System Principles

## 1 System Principles

### 1.1 Starting the System

An Erlang runtime system is started with the command `erl`:

    % erl
    Erlang (BEAM) emulator version 5.2.3.5 [hipe] [threads:0]

    Eshell V5.2.3.5  (abort with ^G)
    1>

`erl` understands a number of command line arguments, see `erl(1)`. A number of them are also described in this chapter.

Application programs can access the values of the command line arguments by calling one of the functions `init:get_argument(Key)`, or `init:get_arguments()`. See `init(3)`.

### 1.2 Restarting and Stopping the System

The runtime system can be halted by calling `halt/0,1`. See `erlang(3)`.

The module `init` contains function for restarting, rebooting and stopping the runtime system. See `init(3)`.

    init:restart()
    init:reboot()
    init:stop()

Also, the runtime system will terminate if the Erlang shell is terminated.

### 1.3 Boot Scripts

The runtime system is started using a boot script. The boot script contains instructions on which code to load and which processes and applications to start.

A boot script file has the extension `.script`. The runtime system uses a binary version of the script. This binary boot script file has the extension `.boot`.

Which boot script to use is specified by the command line flag `-boot`. The extension `.boot` should be omitted. Example, using the boot script `start_all.boot`:

    % erl -boot start_all

If no boot script is specified, it defaults to `ROOT/bin/start`, see Default Boot Scripts below.

The command line flag `-init_debug` makes the `init` process write some debug information while interpreting the boot script:

    % erl -init_debug
    {progress,preloaded}
    {progress,kernel_load_completed}
    {progress,modules_loaded}
    {start,heart}
    {start,error_logger}
    ...

See `script(4)` for a detailed description of the syntax and contents of the boot script.

#### 1.3.1 Default Boot Scripts

Erlang/OTP comes with two boot scripts:

`start_clean.boot`
  Loads the code for and starts the applications Kernel and STDLIB.
`start_sasl.boot`
  Loads the code for and starts the applications Kernel, STDLIB and SASL.

Which of `start_clean` and `start_sasl` to use as default is decided by the user when installing Erlang/OTP using `Install`. The user is asked "Do you want to use a minimal system startup instead of the SASL startup". If the answer is yes, then `start_clean` is used, otherwise `start_sasl` is used. A copy of the selected boot script is made, named `start.boot` and placed in the `ROOT/bin` directory.

#### 1.3.2 User-Defined Boot Scripts

It is sometimes useful or necessary to create a user-defined boot script. This is true especially when running Erlang in embedded mode, see Code Loading Strategy.

It is possible to write a boot script manually. The recommended way to create a boot script, however, is to generate the boot script from a release resource file `Name.rel`, using the function `systools:make_script/1,2`. This requires that the source code is structured as applications according to the OTP design principles. (The program does not have to be started in terms of OTP applications but can be plain Erlang).

Read more about `.rel` files in OTP Design Principles and `rel(4)`.

The binary boot script file `Name.boot` is generated from the boot script file `Name.script` using the function `systools:script2boot(File)`.

### 1.4 Code Loading Strategy

The runtime system can be started in either embedded or interactive mode. Which one is decided by the command line flag `-mode`.

    % erl -mode embedded

Default mode is `interactive`.

- In embedded mode, all code is loaded during system start-up according to the boot script. (Code can also be loaded later by explicitly ordering the code server to do so).
- In interactive mode, code is dynamically loaded when first referenced. When a call to a function in a module is made, and the module is not loaded, the code server searches the code path and loads the module into the system.

Initially, the code path consists of the current working directory and all object code directories under `ROOT/lib`, where `ROOT` is the installation directory of Erlang/OTP. Directories can be named `Name[-Vsn]` and the code server, by default, chooses the directory with the highest version number among those which have the same `Name`. The `-Vsn` suffix is optional. If an `ebin` directory exists under the `Name[-Vsn]` directory, it is this directory which is added to the code path.

The code path can be extended by using the command line flags `-pa Directories` and `-pz Directories`. These will add `Directories` to the head or end of the code path, respectively. Example

    % erl -pa /home/arne/mycode

The code server module `code` contains a number of functions for modifying and checking the search path, see `code(3)`.

### 1.5 File Types

The following file types are defined in Erlang/OTP:

File Types
File Type | File Name/Extension | Documented in
module | `.erl` | Erlang Reference Manual
include file | `.hrl` | Erlang Reference Manual
release resource file | `.rel` | `rel(4)`
application resource file | `.app` | `app(4)`
boot script | `.script` | `script(4)`
binary boot script | `.boot` | -
configuration file | `.config` | `config(4)`
application upgrade file | `.appup` | `appup(4)`
release upgrade file | `relup` | `relup(4)`

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