What is the equivalent of 'update-grub' for RHEL, Fedora, and CentOS systems?

After analyzing the scripts in Fedora, I realize that the configuration file path is read from the symlink /etc/grub2.conf. The correct grub2-mkconfig line is thus:

grub2-mkconfig -o "$(readlink -e /etc/grub2.conf)"

As noted in comments, it might be /etc/grub2.cfg, or /etc/grub2-efi.cfg on a UEFI system. Actually, both links might be present at the same time and pointing to different locations. The -e flag to readlink will error out if the target file does not exist, but on my system both existed… Check your commands, I guess.

Edit the GRUB configuration file with vi or vim, save the changes and close the editor with :wq!.

This is what you need to run to update the GRUB configuration in RHEL or CentOS:

grub2-mkconfig -o /boot/grub2/grub.cfg

On Fedora I use:

grub2-mkconfig -o "$(readlink -e /etc/grub2.cfg)"

because executing with no option to readlink returns a relative path, and grub2-mkconfig gives an error:

$ ls -l /etc/grub2.cfg
lrwxrwxrwx. 1 root root 22 Dec 10  2015 /etc/grub2.cfg -> ../boot/grub2/grub.cfg

$ readlink /etc/grub2.cfg
../boot/grub2/grub.cfg

$ sudo grub2-mkconfig -o "$(readlink /etc/grub2.cfg)"
/usr/sbin/grub2-mkconfig: line 244: ../boot/grub2/grub.cfg.new: No such file or directory

I use the -e option so that if the symlink doesn’t resolve to a file that exists, output displays on stdout so I know something went wrong.

From the man page for readlink:

      -e, --canonicalize-existing
          canonicalize  by  following  every symlink in every component of
          the given name recursively, all components must exist

The update-grub script in Ubuntu is actually just a stub for grub-mkconfig, and it can be adapted to other distros without too much pain. Here it is in its entirety:

#!/bin/sh
set -e
exec grub-mkconfig -o /boot/grub/grub.cfg "$@"

This effectively does what is recommended in the CentOS wiki, and in other answers here – the only difference is that you need to change grub to grub2 in the command and the output path.

(Note that the path will be different for UEFI-based systems – see the wiki link)

In Fedora, the /etc/grub2.cfg symlink points at the BIOS version. On a UEFI system, use:

$ sudo grub2-mkconfig -o /boot/efi/EFI/fedora/grub.cfg

In CentOS:

$ sudo grub2-mkconfig -o /boot/efi/EFI/centos/grub.cfg

Generally, you could use instead:

$ sudo grub2-mkconfig -o $(readlink -f /etc/grub2-efi.cfg)

See https://docs.fedoraproject.org/en-US/fedora/f27/system-administrators-guide/kernel-module-driver-configuration/Working_with_the_GRUB_2_Boot_Loader/index.html#sec-Editing_a_Menu_Entry

This worked on CentOS 8:

sudo grub2-mkconfig --output=/boot/grub2/grub.cfg

On Fedora 32:

sudo grub2-mkconfig -o /boot/efi/EFI/fedora/grub.cfg

Per the RedHat documentation:

Changes to /etc/default/grub require rebuilding the grub.cfg file as
follows:

• On BIOS-based machines, issue the following command as root:

  ~]# grub2-mkconfig -o /boot/grub2/grub.cfg

• On UEFI-based machines, issue the following command as root:

  ~]# grub2-mkconfig -o /boot/efi/EFI/redhat/grub.cfg

In the Fedora 33 guidelines, it gives the following after changing the GRUB configuration:

sudo grub2-mkconfig --output=/boot/grub2/grub.cfg

Boot Loader Specification and blscfg in GRUB

If not, how do the system knows where is the grub configuration file to update when a new kernel version is installed?

Until a few hours ago this was also my understanding of how things work with GRUB. An Oracle Linux 8.4 system we have here just taught me something new. I was a bit irritated staring at the "empty" section where I expected to find the configuration that builds the list of boot entries. I stumbled across blscfg and found this article: Changes/BootLoaderSpecByDefault

HBruijn’s answer makes a lot more sense in that context, even though it was valid before.

Now, you have your grub.cfg file, which can read BLS files and you don’t have to touch grub.cfg for every kernel update. Which is great in from my point of view.

One more attempt at grub-mkconfig

Since there have been many answers posted here from different systems and configurations, let’s have one last shot at this with a regular expression.

# Search for an existing GRUB configuration file symbolic link in '/etc'.
# This should only yield one path.
grub_cfg="$(find "/etc" -maxdepth=1 -regextype posix-egrep 
 -regex "/etc/grub2((|-efi).cfg|.conf)")"

# Resolve symbolic link and generate new grub configuration
sudo grub2-mkconfig -o "$(sudo readlink -e $grub_cfg)"