Minor doc updates

[novaboot.git] / README.pod

=encoding utf8
=head1 NAME

novaboot - Boots a locally compiled operating system on a remote
target or in qemu

=head1 SYNOPSIS

B<novaboot> --help

B<novaboot> [option]... [--] script...

B<./script> [option]...

=head1 DESCRIPTION

Novaboot makes booting of a locally compiled operating system (OS)
(e.g. NOVA or Linux) on remote targets as simple as running a program
locally. It automates things like copying OS images to a TFTP server,
generation of bootloader configuration files, resetting of target
hardware or redirection of target's serial line to stdin/out. Novaboot
is highly configurable and makes it easy to boot a single image on
different targets or different images on a single target.

Novaboot operation is controlled by configuration files, command line
options and by a so called novaboot script, which can be thought as a
generalization of bootloader configuration files (see L</"NOVABOOT
SCRIPT SYNTAX">). Typical way of using novaboot is to make the
novaboot script executable and set its first line to I<#!/usr/bin/env
novaboot>. Then, booting a particular OS configuration becomes the
same as executing a local program – the novaboot script.

Novaboot uses configuration files to, among other things, define
command line options needed for different targets. Users typically use
only the B<-t>/B<--target> command line option to select the target.
Internally, this option expands to the pre-configured options.
Configuration files are searched at multiple places, which allows to
have per-system, per-user or per-project configurations. Configuration
file syntax is described in section L</"CONFIGURATION FILES">.

Simple examples of using C<novaboot>:

=over 3

=item 1.

Run an OS in Qemu. This is can be specified with the B<--qemu> option.
Thus running

 novaboot --qemu myos

(or C<./myos --qemu> as described above) will run Qemu and make it
boot the configuration specified in the F<myos> script.

=item 2.

Create a bootloader configuration file (currently supported
bootloaders are GRUB, GRUB2, ISOLINUX, Pulsar and U-Boot) and copy it
with all other files needed for booting to a remote boot server. Then
use a TCP/IP-controlled relay/serial-to-TCP converter to reset the
target and receive its serial output.

 ./myos --grub2 --server=192.168.1.1:/tftp --iprelay=192.168.1.2

Alternatively, you can put these switches to the configuration file
and run:

 ./myos --target mytarget

=item 3.

Run DHCP and TFTP server on developer's machine to boot the target
from it.

 ./myos --dhcp-tftp

This is useful when no network infrastructure is in place and
the target is connected directly to developer's box.

=item 4.

Create bootable ISO image.

 novaboot --iso -- script1 script2

The created ISO image will have ISOLINUX bootloader installed on it
and the boot menu will allow selecting between I<script1> and
I<script2> configurations.

=back

=head1 PHASES AND OPTIONS

Novaboot performs its work in several phases. Each phase can be
influenced by several command line options, certain phases can be
skipped. The list of phases (in the execution order) is as follows.

=over

=item 1. L<Configuration reading|/Configuration reading phase>

=item 2. L<Command line processing|/Command line processing phase>

=item 3. L<Script preprocessing|/Script preprocessing phase>

=item 4. L<File generation|/File generation phase>

=item 5. L<Target connection|/Target connection check>

=item 6. L<File deployment|/File deployment phase>

=item 7. L<Target power-on and reset|/Target power-on and reset phase>

=item 8. L<Interaction with the bootloader|/Interaction with the bootloader on the target>

=item 9. L<Target interaction|/Target interaction phase>

=back

Each phase is described in the following sections together with the
command line options that control it.

=head2 Configuration reading phase

After starting, novaboot reads configuration files. Their content is
described in section L</"CONFIGURATION FILES">. By default,
configuration is read from multiple locations. First from the system
configuration directory (F</etc/novaboot.d/>), second from the user
configuration file (F<~/.config/novaboot>) and third from F<.novaboot>
files along the path to the current directory. Alternatively, a single
configuration file specified with the B<-c> switch or with the
C<NOVABOOT_CONFIG> environment variable is read. The latter read files
override settings from the former ones.

