In Ubuntu (and I guess in Debian too) there is a system script named update-grub which automatically executes grub-mkconfig -o with the correct path for the GRUB configuration file.
Is there a similar command for Red Hat-based distributions?
If not, how does the system know where the GRUB configuration file is to update when a new kernel version is installed?
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.
Specific actions that need to happen when a RPM package is installed or removed are included within the RPM package itself in pre-install, post-install, pre-uninstall and post-uninstall sections.
For every installed RPM package you can query the RPM database for the exact scripts that are included with the rpm command:
rpm -q --scripts <package-name>
Running that command on a kernel package for CentOS 6 returns among others:
postinstall scriptlet (using /bin/sh):
<snip>
/sbin/new-kernel-pkg --package kernel --install 2.6.32-431.17.1.el6.x86_64 || exit $?
From the manual:
new-kernel-package- tool to script kernel installation
/bin/kernel-install instead. Please edit your answer to include this information for future reference
– pqnet
Aug 26 '14 at 11:26
rpm -q --scripts kernel. The output wasn't that easy to decipher, but it's nice to have a definitive way to find the answer.
– mwfearnley
Oct 21 '21 at 09:08
On Fedora I use:
grub2-mkconfig -o "$(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
readlink part will run withoud priviledges and can't read the symlink as intended.) You'll need to run the whole thing in a root shell, for example sudo -i will give you one. It may still not be the right path on EFI systems though.
– dreua
Apr 08 '23 at 01:09
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 $(readlink -f /etc/grub2-efi.cfg)
Per the RedHat documentation:
Changes to
/etc/default/grubrequire rebuilding thegrub.cfgfile as follows:
On BIOS-based machines, issue the following command as root:
~]# grub2-mkconfig -o /boot/grub2/grub.cfgOn UEFI-based machines, issue the following command as root:
~]# grub2-mkconfig -o /boot/efi/EFI/redhat/grub.cfg
In the Fedora 33 guidelines, it gives the following after changing the GRUB configuration:
sudo grub2-mkconfig --output=/boot/grub2/grub.cfg
Edit the GRUB configuration file with vi or vim, save the changes and close the editor with :wq!.
This is what you need to run to update the GRUB configuration in RHEL or CentOS:
grub2-mkconfig -o /boot/grub2/grub.cfg
The update-grub script in Ubuntu is actually just a stub for grub-mkconfig, and it can be adapted to other distros without too much pain. Here it is in its entirety:
#!/bin/sh
set -e
exec grub-mkconfig -o /boot/grub/grub.cfg "$@"
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)
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.
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 $grub_cfg)"
/etc/grub2.cfg– Anthon Dec 22 '14 at 13:28sudo grub2-mkconfig -o "$(readlink /etc/grub2-efi.cfg)"– Craig Ringer Jun 15 '19 at 04:01