Boot script


The boot script describes how the Erlang runtime system is started. It contains instructions on which code to load and which processes and applications to start.

The command erl -boot Name starts the system with a boot file called Name.boot, which is generated from the Name.script file, using systools:script2boot/1.

The .script file is generated by systools from a .rel file and .app files.


The boot script is stored in a file with the extension .script

The file has the following syntax:

{script, {Name, Vsn},
  {progress, loading},
  {preLoaded, [Mod1, Mod2, ...]},
  {path, [Dir1,"$ROOT/Dir",...]}.
  {primLoad, [Mod1, Mod2, ...]},
  {progress, loaded},
  {kernelProcess, Name, {Mod, Func, Args}},
  {apply, {Mod, Func, Args}},
  {progress, started}]}.    
  • Name = string() defines the name of the system.
  • Vsn = string() defines the version of the system.
  • {progress, Term} sets the "progress" of the initialization program. The function init:get_status() returns the current value of the progress, which is {InternalStatus,Term}.
  • {path, [Dir]} where Dir is a string. This argument sets the load path of the system to [Dir]. The load path used to load modules is obtained from the initial load path, which is given in the script file, together with any path flags which were supplied in the command line arguments. The command line arguments modify the path as follows:

    • -pa Dir1 Dir2 ... DirN adds the directories Dir1, Dir2, ..., DirN to the front of the initial load path.
    • -pz Dir1 Dir2 ... DirN adds the directories Dir1, Dir2, ..., DirN to the end of the initial load path.
    • -path Dir1 Dir2 ... DirN defines a set of directories Dir1, Dir2, ..., DirN which replaces the search path given in the script file. Directory names in the path are interpreted as follows:

      • Directory names starting with / are assumed to be absolute path names.
      • Directory names not starting with / are assumed to be relative the current working directory.
      • The special $ROOT variable can only be used in the script, not as a command line argument. The given directory is relative the Erlang installation directory.
  • {primLoad, [Mod]} loads the modules [Mod] from the directories specified in Path. The script interpreter fetches the appropriate module by calling the function erl_prim_loader:get_file(Mod). A fatal error which terminates the system will occur if the module cannot be located.
  • {kernel_load_completed} indicates that all modules which must be loaded before any processes are started are loaded. In interactive mode, all {primLoad,[Mod]} commands interpreted after this command are ignored, and these modules are loaded on demand. In embedded mode, kernel_load_completed is ignored, and all modules are loaded during system start.
  • {kernelProcess, Name, {Mod, Func, Args}} starts a "kernel process". The kernel process Name is started by evaluating apply(Mod, Func, Args) which is expected to return {ok, Pid} or ignore. The init process monitors the behaviour of Pid and terminates the system if Pid dies. Kernel processes are key components of the runtime system. Users do not normally add new kernel processes.
  • {apply, {Mod, Func, Args}}. The init process simply evaluates apply(Mod, Func, Args). The system terminates if this results in an error. The boot procedure hangs if this function never returns.

In the interactive system the code loader provides demand driven code loading, but in the embedded system the code loader loads all the code immediately. The same version of code is used in both cases. The code server calls init:get_argument(mode) to find out if it should run in demand mode, or non-demand driven mode.