## 3 Creating a First Target System

### 3.1 Introduction

When creating a system using Erlang/OTP, the most simple way is to install Erlang/OTP somewhere, install the application specific code somewhere else, and then start the Erlang runtime system, making sure the code path includes the application specific code.

Often it is not desirable to use an Erlang/OTP system as is. A developer may create new Erlang/OTP compliant applications for a particular purpose, and several original Erlang/OTP applications may be irrelevant for the purpose in question. Thus, there is a need to be able to create a new system based on a given Erlang/OTP system, where dispensable applications are removed, and a set of new applications that are included in the new system. Documentation and source code is irrelevant and is therefore not included in the new system.

This chapter is about creating such a system, which we call a target system.

In the following sections we consider creating target systems with different requirements of functionality:

- a basic target system that can be started by calling the ordinary `erl` script,
- a simple target system where also code replacement in run-time can be performed, and
- an embedded target system where there is also support for logging output from the system to file for later inspection, and where the system can be started automatically at boot time.

We only consider the case when Erlang/OTP is running on a UNIX system.

There is an example Erlang module `target_system.erl` that contains functions for creating and installing a target system. That module is used in the examples below. The source code of the module is listed at the end of this chapter.

### 3.2 Creating a Target System

It is assumed that you have a working Erlang/OTP system structured according to the OTP Design Principles.

Step 1. First create a `.rel` file (see `rel(4)`) that specifies the `erts` version and lists all applications that should be included in the new basic target system. An example is the following `mysystem.rel` file:

    %% mysystem.rel
    {release,
     {"MYSYSTEM", "FIRST"},
     {erts, "5.1"},
     [{kernel, "2.7"},
      {stdlib, "1.10"},
      {sasl, "1.9.3"},
      {pea, "1.0"}]}.

The listed applications are not only original Erlang/OTP applications but possibly also new applications that you have written yourself (here examplified by the application `pea`).

Step 2. From the directory where the `mysystem.rel` file reside, start the Erlang/OTP system:

    erl -pa /home/user/target_system/myapps/pea-1.0/ebin

where also the path to the `pea-1.0` ebin directory is provided.

Step 2. Now create the target system:

    1> target_system:create("mysystem").

The `target_system:create/1` function does the following:

1. Reads the `mysystem.rel` file, and creates a new file `plain.rel` which is identical to former, except that it only lists the `kernel` and `stdlib` applications.
2. From the `mysystem.rel` and `plain.rel` files creates the files `mysystem.script`, `mysystem.boot`, `plain.script`, and `plain.boot` through a call to `systools:make_script/2`.
3. Creates the file `mysystem.tar.gz` by a call to `systools:make_tar/2`. That file has the following contents:

       erts-5.1/bin/
       releases/FIRST/start.boot
       releases/mysystem.rel
       lib/kernel-2.7/
       lib/stdlib-1.10/
       lib/sasl-1.9.3/
       lib/pea-1.0/

   The file `releases/FIRST/start.boot` is a copy of our `mysystem.boot`, and a copy of the original `mysystem.rel` has been put in the `releases` directory.
4. Creates the temporary directory `tmp` and extracts the tar file `mysystem.tar.gz` into that directory.
5. Deletes the `erl` and `start` files from `tmp/erts-5.1/bin`. XXX Why.
6. Creates the directory `tmp/bin`.
7. Copies the previously creates file `plain.boot` to `tmp/bin/start.boot`.
8. Copies the files `epmd`, `run_erl`, and `to_erl` from the directory `tmp/erts-5.1/bin` to the directory `tmp/bin`.
9. Creates the file `tmp/releases/start_erl.data` with the contents "5.1 FIRST".
10. Recreates the file `mysystem.tar.gz` from the directories in the directory `tmp`, and removes `tmp`.

### 3.3 Installing a Target System

Step 3. Install the created target system in a suitable directory.

    3> target_system:install("mysystem", "/usr/local/erl-target").

The function `target_system:install/2` does the following:

1. Extracts the tar file `mysystem.tar.gz` into the target directory `/usr/local/erl-target`.
2. In the target directory reads the file `releases/start_erl.data` in order to find the Erlang runtime system version ("5.1").
3. Substitutes `%FINAL_ROOTDIR%` and `%EMU%` for `/usr/local/erl-target` and `beam`, respectively, in the files `erl.src`, `start.src`, and `start_erl.src` of the target `erts-5.1/bin` directory, and puts the resulting files `erl`, `start`, and `run_erl` in the target `bin` directory.
4. Finally the target `releases/RELEASES` file is created from data in the `releases/mysystem.rel` file.

