ServerBuddies Support Blog

When the kernel loads, it immediately initializes and configures the computer’s memory. Next it configures the various hardware attached to the system, including all processors and I/O subsystems, as well as any storage devices. It then looks for the compressed initrd image in a predetermined location in memory, decompresses it, mounts it, and loads all necessary drivers. Next it initializes file system-related virtual devices, such as LVM or software RAID before unmounting the initrd disk image and freeing up all the memory it once occupied.

After the kernel has initialized all the devices on the system, it creates a root device, mounts the root partition read-only, and frees unused memory.

At this point, with the kernel loaded into memory and operational. However, with no user applications to give the user the ability to provide meaningful input to the system, not much can be done with it.

To set up the user environment, the kernel starts the /sbin/init command.

Add or delete static routes from the Linux IP routing table.

You need to know the network/subnet you wish to reach, also the interface you wish this route to be added to, i.e., which interface to use to reach the subnet.

How to reach another network, 10.20.30.0/24, that is reachable via a router on the 192.168.1.0/24 network, 192.168.1.254.

The following ip route command would add the desired route to the kernel routing table:

Note: eth1 is connected to 192.168.1.0/24

Some common Linux kernel processes

kjournald Commits ext3 journal updates to disk
kswapd Swaps processes when physical memory is low
kreclaimd Reclaims memory pages that haven’t been used recently
ksoftirqd Handles multiple layers of soft interrupts
khubd Configures USB devices

There is one kjournald for each mounted ext3 filesystem.

Among these processes, only init is really a full-fledged user process. The others are actually portions of the kernel that have been dressed up to look like processes for scheduling or architectural reasons.

fsck - check and repair a Linux file system

fsck - check and repair a Linux file system

fsck is used to check and optionally repair one or more Linux file systems. filesys can be a device name (e.g. /dev/hdc1, /dev/sdb2), a mount point (e.g. /, /usr, /home), or an ext2 label or UUID specifier (e.g. UUID=8868abf6-88c5-4a83-98b8-bfc24057f7bd or LABEL=root). Normally, the fsck program will try to handle filesystems on different physical disk drives in parallel to reduce the total amount of time needed to check all of the filesystems.

If no filesystems are specified on the command line, and the -A option is not specified, fsck will default to checking filesystems in /etc/fstab serially.

This is equivalent to the -As options.

The exit code returned by fsck is the sum of the following conditions:

0 - No errors
1 - File system errors corrected
2 - System should be rebooted
4 - File system errors left uncorrected
8 - Operational error
16 - Usage or syntax error
32 - Fsck canceled by user request
128 - Shared library error

The exit code returned when multiple file systems are checked is the bit-wise OR of the exit codes for each file system that is checked.

Email notification everytime users run “sudo” on Linux

For Redhat, Centos Systems make sure that sendmail is running on the machine and configured to send out emails.

Run

Add the following configuration in /etc/sudoers

visudo parses the sudoers file after the edit and will not save the changes if there is a syntax error.