I used this link to successfully setup a Open SSH server on a Debian 8 VM.
Setting up SSH Server on Debian 8
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Author Archives: cyberaka
Using NGINX for Proxy Pass
Lets say I have two servers running on my laptop
1. http://localhost:3000 – This is the UI
2. http://localhost:4000 – This is the web services API
So now I need to setup something like this:
1. http://localhost/ – This should point to UI
2. http://localhost/api/ – This should point to web services API
To set this up I have used NGINX and added this section in the “server” section of the nginx.conf file. On my Mac system this file is located at: /usr/local/etc/nginx/nginx.conf
location /api/ {
proxy_set_header X-Forwarded-Host $host;
proxy_set_header X-Forwarded-Server $host;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_pass http://localhost:4000/;
}
location / {
proxy_set_header X-Forwarded-Host $host;
proxy_set_header X-Forwarded-Server $host;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_pass http://localhost:3000/;
}
To reload NGINX I use the “-s reload” parameter for nginx. On my Mac I execute this command:
/usr/local/Cellar/nginx/1.10.0/bin/nginx -s reload
User input in shell script
I needed to write a small shell script which would allow me to make some decision based on user choices. I needed the user to specify Yes, No or Cancel for an operation. The following piece of hack does the job well.
#! /bin/bash
# define constants
declare -r TRUE=0
declare -r FALSE=1
user_choice() {
local str="$@"
while true
do
# Prompt user, and read command line argument
read -p "$str " answer
# Handle the input we were given
case $answer in
[yY]* ) return $TRUE;;
[nN]* ) return $FALSE;;
[cC]* ) exit;;
* ) echo "Y - Yes, N - No, C - Cancel. Please choose valid option.";;
esac
done
}
if user_choice "Execute Job 1? "; then
echo "Executing Job 1.."
fi
if user_choice "Execute Job 2? "; then
echo "Executing Job 2.."
fi
The following links were referred to achieve this little script:
http://linuxcommand.org/wss0090.php
https://bash.cyberciti.biz/guide/Returning_from_a_function
http://alvinalexander.com/linux-unix/shell-script-how-prompt-read-user-input-bash
Measuring UTF-8 character size
Lets say I type something in Hindi and the outcome is listed below:
तीन व्याकितियों की औसत आयु ३३ वर्ष है.
In Hex View it will look like:
e0 a4 a4 e0 a5 80 e0 a4 a8 20 e0 a4 b5 e0 a5 8d e0 a4 af e0 a4 be e0 a4 95 e0 a4 bf e0 a4 a4 e0 a4 bf e0 a4 af e0 a5 8b e0 a4 82 20 e0 a4 95 e0 a5 80 20 e0 a4 94 e0 a4 b8 e0 a4 a4 20 e0 a4 86 e0 a4 af e0 a5 81 20 e0 a5 a9 e0 a5 a9 20 e0 a4 b5 e0 a4 b0 e0 a5 8d e0 a4 b7 20 e0 a4 b9 e0 a5 88 2e e0 a4 85 e0 a4 97 e0 a4 b0 20 e0 a4 89 e0 a4 a8 e0 a4 95 e0 a5 80 20 e0 a4 86 e0 a4 af e0 a5 81 20 e0 a5 a8 3a e0 a5 a9 3a e0 a5 aa 20 e0 a4 95 e0 a5 87 20 e0 a4 85 e0 a4 a8 e0 a5 81 e0 a4 aa e0 a4 be e0 a4 a4 20 e0 a4 ae e0 a5 87 e0 a4 82 20 e0 a4 b9 e0 a5 8b 2c e0 a4 a4 e0 a5 8b e0 a4 b9 20 e0 a4 89 e0 a4 a8 e0 a4 ae e0 a5 87 20 e0 a4 b8 e0 a5 87 20 e0 a4 b8 e0 a4 ac e0 a4 b8 e0 a5 87 20 e0 a4 ac e0 a5 9c e0 a5 87 20 e0 a4 95 e0 a5 80 20 e0 a4 86 e0 a4 af e0 a5 81 20 e0 a4 95 e0 a4 bf e0 a4 af e0 a5 8d e0 a4 a4 e0 a4 a8 e0 a5 87 20 e0 a4 b5 e0 a4 b0 e0 a5 8d e0 a4 b7 20 e0 a4 b9 e0 a5 8b e0 a4 97 e0 a5 80 3f
The main thing to notice here is that every hindi character is starting with the byte “E0”. This is basically a code point which identifies the code size of the UTF-8 character. The following table appropriate highlights it:
Binary Hex Comments
0xxxxxxx 0x00..0x7F Only byte of a 1-byte character encoding
10xxxxxx 0x80..0xBF Continuation bytes (1-3 continuation bytes)
110xxxxx 0xC0..0xDF First byte of a 2-byte character encoding
1110xxxx 0xE0..0xEF First byte of a 3-byte character encoding
