init(3erl)
NAME
init - 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(5). init also contains functions to restart, reboot, and stop the system.
EXPORTS
boot(BootArgs) -> no_return()
Types:
BootArgs = [binary()]
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:
Flag = atom()
Arg = [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. 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:
Flags = [{Flag :: atom(), Values :: [string()]}]
Returns all command-line flags and 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 = internal_status()
ProvidedStatus = term()
internal_status() = starting | started | stopping
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(3erl).
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:
Id = term()
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:
Status = integer() >= 0 | string()
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(3erl).
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(5).
The flag has also a similar effect on how the code server works;
see code(3erl).
-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(3erl), heart(3erl)
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