Erlang -- System Principles

# 1 System Principles

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1.1 Starting the System

An Erlang runtime system is started with command erl:

    % erl
    Erlang/OTP 17 [erts-6.0] [hipe] [smp:8:8]

    Eshell V6.0  (abort with ^G)
    1>

erl understands a number of command-line arguments, see the erl(1) manual page in ERTS. Some of them are also described in this chapter.

Application programs can access the values of the command-line arguments by calling the function init:get_argument(Key) or init:get_arguments(). See the init(3) manual page in ERTS.

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1.2 Restarting and Stopping the System

The runtime system is halted by calling halt/0,1. For details, see the erlang(3) manual page in ERTS.

The module init contains functions for restarting, rebooting, and stopping the runtime system:

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

For details, see the init(3) manual page in ERTS.

The runtime system terminates if the Erlang shell is terminated.

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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 is to be omitted. For 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.

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,logger}
    ...

For a detailed description of the syntax and contents of the boot script, see the script(4) manual page in SASL.

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Default Boot Scripts

Erlang/OTP comes with these 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.
- no_dot_erlang.boot - Loads the code for and starts the applications Kernel and STDLIB. Skips loading the file .erlang. Useful for scripts and other tools that are to behave the same irrespective of user preferences.

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 directory ROOT/bin.

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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.

A boot script can be written manually. However, it is recommended to create a boot script by generating it 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).

For more information about .rel files, see OTP Design Principles and the rel(4) manual page in SASL.

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

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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 and extra -mode flags are ignored.

The mode properties are as follows:

- In embedded mode, all code is loaded during system startup according to the boot script. (Code can also be loaded later by explicitly ordering the code server to do so.)
- In interactive mode, the 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]. 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, this directory is added to the code path.

The code path can be extended by using the command-line flags -pa Directories and -pz Directories. These add Directories to the head or the 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 the code(3) manual page in Kernel.

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1.5 File Types

The following file types are defined in Erlang/OTP:

File Type | File Name/Extension | Documented in
Module | .erl | Erlang Reference Manual
Include file | .hrl | Erlang Reference Manual
Release resource file | .rel | rel(4) manual page in SASL
Application resource file | .app | app(4) manual page in Kernel
Boot script | .script | script(4) manual page in SASL
Binary boot script | .boot | -
Configuration file | .config | config(4) manual page in Kernel
Application upgrade file | .appup | appup(4) manual page in SASL
Release upgrade file | relup | relup(4) manual page in SASL

Table 1.1: File Types