The system configuration directory is determined by the content of
NOVABOOT_CONFIG_DIR environment variable and defaults to
F</etc/novaboot.d>. Files in this directory with names consisting
solely of English letters, numbers, dashes '-' and underscores '_'
(note that dot '.' is not included) are read in alphabetical order.

Then, the user configuration file is read from
F<$XDG_CONFIG_HOME/novaboot>. If C<$XDG_CONFIG_HOME> environemnt
variable is not set F<~/.config/novaboot> is read instead.

Finally, novaboot searches for files named F<.novaboot> starting from the
directory of the novaboot script (or working directory, see bellow)
and continuing upwards up to the root directory. The found
configuration files are then read in the opposite order (i.e. from the
root directory downwards). This allows to have, for example, a project
specific configuration in F<~/project/.novaboot>.

Note the difference between F<~/.config/novaboot> and F<~/.novaboot>.
The former one is read always, whereas the latter only when novaboot
script or working directory is under the C<$HOME> directory.

In certain cases, the location of the novaboot script cannot be
determined in this early phase. This happens either when the script is
read from the standard input or when novaboot is invoked explicitly as
in the example L</"4."> above. In this case the current working
directory is used as a starting point for configuration file search
instead of the novaboot script directory.

=over 8

=item -c, --config=I<filename>

Use the specified configuration file instead of the default one(s).

=back

=head2 Command line processing phase

=over 8

=item --dump-config

Dump the current configuration to stdout end exit. Useful as an
initial template for a configuration file.

=item -h, --help

Print short (B<-h>) or long (B<--help>) help.

=item -t, --target=I<target>

This option serves as a user configurable shortcut for other novaboot
options. The effect of this option is the same as specifying the
options stored in the C<%targets> configuration variable under key
I<target>. See also L</"CONFIGURATION FILES">.

When this option is not given, novaboot tries to determine the target
to use from either B<NOVABOOT_TARGET> environment variable or from
B<$default_target> configuration file variable.

=back

=head2 Script preprocessing phase

This phases allows to modify the parsed novaboot script before it is
used in the later phases.

=over 8

=item -a, --append=I<parameters>

Append a string to the first C<load> line in the novaboot script. This
can be used to append parameters to the kernel's or root task's
command line. This option can appear multiple times.

=item -b, --bender

Use F<bender> chainloader. Bender scans the PCI bus for PCI serial
ports and stores the information about them in the BIOS data area for
use by the kernel.

=item --chainloader=I<chainloader>

Specifies a chainloader that is loaded before the kernel and other
files specified in the novaboot script. E.g. 'bin/boot/bender
promisc'.

=item --dump

Print the modules to boot and their parameters after this phase
finishes. Then exit. This is useful for seeing the effect of other
options in this section.

=item -k, --kernel=F<file>

Replace the first word on the first C<load> line in the novaboot
script with F<file>.

=item --scriptmod=I<perl expression>

When novaboot script is read, I<perl expression> is executed for every
line (in $_ variable). For example, C<novaboot
--scriptmod=s/sigma0/omega6/g> replaces every occurrence of I<sigma0>
in the script with I<omega6>.

When this option is present, it overrides I<$script_modifier> variable
from the configuration file, which has the same effect. If this option
is given multiple times all expressions are evaluated in the command
line order.

=back

=head2 File generation phase

In this phase, files needed for booting are generated in a so called
I<build directory> (see L</--build-dir>). In most cases configuration
for a bootloader is generated automatically by novaboot. It is also
possible to generate other files using I<heredoc> or I<"<"> syntax in
novaboot scripts. Finally, binaries can be generated in this phases by
running C<scons> or C<make>.

=over 8

=item --build-dir=I<directory>

Overrides the default build directory location.

