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# Introduction

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This section describes the issues that are specific for running Erlang on an UNIX embedded system. It describes the differences in installing and starting Erlang compared to how it is done for a non-embedded system.

For details on how to create a target system, see Creating and Upgrading a Target System in the System Principles section.

When running on Windows, so special considerations need to be made. Starting Erlang should be done via `erlsrv`.

## Installing an Embedded System

This section is about installing an embedded system. The following topics are considered:

- Creating user and installation directory
- Installing an embedded system
- Configuring automatic start at boot
- Changing permission for reboot
- Setting TERM environment variable

Several of the procedures in this section require expert knowledge of the operating system. For most of them super user privilege is needed.

### Creating User and Installation Directory

It is recommended that the embedded environment is run by an ordinary user, that is, a user who does not have super user privileges.

In this section, it is assumed that the username is `otpuser` and that the home directory of that user is:

    /home/otpuser

It is also assumed that in the home directory of `otpuser`, there is a directory named `otp`, the full path of which is:

    /home/otpuser/otp

This directory is the installation directory of the embedded environment.

### Installing an Embedded System

The procedure for installing an embedded system is the same as for an ordinary system (see Installation Guide and Creating and Upgrading a Target System in the System Principles section), except for the following:

- The (compressed) archive file is to be extracted in the installation directory defined above.
- It is not needed to link the start script to a standard directory like `/usr/local/bin`.

### Configuring Automatic Start at Boot

A true embedded system must start when the system boots. This section accounts for the necessary configurations needed to achieve that using `init.d` start scripts.

The embedded system and all the applications start automatically if the script file shown below is added to directory `/etc/rc3.d`. The file must be owned and readable by `root`. Its name cannot be arbitrarily assigned; the following name is recommended:

    S75otp.system

For more details on initialization (and termination) scripts, and naming thereof, see the `init.d` documentation on your OS.

    #!/bin/sh
    #
    #  File name:  S75otp.system
    #  Purpose:    Automatically starts Erlang and applications when the
    #              system starts
    #  Author:     janne@erlang.ericsson.se
    #  Resides in: /etc/rc3.d
    #

    if [ ! -d /usr/bin ]
    then                    # /usr not mounted
            exit
    fi

    killproc() {            # kill the named process(es)
            pid=`/usr/bin/ps -e |
                 /usr/bin/grep -w $1 |
                 /usr/bin/sed -e 's/^  *//' -e 's/ .*//'`
            [ "$pid" != "" ] && kill $pid
    }

    # Start/stop processes required for Erlang

    case "$1" in
    'start')
            # Start the Erlang emulator
            #
            su - otpuser -c "/home/otpuser/otp/bin/start" &
            ;;
    'stop')
            killproc beam
            ;;
    *)
            echo "Usage: $0 { start | stop }"
            ;;
    esac

File `/home/otpuser/otp/bin/start` referred to in the above script is precisely the `start` script described in Starting Erlang. The script variable `$OTPROOT` in that `start` script corresponds to the following example path used in this section:

    /home/otpuser/otp

The `start` script is to be edited accordingly.

Use of the `killproc` procedure in the above script can be combined with a call to `erl_call`, for example:

    $SOME_PATH/erl_call -n Node init stop

To take Erlang down gracefully, see the `erl_call(1)` manual page in `erl_interface` for details on the use of `erl_call`. However, that requires that Erlang runs as a distributed node, which is not always the case.

The `killproc` procedure is not to be removed. The purpose is here to move from run level 3 (multi-user mode with networking resources) to run level 2 (multi-user mode without such resources), in which Erlang is not to run.

### Changing Permissions for Reboot

If the `HEART_COMMAND` environment variable is to be set in the `start` script in Starting Erlang, and if the value is to be set to the path of the `reboot` command, that is:

    HEART_COMMAND=/usr/sbin/reboot

then the ownership and file permissions for `/usr/sbin/reboot` must be changed as follows:

    chown 0 /usr/sbin/reboot
    chmod 4755 /usr/sbin/reboot

See also the `heart` manual page in Kernel.

### Setting TERM Environment Variable

When the Erlang runtime system is automatically started from the `S75otp.system` script, the `TERM` environment variable must be set. The following is a minimal setting:

    TERM=dumb

This is to be added to the `start` script.

## Starting Erlang

This section describes how an embedded system is started. Four programs are involved and they normally reside in the directory `<ERL_INSTALL_DIR>/bin`. The only exception is the `start` program, which can be located anywhere, and is also the only program that must be modified by the user.

In an embedded system, there is usually no interactive shell. However, an operator can attach to the Erlang system by command `to_erl`. The operator is then connected to the Erlang shell and can give ordinary Erlang commands. All interaction with the system through this shell is logged in a special directory.

Basically, the procedure is as follows:

- The `start`) program is called when the machine is started.
- It calls `run_erl`, which sets up things so the operator can attach to the system.
- It calls `start_erl`, which calls the correct version of `erlexec` (which is located in `<ERL_INSTALL_DIR>/erts-EVsn/bin`) with the correct `boot` and `config` files.

## Programs

### start

This program is called when the machine is started. It can be modified or rewritten to suit a special system. By default, it must be called `start` and reside in `<ERL_INSTALL_DIR>/bin`. Another start program can be used, by using configuration parameter `start_prg` in application SASL.

The start program must call `run_erl` as shown below. It must also take an optional parameter, which defaults to `<ERL_INSTALL_DIR>/releases/start_erl.data`.

