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                 module-setup.sh
   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 more generic module.
  20
  21 module-setup.sh:
  22          dracut sources this script to install the functionality that a
  23          module implements onto the initrd.  For the most part, this amounts
  24          to copying files from the host system onto the initrd in a controlled
  25          manner.
  26
  27 install():
  28          This function of module-setup.sh is called to install all
  29          non-kernel files. dracut supplies several install functions that are
  30          specialized for different file types.  Browse through dracut-functions
  31          fore more details.  dracut also provides a $moddir variable if you
  32          need to install a file from the module directory, such as an initrd
  33          hook, a udev rule, or a specialized executable.
  34
  35 installkernel():
  36          This function of module-setup.sh is called to install all
  37          kernel related files.
  38
  39
  40 check():
  41        dracut calls this function to check and see if a module can be installed
  42        on the initrd.
  43
  44        When called without options, check should check to make sure that
  45        any files it needs to install into the initrd from the host system
  46        are present.  It should exit with a 0 if they are, and a 1 if they are
  47        not.
  48
  49        When called with $hostonly set, it should perform the same check
  50        that it would without it set, and it should also check to see if the
  51        functionality the module implements is being used on the host system.
  52        For example, if this module handles installing support for LUKS
  53        encrypted volumes, it should return 0 if all the tools to handle
  54        encrpted volumes are available and the host system has the root
  55        partition on an encrypted volume, 1 otherwise.
  56
  57 depends():
  58        This function should output a list of dracut modules
  59        that it relies upon.  An example would be the nfs and iscsi modules,
  60        which rely on the network module to detect and configure network
  61        interfaces.
  62
  63 Any other files in the module will not be touched by dracut directly.
  64
  65 You are encouraged to provide a README that describes what the module is for.
  66
  67
  68 HOOKS
  69 =====
  70
  71 init has the following hook points to inject scripts:
  72
  73 /lib/dracut/hooks/cmdline/*.sh
  74    scripts for command line parsing
  75
  76 /lib/dracut/hooks/pre-udev/*.sh
  77    scripts to run before udev is started
  78
  79 /lib/dracut/hooks/pre-trigger/*.sh
  80    scripts to run before the main udev trigger is pulled
  81
  82 /lib/dracut/hooks/initqueue/*.sh
  83    runs in parallel to the udev trigger
  84    Udev events can add scripts here with /sbin/initqueue.
  85    If /sbin/initqueue is called with the "--onetime" option, the script
  86    will be removed after it was run.
  87    If /lib/dracut/hooks/initqueue/work is created and udev >= 143 then
  88    this loop can process the jobs in parallel to the udevtrigger.
  89    If the udev queue is empty and no root device is found or no root
  90    filesystem was mounted, the user will be dropped to a shell after
  91    a timeout.
  92    Scripts can remove themselves from the initqueue by "rm $job".
  93
  94 /lib/dracut/hooks/pre-mount/*.sh
  95    scripts to run before the root filesystem is mounted
  96    Network filesystems like NFS that do not use device files are an
  97    exception. Root can be mounted already at this point.
  98
  99 /lib/dracut/hooks/mount/*.sh
 100    scripts to mount the root filesystem
 101    If the udev queue is empty and no root device is found or no root
 102    filesystem was mounted, the user will be dropped to a shell after
 103    a timeout.
 104
 105 /lib/dracut/hooks/pre-pivot/*.sh
 106    scripts to run before latter initramfs cleanups
 107
 108 /lib/dracut/hooks/cleanup/*.sh
 109    scripts to run before the real init is executed and the initramfs
 110    disappears
 111    All processes started before should be killed here.
 112