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.
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 from .app files.
The boot script is stored in a file with extension .script.
The file has the following syntax:
{script, {Name, Vsn},
[
{progress, loading},
{preLoaded, [Mod1, Mod2, ...]},
{path, [Dir1,"$ROOT/Dir",...]}.
{primLoad, [Mod1, Mod2, ...]},
...
{kernel_load_completed},
{progress, loaded},
{kernelProcess, Name, {Mod, Func, Args}},
...
{apply, {Mod, Func, Args}},
...
{progress, started}]}.
- Name =
string():
- Defines the system name.
- Vsn =
string():
- Defines the system version.
- {progress, Term}:
- Sets the "progress" of the initialization program. The
init:get_status/0 function returns the current value of the
progress, which is {InternalStatus,Term}.
- {path, [Dir]}:
- 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 that were supplied in the command-line arguments. The
command-line arguments modify the path as follows:
- *
- -pa Dir1 Dir2 ... DirN adds the directories DirN, DirN-1, ...,
Dir2, Dir1 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 replace 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 erl_prim_loader:get_file(Mod). A fatal error that
terminates the system occurs if the module cannot be located.
- {kernel_load_completed}:
- Indicates that all modules that 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 the "kernel process" Name by evaluating
apply(Mod, Func, Args). The start function is to return {ok,
Pid} or ignore. The init process monitors the behavior
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 evaluates apply(Mod, Func, Args). The system
terminates if this results in an error. The boot procedure hangs if this
function never returns.
Note:
In an interactive system, the code loader provides demand-driven code loading,
but in an embedded system the code loader loads all code immediately. The same
version of code is used in both cases. The code server calls
init:get_argument(mode) to determine if it is to run in demand mode or
non-demand driven mode.