The default build directory location is determined as follows: If the
configuration file defines the C<$builddir> variable, its value is
used. Otherwise, it is the directory that contains the first processed
novaboot script.

See also L</BUILDDIR> variable.

=item -g, --grub[=I<filename>]

Generates grub bootloader menu file. If the I<filename> is not
specified, F<menu.lst> is used. The I<filename> is relative to the
build directory (see B<--build-dir>).

=item --grub-preamble=I<prefix>

Specifies the I<preable> that is at the beginning of the generated
GRUB or GRUB2 config files. This is useful for specifying GRUB's
timeout.

=item --prefix=I<prefix>

Specifies I<prefix> (e.g. F</srv/tftp>) that is put in front of every
file name in generated bootloader configuration files (or in U-Boot
commands).

If the I<prefix> contains string $NAME, it will be replaced with the
name of the novaboot script (see also B<--name>).

If the I<prefix> contains string $BUILDDIR, it will be replaced with
the build directory (see also B<--build-dir>).

=item --grub-prefix

Alias for B<--prefix>.

=item --grub2[=I<filename>]

Generate GRUB2 menu entry in I<filename>. If I<filename> is not
specified F<grub.cfg> is used. The content of the menu entry can be
customized with B<--grub-preamble>, B<--grub2-prolog> or
B<--grub_prefix> options.

In order to use the the generated menu entry on your development
machine that uses GRUB2, append the following snippet to
F</etc/grub.d/40_custom> file and regenerate your grub configuration,
i.e. run update-grub on Debian/Ubuntu.

  if [ -f /path/to/nul/build/grub.cfg ]; then
    source /path/to/nul/build/grub.cfg
  fi

=item --grub2-prolog=I<prolog>

Specifies text that is put at the beginning of the GRUB2 menu entry.

=item -m, --make[=make command]

Runs C<make> to build files that are not generated by novaboot itself.

=item --name=I<string>

Use the name I<string> instead of the name of the novaboot script.
This name is used for things like a title of grub menu or for the
server directory where the boot files are copied to.

=item --no-file-gen

Do not run external commands to generate files (i.e. "<" syntax and
C<run> keyword). This switch does not influence generation of files
specified with "<<WORD" syntax.

=item -p, --pulsar[=mac]

Generates pulsar bootloader configuration file named F<config-I<mac>>
The I<mac> string is typically a MAC address and defaults to
I<novaboot>.

=item --scons[=scons command]

Runs C<scons> to build files that are not generated by novaboot
itself.

=item --strip-rom

Strip I<rom://> prefix from command lines and generated config files.
The I<rom://> prefix is used by NUL. For NRE, it has to be stripped.

=item --gen-only

Exit novaboot after file generation phase.

=back

=head2 Target connection check

If supported by the target, the connection to it is made and it is
checked whether the target is not occupied by another novaboot
user/instance.

=over 8

=item --amt=I<"[user[:password]@]host[:port]>

Use Intel AMT technology to control the target machine. WS management
is used to powercycle it and Serial-Over-Lan (SOL) for input/output.
The hostname or (IP address) is given by the I<host> parameter. If
I<password> is not specified, environment variable AMT_PASSWORD is
used. The I<port> specifies a TCP port for SOL. If not specified, the
default is 16992. Default I<user> is admin.

=item --iprelay=I<addr[:port]>

Use TCP/IP relay and serial port to access the target's serial port
and powercycle it. The IP address of the relay is given by I<addr>
parameter. If I<port> is not specified, it default to 23.

Note: This option is supposed to work with HWG-ER02a IP relays.

=item -s, --serial[=device]

Target's serial line is connected to host's serial line (device). The
default value for device is F</dev/ttyUSB0>.

The value of this option is exported in NB_NOVABOOT environment
variable to all subprocesses run by C<novaboot>.

