These are simple utils for debian-installer:

mapdevfs: Given a devfs device name, returns the name that would be used on
a non devfs system. Only works for some devices, used by eg, boot loader
installers.

log-output: Runs a command, logging any stdout/stderr output to the syslog,
and preserving the command's exit code. If you use the --pass-stdout option,
it will pass stdout through rather than logging it, so that you can redirect
it to a file or pipe it to another process. Note that the command must be
in the path, shell builtins won't work.

anna-install: Each parameter is a udeb that anna should install. If this
command is run before anna has configured a default retriever, the udebs
will be queued for the future anna run. If it's run after a default
retriever is configured, the specified udebs will be installed immediately.

apt-install: The parameter is one or more debs that should be installed into
/target. If the /target environment is not yet set up, the package will be
queued for later installation (at the end of base-installation).
If /target is already set up, the package will be installed straightaway.
Whether Recommends are installed by default depends on the value of the
template base-installer/install-recommends.
The command accepts the following options, which get preserved if packages
are queued for later installation:
* --no-recommends: don't install Recommends, even if that's the default;
* --with-recommends: force installation of Recommends;
* --allow-remove: allow other packages to be removed if necessary.
apt-install makes use of in-target and debconf-apt-progress to allow
configuration questions to be asked and to allow media changes if multiple
CD/DVD sources have been set up.
In case multiple CD/DVD sources are defined in the sources.list and the
installation CD is not already unmounted, apt-install will unmount it
in the D-I environment for the duration of the package installation so as not
to block CD changes by apt in the target environment. Afterwards, if it was
mounted at the start, it will ensure the installation CD/DVD is reloaded.

in-target: Runs the specified command in /target and returns its exit
status. The debconf passthrough frontend is used to make debconf questions
be asked using cdebconf in the installer. This is especially useful for
running things like dpkg-reconfigure, debconf-apt-progress, and tasksel.
The log-output utility is used to log any output; if in-target is called
with the option --pass-stdout, log-output will respect it.

Both apt-install and in-target will by default use the locale selected for
the target system. The environment variable IT_LANG_OVERRIDE can be set to
override this. This is for example needed to avoid locale errors when the
locales package has not yet been installed in the target environment

debconf-get: Prints out the value of a debconf question.

debconf-set: Sets a debconf question to a value.

fetch-url:   fetch-url [-c|-r] <url> <file> [checksum]
Grabs the url and puts it in the file. If the optional checksum is
specified, check that it matches and if not, return an error.
It uses methods for each protocol that it finds in /usr/lib/fetch-url/
(this allows for new protocols to be added).  It returns 0 on success,
non-zero otherwise, although the return codes depend on the error from
the underlying methods, so specific values should not be relied upon
at present.
fetch-url supports the use of `base64` as a fake protocol, which allows one to
specify data in-line (e.g. base64://SGVsbG8gV29ybGQhCg==) which can be useful to
when only a URL pointing to a file is accepted, such as
the keyfile for a repo in the local repos (apt-setup/local<N>/key).
The options modify its behaviour thus (for those methods where they
make sense -- ftp & http currently -- otherwise they're ignored):

  -r	Retry.  For retryable errors on methods that are worth
	retrying (i.e. http/ftp downloads), will repeat the download
	attempt once.

  -c	Continuation Retry.  Likewise, retries, but will attempt to
	continue (i.e. with wget's -c) the download, and will try up
	to 3 times after the initial attempt.
	Only use this if you're downloading something that subsequently
	gets checksummed, as rumour has it that busybox's wget is not
	very bright when it comes to continuing partial downloads.

  It's probably best not to combine these two options, as they're
  currently implemented in nested loops, and the combination will retry
  too many times, and alternate between full and continuation attempts
  in a senseless manner.

register-module: Register a module to go in /etc/modules on the target. May
be called before the target is mounted. Can optionally pass parameters that
should be passed to the module on load; these are added to the appropriate
files in /etc to make modprobe use them. If the -p parameter is passed then
the module is not added to /etc/modules and only the specified parameters
are set for it. The -t parameter is used to specify the type of options
(currently only the default type "options" is used).
The -a parameter makes register-module add a param, without removing any
other params that might have been previously registered.
The -i parameter makes the module be added to the initrd used to boot the
installed system, and loaded during boot. If the script is called with the
-b parameter, the module will be blacklisted.

user-params: Output the parameters the user passed to the kernel on boot,
one per line. This should include only parameters the user entered by hand,
and will skip over parameters used to pre-seed debconf values or set kernel
module-specific parameters. For any of the user's parameters to be
recognised, the kernel command line must include "--" in it, and then all
parameters after the -- are assumed to be user input. Therefore, the
installer's boot loader should pass "--" at the end of its kernel command
line to make this work.

debconf-disconnect: Scripts running under the control of debconf can use
this to execute a command disconnected from debconf, with its standard file
descriptors the same as those of the debconf frontend itself. This is most
useful for starting interactive shells.

update-dev: Make sure that new device nodes have been created after loading
modules.  It is also important for udev because there may be an asynchronous
delay between loading the module and udev getting around to creating the
device node. This allows a process to "wait" until all udev actions have
completed.

search-path: Returns true if a program is found in the path (d-i has no
"which" command); this is deprecated, and new code should simply use
"type PROGRAM >/dev/null 2>&1" instead.

list-devices: List device nodes representing devices of various types,
handling both devfs and udev naming schemes.

block-attr: Get attributes of a block device, using whichever of vol_id or
blkid is available.