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I have dual booted my machine with centos-7 with already installed windows-8 64-bit by default.

If I power on my machine then windows-8 is booting directly without any boot screen.

If I want to boot centos-7 then I need to go to boot options and there I need to select centos7 manually.

When I restart my machine then again windows-8 is booting automatically either I'm in centos or in windows. If I want to restart my centos, after pressing restart button windows is booting automatically. If I want to restart my windows, after pressing restart button windows is booting automatically.

I'm not getting any boot screen which shows the list of operating systems with black background screen.

centos is working fine, windows as well.

This is my problem and I'm not so aware of these things but I have little knowledge in this.

How can I get the boot screen to select or switch between operating systems ? Please help me...

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I guess you installed grub while you installed centos. So, boot into centos and configure your grub. Here you find some help:

Equivalent of update-grub for RHEL/Fedora/CentOS systems?

I think you are searching something like timeout=15 in the grub.conf...

PS: I am not sure if I got you right. I hope I catched the problem...

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As there's not enough info provided I will do a bit of a guess work here and say you have 2 physical HDDs and you have Win8 on 1st and Centos7 on 2nd. You're able to select from which HDD to boot from when in BIOS but if you don't - the Win8 from 1st HDD boots automatically. This is because your 1st HDD (containing Win8) is the default boot device in BIOS. To fix your issue - boot to CentOS7 and list your block storage devices via lsblk and sudo blkid - identify the Win8 HDD (probably /dev/sda) sudo grub2-install /dev/sda (this installs it in the mbr).

TL;DR - boot to Centos and do sudo grub2-install /dev/sda

alternatively skip this all together and select 2nd HDD as a primary boot disk :)

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