=item --stty=I<settings>

Specifies settings passed to C<stty> invoked on the serial line
specified with B<--serial> option. If this option is not given,
C<stty> is called with C<raw -crtscts -onlcr 115200> settings.

=item --remote-cmd=I<cmd>

Command that mediates connection to the target's serial line. For
example C<ssh server 'cu -l /dev/ttyS0'>.

=item --remote-expect=I<string>

Wait for reception of I<string> after establishing the remote
connection.

=item --remote-expect-silent=I<string>

The same as B<--remote-expect> except that the remote output is not
echoed to stdout while waiting for the I<string>. Everything after the
matched string is printed to stdout, so you may want to include line
end characters in the I<string> as well.

=back

=head2 File deployment phase

In some setups, it is necessary to copy the files needed for booting
to a particular location, e.g. to a TFTP boot server or to the
F</boot> partition.

=over 8

=item -d, --dhcp-tftp

Turns your workstation into a DHCP and TFTP server so that the OS can
be booted via PXE BIOS (or similar mechanism) on the test machine
directly connected by a plain Ethernet cable to your workstation.

The DHCP and TFTP servers requires root privileges and C<novaboot>
uses C<sudo> command to obtain those. You can put the following to
I</etc/sudoers> to allow running the necessary commands without asking
for password.

 Cmnd_Alias NOVABOOT = /bin/ip a add 10.23.23.1/24 dev eth0, /bin/ip l set dev eth0 up, /usr/sbin/dhcpd -d -cf dhcpd.conf -lf dhcpd.leases -pf dhcpd.pid, /usr/sbin/in.tftpd --listen --secure -v -v -v --pidfile tftpd.pid *, /usr/bin/touch dhcpd.leases, /usr/bin/pkill --pidfile=dhcpd.pid, /usr/bin/pkill --pidfile=tftpd.pid
 your_login ALL=NOPASSWD: NOVABOOT

=item --tftp

Starts a TFTP server on your workstation. This is similar to
B<--dhcp-tftp> except that DHCP server is not started.

The TFTP server require root privileges and C<novaboot> uses C<sudo>
command to obtain those. You can put the following to I</etc/sudoers>
to allow running the necessary commands without asking for password.

 Cmnd_Alias NOVABOOT =  /usr/sbin/in.tftpd --listen --secure -v -v -v --pidfile tftpd.pid *, /usr/bin/pkill --pidfile=tftpd.pid
 your_login ALL=NOPASSWD: NOVABOOT

=item --tftp-port=I<port>

Port to run the TFTP server on. Implies B<--tftp>.

=item --netif=I<network interface>

Network interface used to deploy files to the target. Default value is
I<eth0>. This influences the configuration of the DHCP server started
by B<--dhcp-tftp> and the value that B<$NB_MYIP> get replaced with in
U-Boot conversation.

=item --iso[=filename]

Generates the ISO image that boots NOVA system via GRUB. If no filename
is given, the image is stored under I<NAME>.iso, where I<NAME> is the name
of the novaboot script (see also B<--name>).

=item --server[=[[user@]server:]path]

Copy all files needed for booting to another location. The files will
be copied (by B<rsync> tool) to the directory I<path>. If the I<path>
contains string $NAME, it will be replaced with the name of the
novaboot script (see also B<--name>).

=item --rsync-flags=I<flags>

Specifies which I<flags> are appended to F<rsync> command line when
copying files as a result of I<--server> option.

=item --concat

If B<--server> is used and its value ends with $NAME, then after
copying the files, a new bootloader configuration file (e.g. menu.lst)
is created at I<path-wo-name>, i.e. the path specified by B<--server>
with $NAME part removed. The content of the file is created by
concatenating all files of the same name from all subdirectories of
I<path-wo-name> found on the "server".

=item --ider

Use Intel AMT technology for IDE redirection. This allows the target
machine to boot from novaboot created ISO image. Implies B<--iso>.

The experimental C<amtider> utility needed by this option can be
obtained from https://github.com/wentasah/amtterm.

