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Daren Eiri

Trained in biological research, but working in IT

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This post assumes that Windows was installed first, and then CentOS was installed second.

Once you install CentOS 7 alongside your Windows OS, you may find that you cannot boot into Windows. The Grub bootloader may only show your Linux OS as your only options to boot from.

centos7 with grub2

To fix this and have the Grub bootloader list your Windows OS, you need to edit the Grub bootloader files. If you have used CentOS is the past (with 6 or earlier), you may find that editing Grub is different. Previously, you would edit /boot/grub/grub.conf. This is no longer the case, as the grub2.cfg file is generated dynamically, based on dependency files. Here’s what you need to edit to configure your bootloader.

  1. Boot into CentOS 7, if you haven’t already.
  2. Determine what partition your Windows OS resides on by running sudo fdisk -l in Terminal. Here’s my output:
     Disk /dev/sda: 320.1 GB, 320072933376 bytes, 625142448 sectors
     Units = sectors of 1 * 512 = 512 bytes
     Sector size (logical/physical): 512 bytes / 512 bytes
     I/O size (minimum/optimal): 512 bytes / 512 bytes
     Disk label type: dos
     Disk identifier: 0xcd8b1219
    
        Device Boot      Start         End      Blocks   Id  System
     /dev/sda1   *        2048     4194303     2096128    7  HPFS/NTFS/exFAT
     /dev/sda2         4194304   360402758   178104227+   7  HPFS/NTFS/exFAT
     /dev/sda3       360402942   625141759   132369409    5  Extended
     /dev/sda5       612595712   625141759     6273024   82  Linux swap / Solaris
     /dev/sda6       360407040   361431039      512000   83  Linux
     /dev/sda7       361433088   612589567   125578240   8e  Linux LVM
     
         + In this example, /dev/sda1 is the recovery partition, and /dev/sda2 is the Windows OS partition. Since partition indexes start at zero, the Windows OS partition will be `hd0,1` (a = 0, 2 = 1; or first disk, second partition) when we edit the Grub file. Make note of this.
    
  3. Open a terminal and navigate to /etc/grub.d/:
     cd  /etc/grub.d/
     
  4. Edit the 40_custom file. You may not see the file if you ls in /grub.d/. That’s okay.
       sudo nano 40_custom
     
  5. You should see the following in the nano text editor:
     #!/bin/sh
     exec tail -n +3 $0
     # This file provides an easy way to add custom menu entries.  Simply type the
     # menu entries you want to add after this comment.  Be careful not to change
     # the 'exec tail' line above.
     
  6. Below the last #, type on a new line:
     menuentry "Windows 7" {
             set root=(hd0,1)
             chainloader +1
             }
     
  7. Finally, run the following to apply the changes to the grub.cfg file:
     grub2-mkconfig --output=/boot/grub2/grub.cfg
     
    • Once you reboot, you should see the option of booting into Windows 7. If a default boot entry into Windows (or something else) is requested, then you need to edit the GRUB_DEFAULT in /etc/default/grub:
       GRUB_DEFAULT="Windows 7"