Linux - Kernel

Tips

Modify sysctl parameters:

/etc/sysctl.conf
/etc/sysctl.d/00-app.conf

Library

List lib from a binary:

ldd /usr/bin/mail

Trace lib call:

ltrace

See libs of a rpm package:

rpm -qR mailx-12.5-19.el7.x86_64

Display loaded libs:

ldconfig -p

Procedures

Rebuilding the initial ramdisk image (RHEL):

# Tested on RHEL 5, 6 & 7

# Sometimes it may be necessary to rebuild the initial ramdisk (also known as initrd or
# initramfs) to include new kernel modules, files, etc.

# RHEL 3, 4 & 5: Rebuilding the initrd
cp /boot/initrd-$(uname -r).img /boot/initrd-$(uname -r).img.bak
mkinitrd -f -v /boot/initrd-$(uname -r).img $(uname -r)

# If we are in a kernel version different to the initrd we are building we must specify
# the full kernel version, without architecture:
mkinitrd -f -v /boot/initrd-2.6.18-348.2.1.el5.img 2.6.18-348.2.1.el5

# For RHEL 6 & 7: Rebuilding the initramfs
cp /boot/initramfs-$(uname -r).img /boot/initramfs-$(uname -r).img.bak
dracut -f

# If we are in a kernel version different to the initrd we are building we must specify
# the full kernel version, including architecture:
dracut -f /boot/initramfs-2.6.32-220.7.1.el6.x86_64.img 2.6.32-220.7.1.el6.x86_64

# *** Consider adding a new grub/grub2 entry pointing to the initial ramdisk image, that
# will allow you to choose the old version at boot time without restoring the backup

Change boot order:

# Verify current version
uname -a

# Display kernel config
cat /etc/sysconfig/kernel

# Display Kernel list
awk -F\' '$1=="menuentry " {print $2}' /etc/grub2.cfg

# Change default
grub2-set-default 2

# Make grub config
grub2-mkconfig -o /boot/grub2/grub.cfg

# Restart
reboot 

# Verify new version
uname -a