### 3.4 Starting a Target System

Now we have a target system that can be started in various ways.

We start it as a basic target system by invoking

    /usr/local/erl-target/bin/erl

where only the `kernel` and `stdlib` applications are started, i.e. the system is started as an ordinary development system. There are only two files needed for all this to work: `bin/erl` file (obtained from `erts-5.1/bin/erl.src`) and the `bin/start.boot` file (a copy of `plain.boot`).

We can also start a distributed system (requires `bin/epmd`).

To start all applications specified in the original `mysystem.rel` file, use the `-boot` flag as follows:

    /usr/local/erl-target/bin/erl -boot /usr/local/erl-target/releases/FIRST/start

We start a simple target system as above. The only difference is that also the file `releases/RELEASES` is present for code replacement in run-time to work.

To start an embedded target system the shell script `bin/start` is used. That shell script calls `bin/run_erl`, which in turn calls `bin/start_erl` (roughly, `start_erl` is an embedded variant of `erl`).

The shell script `start` is only an example. You should edit it to suite your needs. Typically it is executed when the UNIX system boots.

`run_erl` is a wrapper that provides logging of output from the run-time system to file. It also provides a simple mechanism for attaching to the Erlang shell (`to_erl`).

`start_erl` requires the root directory (`"/usr/local/erl-target"`), the releases directory (`"/usr/local/erl-target/releases"`), and the location of the `start_erl.data` file. It reads the run-time system version (`"5.1"`) and release version (`"FIRST"`) from the `start_erl.data` file, starts the run-time system of the version found, and provides `-boot` flag specifying the boot file of the release version found (`"releases/FIRST/start.boot"`).

`start_erl` also assumes that there is `sys.config` in release version directory (`"releases/FIRST/sys.config`). That is the topic of the next section (see below).

The `start_erl` shell script should normally not be altered by the user.

### 3.5 System Configuration Parameters

As was pointed out above `start_erl` requires a `sys.config` in the release version directory (`"releases/FIRST/sys.config"`). If there is no such a file, the system start will fail. Hence such a file has to added as well.

If you have system configuration data that are neither file location dependent nor site dependent, it may be convenient to create the `sys.config` early, so that it becomes a part of the target system tar file created by `target_system:create/1`. In fact, if you create, in the current directory, not only the `mysystem.rel` file, but also a `sys.config` file, that latter file will be tacitly put in the apropriate directory.

### 3.6 Differences from the Install Script

The above `install/2` procedure differs somewhat from that of the ordinary `Install` shell script. In fact, `create/1` makes the release package as complete as possible, and leave to the `install/2` procedure to finish by only considering location dependent files.

### 3.7 Listing of target_system.erl

    -module(target_system).
    -include_lib("kernel/include/file.hrl").
    -export([create/1, install/2]).
    -define(BUFSIZE, 8192).

    %% Note: RelFileName below is the *stem* without trailing .rel,
    %% .script etc.
    %%

    %% create(RelFileName)
    %%
    create(RelFileName) ->
        RelFile = RelFileName ++ ".rel",
        io:fwrite("Reading file: \"~s\" ...~n", [RelFile]),
        {ok, [RelSpec]} = file:consult(RelFile),
        io:fwrite("Creating file: \"~s\" from \"~s\" ...~n",
                  ["plain.rel", RelFile]),
        {release,
         {RelName, RelVsn},
         {erts, ErtsVsn},
         AppVsns} = RelSpec,
        PlainRelSpec = {release,
                        {RelName, RelVsn},
                        {erts, ErtsVsn},
                        lists:filter(fun({kernel, _}) ->
                                             true;
                                        ({stdlib, _}) ->
                                             true;
                                        (_) ->
                                             false
                                     end, AppVsns)
                       },
        {ok, Fd} = file:open("plain.rel", [write]),
        io:fwrite(Fd, "~p.~n", [PlainRelSpec]),
        file:close(Fd),

        io:fwrite("Making \"plain.script\" and \"plain.boot\" files ...~n"),
        make_script("plain"),

        io:fwrite("Making \"~s.script\" and \"~s.boot\" files ...~n",
                  [RelFileName, RelFileName]),
        make_script(RelFileName),

