Using grub2 to recover your system

grub2 is hai­led as the all new, super modu­lar cure-all remedy for all boo­ting pro­blem you’ve had, have and will have. At least that’s the way the deve­l­o­pers and some enthu­si­asts see it, whe­reas most blo­kes who’ve actually had to use it with more than arrow keys and enter will paint a slightly dif­fe­rent picture.

The thing with grub2 is that even though in theory it sounds like the end of all things boo­ting, it’s about as well-documented as the ques­tion for life, the uni­verse, and everything.

And as I today had to try to fight my way through goog­ling for necessary infor­ma­tion again, I’d thought I’d create a quick step-by-step refe­rence with all the most inte­res­ting bits you’ll ever need alre­ady there.

Thusly, the ingre­dients nee­ded to resur­rect your com­pu­ter with grub2. The gist is that you have the goal of boo­ting one spe­ci­fic ope­ra­ting sys­tem on your com­pu­ter, from wher­ein which you’ll use wha­te­ver methods you deem necessary to update your grub in the “right way” — usually a down­grade to an older ver­sion and wait­ing for the dust to blow over.

1. A boo­ting grub2. If your grub2 alre­ady fails to boot because of some ran­dom error, you need to get a grub in smel­ling dis­tance of your BIOS. One of the most pro­ven methods is to
   1. Down­load a USB res­cue image like grml (usually from Your Other Com­pu­ter or that of some­body else)
   2. Put it on an USB stick (dd if=grml-variant_version.iso of=/dev/sdx in most cases, with appro­pria­tely cho­sen variables)
   3. (Re)boot, even­tually adjus­ting the prio­rity for your USB HDD/USB key

   And that’s it, you’re in a grub. Also note that it’s recom­men­da­ble to have an USB stick with a res­cue image lying around for the times when you can’t just easily down­load it.

2. Enter the com­mand line/shell mode by pres­sing ‘c’.
3. Do an ‘ls’, which will give you a lis­ting of reco­gnized devices. Doing an ‘ls device’, e.g. ls (hd0,1) will give you more infor­ma­tion about that device.
4. If the infor­ma­tion by your ls isn’t com­plete, you will have to load some modu­les (by using insmod modulename). Here’s a checklist:
   1. If you do not see any other devices which look like your hard drive(s), e.g. you only have an (hd0) device from your USB medium, then load a device dri­ver. They will allow you to find the actual devices. Exam­ples include:
      • bios­disk
      • scsi
      • fs_uuid
      • pci
      • raid
      • mdraid
      • dm_nv
   2. If you have devices, but no par­ti­ti­ons, you’ll need a par­ti­tion dri­ver. It seems the default grub con­fig does not load any par­ti­tion dri­ver, and debug­ging this is just a bit annoy­ing. But there’s two easy choices for most people:
      • Load the module “part_msdos”.
      • If this doesn’t help, try “part_gpt”.

      These are the two most com­mon par­ti­tion tables (at least for next to ever­yone rea­ding this guide in need) and should help your grub find its par­ti­ti­ons again.

   3. Even­tually, you will also have to load your file­sys­tem dri­vers. I pre­sume you alre­ady know which those are, but for the sake of completion:
      • Almost all Linux use ext2
      • Most cur­rent Win­dows will use ntfs, but fat is also an option.
      • Mac users will use hfsplus for newer sys­tems, hfs for older ones.

   The next step depends on exactly what you want to do. There’s a fork in the road — if you just want to load your pre­viously unboo­ta­ble grub, you will try to load its con­fi­gu­ra­tion file, else you’ll try to boot your ope­ra­ting sys­tem kernel.

   4. To search for a file, you use the search -f filename com­mand, which will give you results on where files of that name are stored. Use root device to set the resul­tant device as the root device for your fur­ther ope­ra­ti­ons. If you only want to load your old grub con­fig, type in configfile filename, whe­reas filename will usually be some­thing like /grub/grub.cfg or /boot/grub/grub.cfg.
   5. Should this fail to resolve your pro­blem, or not be what you’re aiming for, you’ll need to find the ope­ra­ting sys­tem. For most Linu­xens, you’ll pro­bably have a file cal­led /vmlinuz or /boot/vmlinuz to search for. For Win­dows ope­ra­ting sys­tems, look for /Windows/win.ini. For Mac: no clue. When found, set your root device (with root device).
   6. Now methods will become diver­gent, as ope­ra­ting sys­tems dif­fer in the way of boo­ting them.

      Linux

      1. kernel kernel_filename
      2. initrd initrd_filename [most cur­rent ker­nels come with an “initial ram­disk” hol­ding modu­les etc.]
      3. boot — if all goes well, you’re set.

      Win­dows

      1. chainloader +1
      2. boot

      MacOS

      Pro­bably the same as Win­dows, using the chainloader.

   And that’s it. It should cover most cases you’d need to res­tore your capa­bi­lity of boo­ting your ope­ra­ting sys­tem. You’ll pro­bably want to fix/install your boot­loa­der after this, though.

   A hel­pful tool for debug­ging your cur­rent grub state is probe, which will allow you to check what dri­vers are assi­gned to devices.

   Filed under FAQs, Publications · Tagged with boot, grub, HOWTO

   About towo
   towo has been writing stuff on the Internet on and off for years. He also thinks that author blurbs are silly.