There are several ways to run commands repetively in Linux.. (cron, while loop etc). Here is a simple trick. Say you want to check the size of a directory using “du” every 5 seconds, you can use the following command
watch -n 5 du -ch
Check out all the options available in watch by running “man watch” 
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For my notes.. If you ever wanted to check the status of a bonded interface configure in Linux (esp RHEL), you can check the status by running the following command
[root@serverxyz bin]# cat /proc/net/bonding/bond0
i.e. assuming the name of your bond interface is bond0.
Output from the command
Ethernet Channel Bonding Driver: v3.4.0 (October 7, 2008) Bonding Mode: fault-tolerance (active-backup) Primary Slave: eth3 (primary_reselect always) Currently Active Slave: eth3 MII Status: up MII Polling Interval (ms): 100 Up Delay (ms): 0 Down Delay (ms): 0 Slave Interface: eth3 MII Status: up Link Failure Count: 0 Permanent HW addr: 00:10:18:6e:b8:1a Slave Interface: eth0 MII Status: up Link Failure Count: 0 Permanent HW addr: 00:21:5e:11:34:32
The configuration files involved are
/etc/sysconfig/network-scripts/ifcfg-bond0 (Bond Interface)
DEVICE=bond0 IPADDR=10.10.40.26 NETMASK=255.255.255.0 ONBOOT=yes BOOTPROTO=none USERCTL=no GATEWAY=10.10.40.1 NETWORK=10.10.40.0 BROADCAST=10.10.40.255 TYPE=Ethernet
/etc/sysconfig/network-scripts/ifcfg-eth3 (Primary Interface)
DEVICE=eth3 BOOTPROTO=none ONBOOT=yes HWADDR=00:10:18:6e:b8:1a MASTER=bond0 SLAVE=yes TYPE=Ethernet USERCTL=no
/etc/sysconfig/network-scripts/ifcfg-eth0 (Secondary Interface)
DEVICE=eth0 HWADDR=00:21:5e:11:34:32 USERCTL=no ONBOOT=yes MASTER=bond0 SLAVE=yes BOOTPROTO=none TYPE=Ethernet
We run multiple applications in Jboss at my work and one of the applications used to take an inordinate time to come up. A typical application would take < 1 minute to get deployed and this particular application for some reason was taking ~7-8 minutes. We initially thought it was a bug in the code and gave hell to our development team .. But on closer investigation, we found out that a feature we enabled in the Jboss server settings which allows content to be hosted on network storage was causing the issue.
I blogged the feature in Jboss to follow sym links here (http://kudithipudi.org/2008/07/25/howto-configure-jboss-to-follow-symbolic-links/). So essentially when Jboss was started, it was checking all the content in these network path to check for applications to deploy. And traversing a network share with 1000s of directories isn’t fun ..
We fixed it by making a simple edit to the start up script. Here’s the psuedo code for the script
And now the application starts in less than a minute .
I guess there might be other elegant ways to do this. i.e. Configure Jboss to only deploy certain applications, but this did the trick for us .
We ran into an issue at work recently, which prompted us to do some performance testing on some of our Linux servers. The requirement was to stress test the key components of the server (CPU, RAM, HDD, Network) and prove that different servers with the same configuration were capable of performing identically. Pretty simple right .. The challenge was to find tools that could be run to stress test each of the components. There were a lot of tools for CPU and memory (RAM) testing, but not a lot for network and hard drive (HDD) testing. After searching high and low, we found a couple of tools, that I wanted to document here for future reference.
I found a pretty interesting tool called Iozone written by William Norcott (Oracle) and Don Capps. You can get the source code and builds for major OSs at http://iozone.org . Despite installing the program using RPM, we were not able to run the program without specifying the complete path.
There are a ton of options for the program, but the easiest method to run it was in automated mode with the output going to an Excel spreadsheet (more like a glorified CSV file ). Here is the command we used
/opt/iozone/bin/iozone -a -Rb output_excel_file.xls
The “-a” is to tell the program in automated mode and the “-Rb” is to tell the program to format the output in Excel format. And you can then go ahead and open the spreadsheet in Excel and create 3D graphs to check and compare the output.
Most of the information out there in terms of testing the network stack of a machine is either to copy large files over a network share or via FTP. We didn’t find that was enough to really max out a Gigport since there were protocol limitations that didn’t allow us to saturate the network port. After some searching, we stumbled across a tool called “ettcp” on Sourceforge. ettcp itself is an offshoot of ttcp. ttcp (stands for test tcp) was created to test network performance between two nodes. I couldn’t find any place to download ttcp itself, but you can download ettcp at http://ettcp.sourceforge.net/.
We used a server, to act as a common receiver for all the servers we intended to do a performance test on. Here are the commands we used to run the test
RECEIVER (Common Server)
./ettcp -r -s -f M
The options are
TRANSMITTER (Test Servers)
./ettcp -t -s receiver_hostname -n 10000000 -f M
the options are
Adding the following options to the [Global] section in the samba (open source software providing windows compatible file sharing for *nix operating systems) will most likely speed your throughput
socket options = TCP_NODELAY IPTOS_LOWDELAY SO_SNDBUF=8192 SO_RCVBUF=8192The receive and send buffer seem to do the trick.
I ran across a cached Google entry for a good collection of awk one liners by Eric Pemant. Since I thought the original site was down, I decided to host it on my domain at http://kudithipudi.org/misc/awk_one_liners.txt. But I did some digging and looks like Eric has his own site now and hosts a lot more resources related to awk at http://www.pement.org/awk.htm. Thx for the great collection Eric..
There are a ton of scripts to find how many IP addresses  are configured on a system, but I could not find one, whic would show me which particular network interface an IP address was configured on. Here is a one liner, that will give you this information in Linux
/sbin/ifconfig | grep -B1 10.10.10.10 | awk '{if (NR==1) print $1}'
The same script can be changes a bit to support other operating systems too. Essentially, I am doing a grep (search) of the output of ifconfig, which shows all the network information on the system for a particular IP. At the same time, I am using the -B1 option, which will show the line above the matching line. Finally, I am piping this to awk and printing the first row in the first column.
Say you need to sudo as a particular user and run a command and at the same time you need to pass an environmental variable to the command, you can do it by passing the command in doublequotes.
For example, I want to start Oracle while I am logged in as another user (vinay), I can start the database using dbstart by issues
"sudo su - oracle -c "dbstart /$ORACLE_HOME"
$ORACLE_HOME is an environmental variable listed under user oracle’s environment.
Needless to say, you need to ensure that you have sudo configured to allow your userID to su to oracle.
I ran into a challenge at work, where we had to allow e-mail delivery for certain domains, but block all other domains. But at the same time, we had to ensure that the clients sending e-mails did not get a delivery error. We were using Postfix as the MTA running on Redhat Linux. Here’s how I resolved it
DOMAIN1 :
DOMAIN2 :
* discard:
This configuraiton will ensure that all e-mails address to DOMAIN1 and DOMAIN2 are delivered normally, but the rest of the e-mails are silently discarded.
Note : Ensure that you follow the syntax for where to place the : verbatim.
For my reference.. good article about command line usage of grep . http://arstechnica.com/open-source/news/2009/05/command-line-made-easy-five-simple-recipes-for-grep.ars . Older how to articles on my site related to this topic are here and here.