README.modules

   1 Most of the functionality that dracut implements are actually implemented
   2 by dracut modules.  Dracut modules live in modules.d, and have the following
   3 structure:
   4
   5 dracut_install_dir/modules.d/
   6         00modname/
   7                 install
   8                 check
   9                 <other files as needed by the hook>
  10
  11 00modname: The name of the module prefixed by a two-digit numeric sort code.
  12            The numeric code must be present and in the range of 00 - 99.
  13            Modules with lower numbers are installed first.  This is important
  14            because the dracut install functions (which install files onto
  15            the initrd) refuse to overwrite already installed files. This makes
  16            it easy for an earlier module to override the functionality of a
  17            later module, so that you can have a distro or system specific
  18            module override or modify the functionality of a generic module
  19            without having to patch the mode generic module.
  20
  21 install: dracut sources this script to install the functionality that a
  22          module implements onto the initrd.  For the most part, this amounts
  23          to copying files from the host system onto the initrd in a controlled
  24          manner.  dracut supplies several install functions that are
  25          specialized for different file types.  Browse through dracut-functions
  26          fore more details.  dracut also provides a $moddir variable if you
  27          need to install a file from the module directory, such as an initrd
  28          hook, a udev rule, or a specialized executable.
  29
  30 check: Dracut calls this program to check and see if a module can be installed
  31        on the initrd.
  32
  33        When called without options, check should check to make sure that
  34        any files it needs to install into the initrd from the host system
  35        are present.  It should exit with a 0 if they are, and a 1 if they are
  36        not.
  37
  38        When called with -H, it should perform the same check that it would
  39        without any options, and it should also check to see if the
  40        functionality the module implements is being used on the host system.
  41        For example, if this module handles installing support for LUKS
  42        encrypted volumes, it should return 0 if all the tools to handle
  43        encrpted volumes are available and the host system has the root
  44        partition on an encrypted volume, 1 otherwise.
  45
  46        Check may take additional options in the future.
  47        We will most likely grow a module dependency checking system in the
  48        near future, and check will need to handle a -d option when we do.
  49
  50 Any other files in the module will not be touched by dracut directly.
  51
  52 You are encouraged to provide a README that descrobes what the module is for.