11110xxx 0xF0..0xF4 First byte of a 4-byte character encoding
Reference: https://stackoverflow.com/questions/5290182/how-many-bytes-does-one-unicode-character-take/33349765#33349765
Embedding images directly into HTML
I needed to email an HTML report and I was not happy with the fact that we have to create zip file containing the HTML, Images etc. I knew it was possible to embed base64 data directly into HTML I started looking for some PoC. My search took me to an online base64 encoder and decoder. This gave me the idea to convert the external image files into base64 string and embed it directly into HTML. On some search I landed up Apache Commons Codec Library which contains a Base64 class which can be used to covert an input stream into a base64 string.
No line wrapping in Vi
The vi editor in Linux does line wrapping by default. To turn this off for some files with huge lines I use the following command from vi command prompt:
To disable wrap.
:set nowrap
To enable wrap.
:setwrap
Vi Cheat Sheets
I use this ultimate cheat sheet for vi command on Linux whenever in doubt:
JSON to Java Code Generator
I recently received a couple of REST Web Service endpoints where the implementation technology was not Java. So I had to either rollout my own object and then rely on the object mapper to map the JSON to Java or come up with an approach to generate the relevant POJO file. On a little bit of looking around I found out the following two links for generating Java Code from JSON.
http://json2java.azurewebsites.net/
https://javafromjson.dashingrocket.com/
The POJO generated were decent and got the job done. I didn’t have to write the POJO myself and the code generated by these websites did the job well.
Another link recommended in the posts comments.
http://www.freecodeformat.com/json2pojo.php (Thanks Daniel)
I found out the 1st two links are no longer working. So I found out about another tool based on Visual Studio Code. Use the following extension for solving this use case.
https://marketplace.visualstudio.com/items?itemName=quicktype.quicktype
Delete Untracked File Using Git
Sometimes we want to clean out our Git workspace and remove any file which is untracked. A hard reset usually does the job for modified files as shown below:
git reset --hard
But hard reset doesn’t fit in case of untracked files. The following command will do the job:
git clean -d -fx ""
-x means ignored files are also removed as well as files unknown to git.
-d means remove untracked directories in addition to untracked files.
-f is required to force it to run.
Reference:
https://www.kernel.org/pub/software/scm/git/docs/git-clean.html
Leveraging Tee and Grep for filtering program output
I have a utility which compiles my UI code and minifies them. It works pretty well and doesn’t requires much supervision. Trouble starts if the code compilation fails and it gets ignored in the huge log file which is generated by this utility. I use the following approach to filter out errors and yet preserving the output of the utility program for any diagnostic in case of any error.
./my_utility.sh | tee output.txt | grep error
Basically what happens is that “my_utility.sh” generates a lot of output which is piped into the “tee” command which in turn dumps it into a text file named “output.txt” and then the same output is again piped into the “grep” command which looks for any error text in the output. If the error is found it is outputted on the console.
So the end result is that in 99% cases I don’t see any output from the utility as I don’t want to see debug output. However the moment an error happens it gets flagged on console and I have the “output.txt” file to audit what went wrong and the root cause of the error.