=back

=head2 Target power-on and reset phase

At this point, the target is reset (or switched on/off). There is
several ways how this can be accomplished. Resetting a physical target
can currently be accomplished by the following options: B<--amt>,
B<--iprelay>, B<--reset-cmd>.

=over 8

=item --on, --off

Switch on/off the target machine and exit. The script (if any) is
completely ignored. Currently it works only with B<--iprelay> or
B<--amt>.

=item -Q, --qemu[=I<qemu-binary>]

Boot the configuration in qemu. Optionally, the name of qemu binary
can be specified as a parameter.

=item --qemu-append=I<flags>

Append I<flags> to the default qemu flags (QEMU_FLAGS variable or
C<-cpu coreduo -smp 2>).

=item -q, --qemu-flags=I<flags>

Replace the default qemu flags (QEMU_FLAGS variable or C<-cpu coreduo
-smp 2>) with I<flags> specified here.

=item --reset-cmd=I<cmd>

Command that resets the target.

=item --no-reset, --reset

Disable/enable resetting of the target.

=back

=head2 Interaction with the bootloader on the target

=over 8

=item --uboot[=I<prompt>]

Interact with U-Boot bootloader to boot the thing described in the
novaboot script. I<prompt> specifies the U-Boot's prompt (default is
"=> ", other common prompts are "U-Boot> " or "U-Boot# ").
Implementation of this option is currently tied to a particular board
that we use. It may be subject to changes in the future!

=item --no-uboot

Disable U-Boot interaction previously enabled with B<--uboot>.

=item --uboot-init

Command(s) to send the U-Boot bootloader before loading the images and
booting them. This option can be given multiple times. After sending
commands from each option novaboot waits for U-Boot I<prompt>.

If the command contains string I<$NB_MYIP> then this string is
replaced by IPv4 address of eth0 interface (see also B<--netif>).
Similarly I<$NB_PREFIX> is replaced with prefix given by B<--prefix>.

See also C<uboot> keyword in L</"NOVABOOT SCRIPT SYNTAX">).

=item --uboot-addr I<name>=I<address>

Load address of U-Boot's C<tftpboot> command for loading I<name>,
where name is one of I<kernel>, I<ramdisk> or I<fdt> (flattened device
tree).

The default addresses are ${I<name>_addr_r}, i.e. U-Boot environment
variables used by convention for this purpose.

=item --uboot-cmd=I<command>

Specifies U-Boot command used to execute the OS. If the command
contains strings C<$kernel_addr>, C<$ramdisk_addr>, C<$fdt_addr>,
these are replaced with the addresses configured with B<--uboot-addr>.

The default value is

    bootm $kernel_addr $ramdisk_addr $fdt_addr

or the C<UBOOT_CMD> variable if defined in the novaboot script.

=back

=head2 Target interaction phase

In this phase, target's serial output is redirected to stdout and if
stdin is a TTY, it is redirected to the target's serial input allowing
interactive work with the target.

=over 8

=item --exiton=I<string>

When I<string> is sent by the target, novaboot exits. This option can
be specified multiple times, in which case novaboot exits whenever
either of the specified strings is sent.

If I<string> is C<-re>, then the next B<--exiton>'s I<string> is
treated as regular expression. For example:

    --exiton -re --exiton 'error:.*failed'

=item --exiton-re=I<regex>

The same as --exiton -re --exiton I<regex>.

=item --exiton-timeout=I<seconds>

By default B<--exiton> waits for the string match forever. When this
option is specified, "exiton" timeouts after the specifies number of
seconds and novaboot returns non-zero exit code.

=item -i, --interactive

Setup things for interactive use of target. Your terminal will be
switched to raw mode. In raw mode, your system does not process input
in any way (no echoing of entered characters, no interpretation
special characters). This, among others, means that Ctrl-C is passed
to the target and does no longer interrupt novaboot. Use "~~."
sequence to exit novaboot.

