System Principles

Starting the System

An Erlang runtime system is started with command erl:

% erl Erlang/OTP 27 [erts-15.0] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:1] [jit] Eshell V15.0 (press Ctrl+G to abort, type help(). for help) 1>

erl understands a number of command-line arguments; see erl in the ERTS application. Some arguments are also described in this chapter.

Application programs can access the values of the command-line arguments by calling one of the following functions:

• init:get_argument(Key)
• init:get_arguments()
• init:get_plain_arguments()

Restarting and Stopping the System

The runtime system is halted by calling halt/0,1,2.

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

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

The runtime system terminates if the Erlang shell is terminated.

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, where ROOT is the installation directory of Erlang/OTP. See Default Boot Scripts.

When the command-line flag -init_debug is used, the init process will output 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 script in the SASL application.

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, start_clean is used, otherwise start_sasl is used. The chosen boot script is copied and renamed as start.boot, then placed into directory ROOT/bin.

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.

While it is possible to manually create a boot script, it is preferable to generate it from a release resource file called 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.

For more information about .rel files, see Release Handling and the rel page in SASL.

To generate the binary boot script file Name.boot the boot script file Name.script, use the systools:script2boot(File) function.

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

The default mode is interactive. If more than one -mode flag is given, the first one will be used.

The mode properties are as follows:

• In embedded mode, all code is loaded during system startup according to the boot script. (Code can be loaded later by explicitly ordering the code server to load it.)

• In interactive mode, code is dynamically loaded when first required, which means that when an attempt is made to call a function in a module that 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], where the -Vsn suffix is optional. By default, the code server chooses the directory with the highest version number among those which have the same Name. 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 module contains a number of functions for modifying and querying the search path.

File Types

The following file types are defined in Erlang/OTP:

Table: File Types