Monitoring Commands in RedHat Linux Servers

Important Monitoring commands in Linux

For a system and network administrator, it’s very tough to debug and monitor the Linux servers activities and performance daily. In this tutorial, I have compiled some important monitoring commands that might be useful for the Linux/UNIX administrators. All these commands are available under all flavors of UNIX and these commands are very much useful in probing the cause for the errors.

1.vmstat(Virtual Memory Statistics):

This command will display the statistics of virtual memory, CPU activity, IO Blocks, Kernel threads and many more.

Some Linux distribution will not be having this command by default, You will need to install the systat package which contains the vmstat command.

#vmstat

2. To check the Active and Inactive Memory Details:
#vmstat -a

From the above output, you can check the active and inactive memory details, the column si and so indicates the following meaning,

si = Swapped in every second from disk in kilobytes

so = Swapped out every second to disk in kilobytes

free = Total free memory spaces
3.lsof(List of Open Files):

This command is very much useful in analyzing which processes are accessing and opening the files, and the open files include are Disk files, Pipes, Devices, Network sockets. For example when you trying to unmount a filesystem and if it not unmounting which means some process is accessing that filesystem, to check which processes are accessing the filesystem we can run this “lsof” command to get the full report. With this command, we can easily identify which files are in use

4. To list all open files
#lsof

From the above output,t it showing the long listing of open files

FD =File Descriptor and under this we will have some values ,

CWD =Current working directory

rtd = Root directory

mem = Memory mapped file

txt = Program text(Data and code)

TYPE of files and its identification

DIR =Directory

REG = Regular file

To learn more about “lsof” command visit this link Importance of lsof command

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5.tcpdump (Network Packet Analyzer):

The tcpdump is the most useful command line  Network packet analyzer or packets sniffer program which is very much useful in capturing the TCP/IP packets that received or transferred on a specified network adapter over a network. This tool has also an option to save the captured data too a file for further analysis.

6. To capture the packets from a specific interface :
#tcpdump   -i   eth0

eth0 = Logical name of the network adapter,0 indicates the first  adapter 

Cancel the program by pressing ctrl+c, you will see the below output,

Note: This command saves the output in "pcap" format which can be viewed only by the "tcpdump" command
7. To capture only “N” number of packets:

By default the “tcpdump” command captures all the packets for the specified interface until you cancel the program, now by using one special option “-c”  you can capture the specified number of packets.

Below example  captures only 4 packets

#tcpdump   -c 4 -i eth0

8. To check the Number of Interfaces in you Server, run the following command
#tcpdump  -D

8.To capture and save the Packets in a File:
#tcpdump   -w   mylog.pcap    -i   eth0

mylog.pcap= filename along with the extension .pcap

9.To View the Captured Packet Files
#tcpdump   -r    mylog.pcap

10.To Capture Packets from a specific Port:

For example, To capture the packets from the “ssh” port, run the following command,

#tcpdump   -i  eth0  port  22

11.Netstat(Network Statistics):

This command is very much useful in monitoring the Incoming and outgoing packets and also you can monitor the interface statistics. When you are having connectivity issues to your server the first most thing is you need to check the port is in listening or non-listening state, that can be done by using the netstat command. This command is very much useful for the network administrators to check and analyze the network related problems.

12. To check all Listening ports of TCP and UDP Connections:
#netstat -a  |more

From the above output from the IP 192.168.1.175, one client is connected to my server via ssh port and the connection status is ESTABLISHED

13.To List only TCP connection details
#netstat -at

14.To Display the Full Statistics by Protocols:

By default, the statistics can be displayed only for the TCP, UDP, ICMP, and IP protocols, The -s option is used to specify a set of protocols

#netstat  -s

You can check the full statistics by protocols like Number of active connections, the total number of packets received, dropped and many more.

15.To display the statistics by TCP Protocols.
#netstat  -st

You can check the total number of active connections and failed attempts via this protocol and many more you can get from this command.

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16.IOTOP Command:

This command is very much similar to the “top” command, the only difference is with iotop you can check the real-time disk I/O and processe. This command is useful to find the exact process and high used Disk read/write processes

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Hello readers! Let me introduce my self first. My name is Vasanth Nirmal Singh J S having 9+ years of experience in IT on all flavours of Unix operating systems ,Storage's and many more .. I would like to share my technical experience i have come across - can be help to other people. So in this blog, I'll post my thoughts related to ITIS. I'll share experiences that I've had while working in different environments. You can expect content related to Unix,Solaris,Linux,EMC Storeages,HP-UX and many others. I hope this blog can be useful for you! Your comments will be appreciated!

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