        TarFileName = io_lib:fwrite("~s.tar.gz", [RelFileName]),
        io:fwrite("Creating tar file \"~s\" ...~n", [TarFileName]),
        make_tar(RelFileName),

        io:fwrite("Creating directory \"tmp\" ...~n"),
        file:make_dir("tmp"),

        io:fwrite("Extracting \"~s\" into directory \"tmp\" ...~n", [TarFileName]),
        extract_tar(TarFileName, "tmp"),

        TmpBinDir = filename:join(["tmp", "bin"]),
        ErtsBinDir = filename:join(["tmp", "erts-" ++ ErtsVsn, "bin"]),
        io:fwrite("Deleting \"erl\" and \"start\" in directory \"~s\" ...~n",
                  [ErtsBinDir]),
        file:delete(filename:join([ErtsBinDir, "erl"])),
        file:delete(filename:join([ErtsBinDir, "start"])),

        io:fwrite("Creating temporary directory \"~s\" ...~n", [TmpBinDir]),
        file:make_dir(TmpBinDir),

        io:fwrite("Copying file \"plain.boot\" to \"~s\" ...~n",
                  [filename:join([TmpBinDir, "start.boot"])]),
        copy_file("plain.boot", filename:join([TmpBinDir, "start.boot"])),

        io:fwrite("Copying files \"epmd\", \"run_erl\" and \"to_erl\" from \n"
                  "\"~s\" to \"~s\" ...~n",
                  [ErtsBinDir, TmpBinDir]),
        copy_file(filename:join([ErtsBinDir, "epmd"]),
                  filename:join([TmpBinDir, "epmd"]), [preserve]),
        copy_file(filename:join([ErtsBinDir, "run_erl"]),
                  filename:join([TmpBinDir, "run_erl"]), [preserve]),
        copy_file(filename:join([ErtsBinDir, "to_erl"]),
                  filename:join([TmpBinDir, "to_erl"]), [preserve]),

        StartErlDataFile = filename:join(["tmp", "releases", "start_erl.data"]),
        io:fwrite("Creating \"~s\" ...~n", [StartErlDataFile]),
        StartErlData = io_lib:fwrite("~s ~s~n", [ErtsVsn, RelVsn]),
        write_file(StartErlDataFile, StartErlData),

        io:fwrite("Recreating tar file \"~s\" from contents in directory "
                  "\"tmp\" ...~n", [TarFileName]),
        {ok, Tar} = erl_tar:open(TarFileName, [write, compressed]),
        {ok, Cwd} = file:get_cwd(),
        file:set_cwd("tmp"),
        erl_tar:add(Tar, "bin", []),
        erl_tar:add(Tar, "erts-" ++ ErtsVsn, []),
        erl_tar:add(Tar, "releases", []),
        erl_tar:add(Tar, "lib", []),
        erl_tar:close(Tar),
        file:set_cwd(Cwd),
        io:fwrite("Removing directory \"tmp\" ...~n"),
        remove_dir_tree("tmp"),
        ok.

    install(RelFileName, RootDir) ->
        TarFile = RelFileName ++ ".tar.gz",
        io:fwrite("Extracting ~s ...~n", [TarFile]),
        extract_tar(TarFile, RootDir),
        StartErlDataFile = filename:join([RootDir, "releases", "start_erl.data"]),
        {ok, StartErlData} = read_txt_file(StartErlDataFile),
        [ErlVsn, RelVsn| _] = string:tokens(StartErlData, " \n"),
        ErtsBinDir = filename:join([RootDir, "erts-" ++ ErlVsn, "bin"]),
        BinDir = filename:join([RootDir, "bin"]),
        io:fwrite("Substituting in erl.src, start.src and start_erl.src to\n"
                  "form erl, start and start_erl ...\n"),
        subst_src_scripts(["erl", "start", "start_erl"], ErtsBinDir, BinDir,
                          [{"FINAL_ROOTDIR", RootDir}, {"EMU", "beam"}],
                          [preserve]),
        io:fwrite("Creating the RELEASES file ...\n"),
        create_RELEASES(RootDir,
                        filename:join([RootDir, "releases", RelFileName])).

    %% LOCALS

    %% make_script(RelFileName)
    %%
    make_script(RelFileName) ->
        Opts = [no_module_tests],
        systools:make_script(RelFileName, Opts).

    %% make_tar(RelFileName)
    %%
    make_tar(RelFileName) ->
        RootDir = code:root_dir(),
        systools:make_tar(RelFileName, [{erts, RootDir}]).