=item --no-interaction, --interaction

Skip resp. force target interaction phase. When skipped, novaboot exits
immediately when boot is initiated.

=item --expect=I<string>

When I<string> is received from the target, send the string specified
with the subsequent B<--send*> option to the target.

=item --expect-re=I<regex>

When target's output matches regular expression I<regex>, send the
string specified with the subsequent B<--send*> option to the target.

=item --expect-raw=I<perl-code>

Provides direct control over Perl's Expect module.

=item --send=I<string>

Send I<string> to the target after the previously specified
B<--expect*> was matched in the target's output. The I<string> may
contain escape sequences such as "\n".

Note that I<string> is actually interpreted by Perl, so it can contain
much more that escape sequences. This behavior may change in the
future.

Example: C<--expect='login: ' --send='root\n'>

=item --sendcont=I<string>

Similar to B<--send> but continue expecting more input.

Example: C<--expect='Continue?' --sendcont='yes\n'>

=item --final-eol, --no-final-eol

By default, B<novaboot> always prints an end-of-line character at the
end of its execution in order to ensure that the output of programs
started after novaboot appears at the beginning of the line. When this
is not desired B<--no-final-eol> option can be used to override this
behavior.

=back

=head1 NOVABOOT SCRIPT SYNTAX

The syntax tries to mimic POSIX shell syntax. The syntax is defined
with the following rules.

Lines starting with "#" and empty lines are ignored.

Lines that end with "\" are concatenated with the following line after
removal of the final "\" and leading whitespace of the following line.

Lines of the form I<VARIABLE=...> (i.e. matching '^[A-Z_]+=' regular
expression) assign values to internal variables. See L</VARIABLES>
section.

Lines starting with C<load> keyword represent modules to boot. The
word after C<load> is a file name (relative to the build directory
(see B<--build-dir>) of the module to load and the remaining words are
passed to it as the command line parameters.

When the C<load> line ends with "<<WORD" then the subsequent lines
until the line containing solely WORD are copied literally to the file
named on that line. This is similar to shell's heredoc feature.

When the C<load> line ends with "< CMD" then command CMD is executed
with F</bin/sh> and its standard output is stored in the file named on
that line. The SRCDIR variable in CMD's environment is set to the
absolute path of the directory containing the interpreted novaboot
script.

Lines starting with C<run> keyword contain shell commands that are run
during file generation phase. This is the same as the "< CMD" syntax
for C<load> keyboard except that the command's output is not
redirected to a file. The ordering of commands is the same as they
appear in the novaboot script.

Lines starting with C<uboot> represent U-Boot commands that are sent
to the target if B<--uboot> option is given. Having a U-Boot line in
the novaboot script is the same as passing an equivalent
B<--uboot-init> option to novaboot. The C<uboot> keyword can be
suffixed with timeout specification. The syntax is C<uboot:Ns>, where
C<N> is the whole number of seconds. If the U-Boot command prompt does
not appear before the timeout, novaboot fails. The default timeout is
10 seconds.

Example (Linux):

  #!/usr/bin/env novaboot
  load bzImage console=ttyS0,115200
  run  make -C buildroot
  load rootfs.cpio < gen_cpio buildroot/images/rootfs.cpio "myapp->/etc/init.d/S99myapp"

Example (NOVA User Land - NUL):

  #!/usr/bin/env novaboot
  WVDESC=Example program
  load bin/apps/sigma0.nul S0_DEFAULT script_start:1,1 \
                           verbose hostkeyb:0,0x60,1,12,2
  load bin/apps/hello.nul
  load hello.nulconfig <<EOF
  sigma0::mem:16 name::/s0/log name::/s0/timer name::/s0/fs/rom ||
  rom://bin/apps/hello.nul
  EOF

This example will load three modules: F<sigma0.nul>, F<hello.nul> and
F<hello.nulconfig>. sigma0 receives some command line parameters and
F<hello.nulconfig> file is generated on the fly from the lines between
C<<<EOF> and C<EOF>.