This program is to set static parameters and environment variables such as `-sname Name` and `HEART_COMMAND` to reboot the machine.

The `<RELDIR>` directory is where new release packets are installed, and where the release handler keeps information about releases. For more information, see the `release_handler` manual page in SASL.

The following script illustrates the default behaviour of the program:

    #!/bin/sh
    # Usage: start [DataFile]
    #
    ROOTDIR=/usr/local/otp

    if [ -z "$RELDIR" ]
    then
       RELDIR=$ROOTDIR/releases
    fi

    START_ERL_DATA=${1:-$RELDIR/start_erl.data}

    $ROOTDIR/bin/run_erl /tmp/ $ROOTDIR/log "exec $ROOTDIR/bin/start_erl \
                         $ROOTDIR $RELDIR $START_ERL_DATA" > /dev/null 2>&1 &

The following script illustrates a modification where the node is given the name `cp1`, and where the environment variables `HEART_COMMAND` and `TERM` have been added to the previous script:

    #!/bin/sh
    # Usage: start [DataFile]
    #
    HEART_COMMAND=/usr/sbin/reboot
    TERM=dumb
    export HEART_COMMAND TERM

    ROOTDIR=/usr/local/otp

    if [ -z "$RELDIR" ]
    then
       RELDIR=$ROOTDIR/releases
    fi

    START_ERL_DATA=${1:-$RELDIR/start_erl.data}

    $ROOTDIR/bin/run_erl /tmp/ $ROOTDIR/log "exec $ROOTDIR/bin/start_erl \
          $ROOTDIR $RELDIR $START_ERL_DATA -heart -sname cp1" > /dev/null 2>&1 &

If a diskless and/or read-only client node is about to start, file `start_erl.data` is located in the client directory at the master node. Thus, the `START_ERL_DATA` line is to look like:

    CLIENTDIR=$ROOTDIR/clients/clientname
    START_ERL_DATA=${1:-$CLIENTDIR/bin/start_erl.data}

### run_erl

This program is used to start the emulator, but you will not be connected to the shell. `to_erl` is used to connect to the Erlang shell.

    Usage: run_erl pipe_dir/ log_dir "exec command [parameters ...]"

Here:

- `pipe_dir/` is to be `/tmp/` (`to_erl` uses this name by default).
- `log_dir` is where the log files are written.
- `command [parameters]` is executed.
- Everything written to `stdin` and `stdout` is logged in `log_dir`.

Log files are written in `log_dir`. Each log file has a name of the form `erlang.log.N`, where N is a generation number, ranging from 1 to 5. Each log file holds up to 100 kB text. As time goes by, the following log files are found in the log file directory:

    erlang.log.1
    erlang.log.1, erlang.log.2
    erlang.log.1, erlang.log.2, erlang.log.3
    erlang.log.1, erlang.log.2, erlang.log.3, erlang.log.4
    erlang.log.2, erlang.log.3, erlang.log.4, erlang.log.5
    erlang.log.3, erlang.log.4, erlang.log.5, erlang.log.1
    ...

The most recent log file is the rightmost in each row. That is, the most recent file is the one with the highest number, or if there are already four files, the one before the skip.

When a log file is opened (for appending or created), a time stamp is written to the file. If nothing has been written to the log files for 15 minutes, a record is inserted that says that we are still alive.

For more details see `run_erl` in the ERTS documentation.

### to_erl

This program is used to attach to a running Erlang runtime system, started with `run_erl`.

    Usage: to_erl [pipe_name | pipe_dir]

Here `pipe_name` defaults to `/tmp/erlang.pipe.N`.

To disconnect from the shell without exiting the Erlang system, type `Ctrl-D`.

### start_erl

This program starts the Erlang emulator with parameters `-boot` and `-config` set. It reads data about where these files are located from a file named `start_erl.data`, which is located in `<RELDIR>`. Each new release introduces a new data file. This file is automatically generated by the release handler in Erlang.

The following script illustrates the behaviour of the program:

    #!/bin/sh
    #
    # This program is called by run_erl. It starts
    # the Erlang emulator and sets -boot and -config parameters.
    # It should only be used at an embedded target system.
    #
    # Usage: start_erl RootDir RelDir DataFile [ErlFlags ...]
    #
    ROOTDIR=$1
    shift
    RELDIR=$1
    shift
    DataFile=$1
    shift

    ERTS_VSN=`awk '{print $1}' $DataFile`
    VSN=`awk '{print $2}' $DataFile`

    BINDIR=$ROOTDIR/erts-$ERTS_VSN/bin
    EMU=beam
    PROGNAME=`echo $0 | sed 's/.*\///'`
    export EMU
    export ROOTDIR
    export BINDIR
    export PROGNAME
    export RELDIR

    exec $BINDIR/erlexec -boot $RELDIR/$VSN/start -config $RELDIR/$VSN/sys $*

If a diskless and/or read-only client node with the SASL configuration parameter `static_emulator` set to `true` is about to start, the `-boot` and `-config` flags must be changed.

As such a client cannot read a new `start_erl.data` file (the file cannot be changed dynamically). The boot and config files are always fetched from the same place (but with new contents if a new release has been installed).

The `release_handler` copies these files to the `bin` directory in the client directory at the master nodes whenever a new release is made permanent.

Assuming the same `CLIENTDIR` as above, the last line is to look like:

    exec $BINDIR/erlexec -boot $CLIENTDIR/bin/start \
         -config $CLIENTDIR/bin/sys $*

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