    %% extract_tar(TarFile, DestDir)
    %%
    extract_tar(TarFile, DestDir) ->
        erl_tar:extract(TarFile, [{cwd, DestDir}, compressed]).

    create_RELEASES(DestDir, RelFileName) ->
        release_handler:create_RELEASES(DestDir, RelFileName ++ ".rel").

    subst_src_scripts(Scripts, SrcDir, DestDir, Vars, Opts) ->
        lists:foreach(fun(Script) ->
                              subst_src_script(Script, SrcDir, DestDir,
                                               Vars, Opts)
                      end, Scripts).

    subst_src_script(Script, SrcDir, DestDir, Vars, Opts) ->
        subst_file(filename:join([SrcDir, Script ++ ".src"]),
                   filename:join([DestDir, Script]),
                   Vars, Opts).

    subst_file(Src, Dest, Vars, Opts) ->
        {ok, Conts} = read_txt_file(Src),
        NConts = subst(Conts, Vars),
        write_file(Dest, NConts),
        case lists:member(preserve, Opts) of
            true ->
                {ok, FileInfo} = file:read_file_info(Src),
                file:write_file_info(Dest, FileInfo);
            false ->
                ok
        end.

    %% subst(Str, Vars)
    %% Vars = [{Var, Val}]
    %% Var = Val = string()
    %% Substitute all occurrences of %Var% for Val in Str, using the list
    %% of variables in Vars.
    %%
    subst(Str, Vars) ->
        subst(Str, Vars, []).

    subst([$%, C| Rest], Vars, Result) when $A =< C, C =< $Z ->
        subst_var([C| Rest], Vars, Result, []);
    subst([$%, C| Rest], Vars, Result) when $a =< C, C =< $z ->
        subst_var([C| Rest], Vars, Result, []);
    subst([$%, C| Rest], Vars, Result) when  C == $_ ->
        subst_var([C| Rest], Vars, Result, []);
    subst([C| Rest], Vars, Result) ->
        subst(Rest, Vars, [C| Result]);
    subst([], _Vars, Result) ->
        lists:reverse(Result).

    subst_var([$%| Rest], Vars, Result, VarAcc) ->
        Key = lists:reverse(VarAcc),
        case lists:keysearch(Key, 1, Vars) of
            {value, {Key, Value}} ->
                subst(Rest, Vars, lists:reverse(Value, Result));
            false ->
                subst(Rest, Vars, [$%| VarAcc ++ [$%| Result]])
        end;
    subst_var([C| Rest], Vars, Result, VarAcc) ->
        subst_var(Rest, Vars, Result, [C| VarAcc]);
    subst_var([], Vars, Result, VarAcc) ->
        subst([], Vars, [VarAcc ++ [$%| Result]]).

    copy_file(Src, Dest) ->
        copy_file(Src, Dest, []).

    copy_file(Src, Dest, Opts) ->
        {ok, InFd} = file:rawopen(Src, {binary, read}),
        {ok, OutFd} = file:rawopen(Dest, {binary, write}),
        do_copy_file(InFd, OutFd),
        file:close(InFd),
        file:close(OutFd),
        case lists:member(preserve, Opts) of
            true ->
                {ok, FileInfo} = file:read_file_info(Src),
                file:write_file_info(Dest, FileInfo);
            false ->
                ok
        end.

    do_copy_file(InFd, OutFd) ->
        case file:read(InFd, ?BUFSIZE) of
            {ok, Bin} ->
                file:write(OutFd, Bin),
                do_copy_file(InFd, OutFd);
            eof  ->
                ok
        end.

    write_file(FName, Conts) ->
        {ok, Fd} = file:open(FName, [write]),
        file:write(Fd, Conts),
        file:close(Fd).

    read_txt_file(File) ->
        {ok, Bin} = file:read_file(File),
        {ok, binary_to_list(Bin)}.

    remove_dir_tree(Dir) ->
        remove_all_files(".", [Dir]).

    remove_all_files(Dir, Files) ->
        lists:foreach(fun(File) ->
                              FilePath = filename:join([Dir, File]),
                              {ok, FileInfo} = file:read_file_info(FilePath),
                              case FileInfo#file_info.type of
                                  directory ->
                                      {ok, DirFiles} = file:list_dir(FilePath),
                                      remove_all_files(FilePath, DirFiles),
                                      file:del_dir(FilePath);
                                  _ ->
                                      file:delete(FilePath)
                              end
                      end, Files).

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