Find out which Java thread is causing the high CPU usage

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You see your java application is consuming high CPU but you don't know specifically which thread is consuming CPU out of thousand threads.

The solution is very simple.
1. First we need to find out native thread ID that is using high CPU. Issue top -H command on terminal and it will show all threads and the percentage of the CPU they are consuming. Find out your application thread from the list and note down native thread id which will be shown in the first column named PID.

2. Take thread dump of your application.

3. Thread dump contains native thread ID but in hexadecimal format. So convert the decimal thread ID we have noted in the first step to hexadecimal value and search for this value in the thread dump and you will find the thread's stack trace.

 You can see more detailed explanation in the source link below. It gives an example program and also details how to do this in both Unix and Windows.

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Source: https://blogs.oracle.com/jiechen/entry/analysis_against_jvm_thread_dump

To get http status code from curl call

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We can use below command to get http response code of a URL from bash script.

status_code=$(curl --write-out "\n%{http_code}\n" --silent --output /dev/null --insecure https://www.google.co.in/)

silent - to stop progress information in output
output - we are re-directing normal output to null device
write-out - whatever things we needed can be given here. Here we are printing http status code. It will be printed after completion, i.e., after printing all output.
insecure - this allows connections to SSL sites without certs

References: http://stackoverflow.com/a/2220646/324900

Overriding logback file embedded in jar file

I was trying to run a jar file by setting it up in cronjob. Now this jar file contains a logback configuration file included in it which is causing me problem because all logs are being written to a specific file name, like processnamelogs.log. All runs of jar file will append to the same log file.

Now this is not what I wanted since the log file is going to grow huge in few runs. Also if all runs of jar file are writing to same log file it will be difficult to setup a monitoring script to check for errors.

What I wanted is that each run of jar file writes to a separate log file with its name containing start time of that run. So it will be easy to go through logs in case of errors. Also I can easily setup a monitoring script to only search in the log files with current date in their names.

So I need to write my own logback configuration file and a way to make sure the jar file uses my configuration file instead of embedded one. I have searched and got one system property (logback.configurationFile) which does what I wanted.

e.g.,
  java  -Xms128m -Xmx512m -XX:MaxPermSize=256m -jar -Dlogback.configurationFile=./mylogback.xml ./process.jar

mylogback.xml

<configuration>

  <!-- Insert the current time formatted as "yyyyMMdd'T'HHmmss" under
       the key "bySecond" into the logger context. This value will be
       available to all subsequent configuration elements. -->
  <timestamp key="bySecond" datePattern="yyyyMMdd'T'HHmmss"/>

  <appender name="FILE" class="ch.qos.logback.core.FileAppender">
    <!-- use the previously created timestamp to create a uniquely
         named log file -->
    <file>log-${bySecond}.txt</file>
    <encoder>
      <pattern>%logger{35} - %msg%n</pattern>
    </encoder>
  </appender>

  <root level="DEBUG">
    <appender-ref ref="FILE" />
  </root>
</configuration>

References:

• http://logback.qos.ch/manual/configuration.html#configFileProperty
• http://logback.qos.ch/manual/appenders.html#uniquelyNamed

find out number of occurrences of a character in mysql

How to find out count of a specific character or a char-sequence in mysql?

There is no simple method available to do that. However we can use below method to find out.

select length(x) - length(replace(x, ":", ""))

What is Heartbleed Bug

It is basically a security bug (buffer overread) which is in the Open SSL implementation which is used by a looooooooot of websites on the internet to secure ( :) ) the communication between browser and website.

Jacob on stackexchange explained this issue in detail. Read his answer at superuser.com

to view only directories in Unix

If you have ever wanted to show a list of subdirectories underneath the current directory while at the terminal prompt, there’s a simple command that can help.
Simply run the following command from the bash terminal prompt, which filters the file listing to only show folders:

ls -l | grep ^d

Since typing that whole command every time you want to show a list of directories might get a little tiring, you can use the alias feature to assign it to something you can more easily remember:

alias lsd="ls -l | grep ^d"

Now when you type “lsd” at the prompt, you will see a list containing only the directories, which can be very useful when you are trying to navigate through a messy set of folders.

Some useful VI editor commands

J – to join two lines
Deletion
dl – single character, dw – Single word, dd – single line, d$ – to the end of the line
Editing
i – insert mode, A – append to the end of the line, o – insert in next line
Misc
:set list – to see special characters
:%s/str1/str2/g – search and replace

Some useful variables in Unix

$$ = The PID number of the process executing the shell.
$? = Exit status variable.
$0 = The name of the command you used to call a program.
$1 = The first argument on the command line.
$2 = The second argument on the command line.
$n = The nth argument on the command line.
$* = All the arguments on the command line.
$# The number of command line arguments.

Read password from shell script

Since you don't want password to be not displayed on terminal you need to disable echo before reading password. Once password is read you need to enable it again.

#————————————————————
#  Read Password
#————————————————————
/usr/ucb/echo -n “Please Enter Password For User ${SUSER}: “
stty -echo
read SPSWD
stty echo
clear