When working with CentOS 6 systems, you'll notice the default kernel boot parameters include rhgb (Red Hat Graphical Boot) and quiet, which suppress boot messages. For server environments and automated deployments, we often need to remove these for better visibility into the boot process.

Unlike Debian-based systems that use /etc/default/grub, CentOS 6 handles kernel parameters differently. The key files involved are:

/boot/grub/grub.conf
/etc/grub.conf (symlink to /boot/grub/grub.conf)
/etc/sysconfig/grub

Here's a robust script to modify parameters for all current and future kernels:

#!/bin/bash

# Backup original configuration
cp /boot/grub/grub.conf /boot/grub/grub.conf.bak

# Remove rhgb and quiet parameters
sed -i 's/\brhgb\b//g; s/\bquiet\b//g' /boot/grub/grub.conf

# Ensure new kernels inherit these settings
echo 'GRUB_CMDLINE_LINUX=""' > /etc/sysconfig/grub

# Rebuild grub configuration
/sbin/grub-mkconfig -o /boot/grub/grub.conf

The key to making changes persist across kernel updates lies in the /etc/sysconfig/grub file. New kernels installed via yum will inherit settings from this file when generating their boot entries.

After running the script, verify the changes:

grep -E 'kernel|initrd' /boot/grub/grub.conf

You should see kernel lines without rhgb or quiet parameters. For a complete test, reboot the system and observe the boot messages.

If you're working with newer CentOS versions that include grubby, you can use:

grubby --update-kernel=ALL --remove-args="rhgb quiet"

When implementing this in automated deployments:

• Always create backups before modifying boot configurations
• Include verification steps in your scripts
• Handle cases where files might be symlinks

In CentOS 6, the bootloader configuration differs significantly from Debian-based systems. Unlike Ubuntu's /etc/default/grub approach, CentOS stores kernel parameters in /boot/grub/grub.conf (or menu.lst for legacy systems). The challenge is modifying this configuration programmatically for both current and future kernels.

Here's a script that permanently removes rhgb and quiet parameters while maintaining future compatibility:

#!/bin/bash
# Backup original grub.conf
cp /boot/grub/grub.conf /boot/grub/grub.conf.bak

# Remove rhgb and quiet from all kernel entries
sed -i 's/\brhgb\b//g; s/\bquiet\b//g' /boot/grub/grub.conf

# Create a persistent configuration template
if [ ! -f /etc/grub.conf.template ]; then
    cp /boot/grub/grub.conf /etc/grub.conf.template
fi

# Install script to maintain changes during kernel updates
cat << 'EOF' > /etc/kernel/postinst.d/99-update-grub
#!/bin/sh
if [ -f /etc/grub.conf.template ]; then
    cp /etc/grub.conf.template /boot/grub/grub.conf
    for kernel in /boot/vmlinuz-*; do
        version=${kernel#*-}
        if ! grep -q "initrd /boot/initrd-${version}" /boot/grub/grub.conf; then
            sed -i "/^title.*${version}/a \ \ \ \ initrd /boot/initrd-${version}" /boot/grub/grub.conf
        fi
    done
fi
EOF

chmod +x /etc/kernel/postinst.d/99-update-grub

The above solution creates a post-installation hook that maintains your configuration. For systems using GRUB2 (which CentOS 6 doesn't by default), you would need a different approach:

# For GRUB2 systems (e.g., CentOS 7+)
grubby --update-kernel=ALL --remove-args="rhgb quiet"

After implementation, verify your changes by:

grep -E 'kernel|initrd' /boot/grub/grub.conf
cat /proc/cmdline

Reboot and check the kernel command line parameters are indeed removed.

While not directly related to boot parameters, some settings can be adjusted via sysctl for runtime configuration:

# Example of persistent sysctl changes
echo "kernel.printk = 4 4 1 7" >> /etc/sysctl.conf
sysctl -p