Coordination of system startup.

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.

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.

• -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.

  It defaults to strict from OTP 27 and this option is scheduled for removal in OTP 28.

• -epmd_module Module - This flag is deprecated and has been replaced by the kernel application parameter epmd_module.

• -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.

  Example:

  % erl -extra +A 1 --
  ...
  1> init:get_plain_arguments().
  ["+A","1","--"]

  The -extra flag can be passed on the command line, through ERL_*FLAGS or -args_file. It only effects the remaining command-line flags in the entity in which it is passed. If multiple -extra flags are passed they are concatenated using the same order rules as ERL_*FLAGS or -args_file in which they are given.

  Example:

  % ERL_AFLAGS="-extra a" ERL_ZFLAGS="-extra d" erl -extra b -extra c
  ...
  1> init:get_plain_arguments().
  ["a","b","-extra","c","d"]

• -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 strings. If an exception is raised, Erlang stops with an error message.

  Example:

  % erl -S httpd serve --port 8080 /var/www/html

  This starts the Erlang runtime system and evaluates the function httpd:serve(["--port", "8080", "/var/www/html"]). All arguments up to the end of the command line will be passed to the called function.

  The function is 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.

  The -S flag is only allowed on the command line. If passed through ERL_*FLAGS or -args_file it will be parsed as a normal command line flag.

• -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.

  Note

  This flag will not forward arguments beginning with a hyphen (-) to the specified function, as these will be interpreted as flags to the runtime. If the function uses flags in this form, it is advised to use -S instead.

• -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.

  Note

  This flag will not forward arguments beginning with a hyphen (-) to the specified function, as these will be interpreted as flags to the runtime. If the function uses flags in this form, it is advised to use -S instead, with the additional caveat that arguments are passed as strings instead of atoms.

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, heart