=head2 VARIABLES

The following variables are interpreted in the novaboot script:

=over 8

=item BUILDDIR

Novaboot chdir()s to this directory before file generation phase. The
directory name specified here is relative to the build directory
specified by other means (see L</--build-dir>).

=item EXITON

Assigning this variable has the same effect as specifying L</--exiton>
option.

=item HYPERVISOR_PARAMS

Parameters passed to hypervisor. The default value is "serial", unless
overridden in configuration file.

=item KERNEL

The kernel to use instead of the hypervisor specified in the
configuration file with the C<$hypervisor> variable. The value should
contain the name of the kernel image as well as its command line
parameters. If this variable is defined and non-empty, the variable
HYPERVISOR_PARAMS is not used.

=item NO_BOOT

If this variable is 1, the system is not booted. This is currently
only implemented for U-Boot bootloader where it is useful for
interacting with the bootloader without booting the system - e.g. for
flashing.

=item QEMU

Use a specific qemu binary (can be overridden with B<-Q>) and flags
when booting this script under qemu. If QEMU_FLAGS variable is also
specified flags specified in QEMU variable are replaced by those in
QEMU_FLAGS.

=item QEMU_FLAGS

Use specific qemu flags (can be overridden with B<-q>).

=item UBOOT_CMD

See L</--uboot-cmd>.

=item WVDESC

Description of the WvTest-compliant program.

=item WVTEST_TIMEOUT

The timeout in seconds for WvTest harness. If no complete line appears
in the test output within the time specified here, the test fails. It
is necessary to specify this for long running tests that produce no
intermediate output.

=back

=head1 CONFIGURATION FILES

Novaboot can read its configuration from one or more files. By
default, novaboot looks for files in F</etc/novaboot.d>, file
F<~/.config/novaboot> and files named F<.novaboot> as described in
L</Configuration reading phase>. Alternatively, configuration file
location can be specified with the B<-c> switch or with the
NOVABOOT_CONFIG environment variable. The configuration file has Perl
syntax (i.e. it is better to put C<1;> as the last line) and should set
values of certain Perl variables. The current configuration can be
dumped with the B<--dump-config> switch. Some configuration variables
can be overridden by environment variables (see below) or by command
line switches.

Supported configuration variables include:

=over 8

=item $builddir

Build directory location relative to the location of the configuration
file.

=item $default_target

Default target (see below) to use when no target is explicitly
specified with the B<--target> command line option or
B<NOVABOOT_TARGET> environment variable.

=item %targets

Hash of target definitions to be used with the B<--target> option. The
key is the identifier of the target, the value is the string with
command line options. For instance, if the configuration file contains:

 $targets{'mybox'} = '--server=boot:/tftproot --serial=/dev/ttyUSB0 --grub',

then the following two commands are equivalent:

 ./myos --server=boot:/tftproot --serial=/dev/ttyUSB0 --grub
 ./myos -t mybox

=back

=head1 ENVIRONMENT VARIABLES

Some options can be specified not only via config file or command line
but also through environment variables. Environment variables override
the values from configuration file and command line parameters
override the environment variables.

=over 8

=item NOVABOOT_CONFIG

Name of the novaboot configuration file to use instead of the default
one(s).

=item NOVABOOT_CONFIG_DIR

Name of the novaboot configuration directory. When not specified
F</etc/novaboot.d> is used.

=item NOVABOOT_TARGET

Name of the novaboot target to use. This overrides the value of
B<$default_target> from the configuration file and can be overriden
with the B<--target> command line option.

=item NOVABOOT_BENDER

Defining this variable has the same meaning as B<--bender> option.

=back

=head1 AUTHORS

Michal Sojka <sojka@os.inf.tu-dresden.de>