Troubleshooting boot issues in Linux

Missing boot loader	Error:      - this means your boot loader cannot be found   - it might be removed (rm -fr /boot/grub2)   - or corrupted (dd if=/dev/zero of=/dev/sda bs=1 count=446) Solution: 1. Insert installation disc and boot into rescue mode 2. Acces /: # chroot /mnt/sysimage 3. Rebuild grub: # grub2-install /dev/sda 4. Check if grub.cfg is present, if not, recreate it: # grub2-mkconfig -o /boot/grub2/grub.cfg 5. # exit 6. # exit
Missing initramfs/initrd	Error: - initramfs might be deleted Solution: 1. Insert installation disc and boot into rescue mode 2. Access /: # chroot /mnt/sysimage 3. Check if initramfs is really gone: # cd /boot && ls -l *initramfs* 4. Build an initramfs: # dracut 5. # exit 6. # exit
Missing kernel	Error:    - vmlinuz* might be deleted Solution: 1. Insert installation disc and boot into rescue mode 2. Copy kernel rpm to sysimage: # cp /run/installer/repo/Packages/kernel-3.10*rpm /mnt/sysimage/tmp 3. Switch to /: # chroot /mnt/sysimage 4. Install unpatched/base kernel: # rpm -ivh /tmp/kernel-3*rpm --force 5. # exit 6. # exit
GRUB 2 config file is missing but you don't have a rescue disc	Error: A reboot will drop you to this shell   - if GRUB cannot find the config file (grub.cfg), it will drop you down to a shell shown above   - from that shell, you will be able to enter commands interactively Solution: 1. Since we don't have a rescue disc, we cannot boot into rescue mode and recreate grub.cfg 2. So we are left with a grub shell 3. Determine what partitions holds the grub.cfg, kernel, and initrd       grub> ls (hd0)/       grub> ls (hd0,1)/       grub> ls (hd0,msdos2)/       grub> ls (hd0,msdos1)/ 4. If you got it, set it as root partition       grub> set root=(hd0,msdos1) 5. Set the path to kernel and specify location of root filesystem       grub> linux /vmlinuz-3.10.0-514.el7.x86_64 root=/dev/mapper/cl-root 6. Set the initramfs path       grub> initrd /initramfs-3.10.0-514.el7.x86_64.img 7. Boot now       grub> boot 8. Once you are able to login, you can now recreate grub.cfg       # grub2-mkconfig -o /boot/grub2/grub.cfg 9. You have now a working grub menu       # systemctl reboot
some modules are missing from the GRUB 2 folder	Error:    - this is the grub rescue shell Solution: Most of the minimal modules that is used by Grub has been corrupted so best thing to do is a rebuild grub from resuce disk.
Recreating GRUB 1 config file in RHEL 6	If /boot/grub/grub.conf is lost, there is no "grub-mkconfig" command to help you. So you need to recreate it from scratch. You can do it in several ways. * Recreating grub.conf from rescue mode * 1. Insert installation disc and boot from rescue mode 2. Go and access your files: # chroot /mnt/sysimage 3. Create grub.conf: # /boot/grub/grub.conf 4. Enter the following: default=0 timeout=5 splashimage=(hd0,0)/grub/splash.xmp.gz title Red Hat 6 root (hd0,0)   kernel /vmlinuz-2.6.0-514.el7.x86_64 root=/dev/mapper/VolGroup-lv_root   initrd /initrd-2.6.0-514.el7.x86_64.img 5. Save and quit 6. Exit chrooted environment: # exit 7. Exit rescue mode to reboot: # exit * Via grub shell method* 1. When you reboot while grub.conf is missing, you will be dropped to grub shell 2. Enter the following commands from grub shell   grub> root (hd0,0)   grub> kernel /vmlinuz-2.6.0-514.el7.x86_64 root=/dev/mapper/VolGroup-lv_root   grub> /initrd-2.6.0-514.el7.x86_64.img   grub> boot 3. Onced you are on the OS shell, recreate grub.conf   # vi /boot/grub/grub.conf   # enter the following: default=0 timeout=5 splashimage=(hd0,0)/grub/splash.xmp.gz title Red Hat 6 root (hd0,0)   kernel /vmlinuz-2.6.0-514.el7.x86_64 root=/dev/mapper/VolGroup-lv_root   initrd /initrd-2.6.0-514.el7.x86_64.img 4. Save and quit 5. Reboot
Non-existent device in /etc/fstab	Problem: If you have a non-existent entry in fstab, let's say you removed a filesystem from LVM but forgot to remove it from /etc/fstab, your next boot will look like this: This is your clue - during bootup, systemd/init will tell you that it cannot find a specific device. So you need to check fstab for non-exsiting devices. Then you will be dropped to emergency shell: Solution: 1. Provide the root password and enter 2. Check fstab for non existent devices by: # mount -a 3. Remove or comment it from fstab 4. Check again if all are OK: # mount -a 5. Reboot: # systemctl reboot
Missing /etc/grub.d contents	Reinstall grub2-tools: # yum reinstall grub2-tools -y
Wrong kernel boot parameters	These are the errors/issues you will encounter when the corresponding boot parameter is incorrect/missing # wrong root=/path/to/rootfilesystem   -> your kernel might panic (seen on centos 7)   -> or you will see dracut timing out (seen on centos 7)