Linux Administrator Interview Questions & Answers

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To change the run level we have to edit the file "/etc/inittab" and change initdefault entry ( id:5:initdefault:). Using 'init' command we change the run level temporary like 'init 3′ , this command will move the system in runlevl 3.

To set this value permanently for a user, it has to be put in the appropriate profile file which depends on the default shell of the user.

umask stands for 'User file creation mask', which determines the settings of a mask that controls which file permissions are set for files and directories when they are created.

The '/lib/modules/kernel-version/' directory stores all kernel modules or compiled drivers in Linux operating system. Also with 'lsmod' command we can see all the installed kernel modules.

Using fdisk utility we can create partitions from the raw disk.Below are the steps to create partition from the raw dsik :
☛ fdisk /dev/hd* (IDE) or /dev/sd* (SCSI)
☛ Type n to create a new partition
☛ After creating partition , type w command to write the changes to the partition table.

The logical Steps to reduce size of LVM partition :
☛ Umount the filesystem using umount command,
☛ use resize2fs command , e.g resiz2fs /dev/mapper/myvg-mylv 10G
☛ Now use the lvreduce command , e.g lvreduce -L 10G /dev/mapper/myvg-mylv
Above Command will shrink the size & will make the filesystem size 10GB.

☛ Use the lvextend command (lvextend -L +100M /dev/<Name of the LVM Partition> , in this example we are extending the size by 100MB.
☛ resize2fs /dev/<Name of the LVM Partition>
☛ check the size of partition using 'df -h' command

Sar Collect, report, or save system activity information. The default version of the sar command (CPU utilization report) might be one of the first facilities the user runs to begin system activity investigation, because it monitors major system resources. If CPU utilization is near 100 percent (user + nice + system), the workload sampled is CPU-bound.

Using 'free' & 'vmstat' command we can display the physical and virtual memory statistics respectively.With the help of 'sar' command we see the CPU utilization & other stats.

LVM stands for Logical Volume Manager, to re-size filesystem's size online we required LVM partition in Linux. Size of LVM partition can be extended and reduced using the lvextend & lvreduce commands respectively.

To share a directory using nfs , first edit the configuration file '/etc/exportfs' , add a entry like
'/<directory-name> <ip or Network>(Options)' and then restart the nfs service.

Using 'showmount' command we can see what directories are shared via nfs e.g 'showmount -e <ip address of nfs server>'.Using mount command we can mount the nfs share on linux machine.

Service: Port
SMTP: 25
SSH: 22
Squid: 3128
DNS: 53
FTP: 20 (data transfer) , 21 ( Connection established)
DHCP: 67/UDP(dhcp server) , 68/UDP(dhcp client)

Network bonding is the aggregation of multiple Lan cards into a single bonded interface to provide fault tolerance and high performance. Network bonding is also known as NIC Teaming.

☛ balance-rr or 0 - round-robin mode for fault tolerance and load balancing.
☛ active-backup or 1 - Sets active-backup mode for fault tolerance.
☛ balance-xor or 2 - Sets an XOR (exclusive-or) mode for fault tolerance and load balancing.
☛ broadcast or 3 - Sets a broadcast mode for fault tolerance. All transmissions are sent on all slave interfaces.
☛ 802.3ad or 4 - Sets an IEEE 802.3ad dynamic link aggregation mode. Creates aggregation groups that share the same speed & duplex settings.
☛ balance-tlb or 5 - Sets a Transmit Load Balancing (TLB) mode for fault tolerance & load balancing.
☛ balance-alb or 6 - Sets an Active Load Balancing (ALB) mode for fault tolerance & load balancing.

By using the command 'cat /proc/net/bonding/bond0′ , we can check which mode is enabled and what lan cards are used in this bond. In this example we have one only one bond interface but we can have multiple bond interface like bond1,bond2 and so on.

By using the Commands 'netstat -nr' and 'route -n' we can see the default route and routing tables.

Use the Command 'netstat -listen' and 'lsof -i'

With the help of command 'chkconfig -list | grep 5:on' we can list all the service that are enabled in run level5. For other run levels just replace 5 with the respective run level.

We can enable a service using the Command 'chkconfig <Service-Name> on -level 3′

We should never upgrade Linux Kernel , always install the new New kernel using rpm command because upgrading a kenel can make your linux box in a unbootable state.

There are two ways to scan newly assigned luns :
Method 1: if sg3 rpm is installed , then run the command 'rescan-scsi-bus.sh'
Method 2: Run the Command , echo " - - - " > /sys/class/scsi_host/hostX/scan

We can find the WWN numbers of HBA cards using the command 'systool -c fc_host -v | grep port_name'

To Set the kernel parameters in linux , first edit the file '/etc/sysctl.conf' after making the changes save the file and run the command 'sysctl -p' , this command will make the changes permanently without rebooting the machine.

Puppet is an open-source & enterprise software for configuration management toll in UNIX like operating system. Puppet is a IT automation software used to push configuration to its clients (puppet agents) using code. Puppet code can do a variety of tasks from installing new software, to check file permissions, or updating user accounts & lots of other tasks.