return blocks 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 that does not return blocks further processing; to avoid this, use some variant of spawn in such cases.

Because of the limited length of atoms, it is recommended to use -run 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)

© 2010–2017 Ericsson AB
Licensed under the Apache License, Version 2.0.