Simplifying Docker Deployment with PM2

As you know, PM2 is a daemon process manager that allows you to keep applications online. Many times, you may be running your service inside a Docker image. The service can be written in any language, such as Node.js, Java, etc. Below is the shell script that can be used to deploy your service. It can be added and used in your CI/CD pipeline.

LINUX - Shell script to detect file change

Below is the simple shell script which can be used to detect the file change in a particular directory. It can detect file modified, file touched, new file created, file deleted / moved, files mode change (chmod) / files owner change (chown). It runs continuously after a sleep of specified interval and compare the list of files against the list of files prior to sleep.

Please see the self explanatory shell script below.

Shell script - function to split a string based on a delimiter

#!/bin/ksh

#function to split a string based on a delimiter
function split {

        string=$1
        delimiter=$2

echo "split string '"$string"' with delimiter '"$delimiter"'"
        for word in $(echo $string | tr "$delimiter" "\n")
        do
                echo $word
        done
}

#call the split function with arguments string and delimiter
split "transient;private;protected;public;synchronized;native" ";"

split "float|short|char|double" "|"

UNIX/LINUX - How to encrypt/decrypt a text file

crypt command can be used to encrypt/decrypt a particular text file in UNIX/LINUX. It is a simple encryption algorithm which works on a secret-key basis. It encrypts or decrypts a stream of data from standard input, and writes the result to standard output. Encryption and decryption is a symmetric process.

Encryption
This will create an encrypted file enTextFile.txt  from a text file textFile.txt. You can use either of the commands both does one and the same task.

crypt ENCRYPTION_KEY < textFile.txt > enTextFile.txt

OR

cat textFile.txt | crypt > enTextFile.txt

Decryption
This will decrypt an encrypted file enTextFile.txt and store the output in textFile.txt

crypt ENCRYPTION_KEY < enTextFile.txt (to show data on console)
crypt ENCRYPTION_KEY < enTextFile.txt > textFile.txt

OR

cat enTextFile.txt | crypt > textFile.txt

Linux - Copy/paste block of codes from Windows editor to Vi editor

Below is the command used to Copy/paste a block of code(s) from Windows editor to Vi editor. Basically to replace complete text from a particular file.

First copy the block of code(s) from windows editor then go to Vi editor and use the following command sequentially as given below

<ESC>
d
Shift+g
i
Shift+Insert
<ESC>

Above command says delete (d) the codes form specific line number (0 in this case) upto end of the file i.e. last line (note - capital G, hence Shift+g). Then go to insert mode (i) and paste the copied lines.

Linux - Block of comments in shell script

There is no block comment on shell script however there are few options that you can try out -

Method 1

#!/bin/bash
echo "This is a statement"
: << 'COMMENTS'
echo "The code that you want be commented out goes here"
echo "This echo statement will not be called"
COMMENTS
echo "This statement is outside the comment block"

At the beginning of the block you wish to comment out put -
: << 'COMMENTS'

The '' around the COMMENTS delimiter is important, otherwise things inside the block will be parsed and executed. I have used 'COMMENTS' as it should be a string of characters which is not used in you code, modify it in your code if needed.

And at the end of the block put -
COMMENTS

Method 2

#!/bin/bash
echo "This is a statement"
if [ 1 -eq 0 ]; then
echo "The code that you want be commented out goes here"
echo "This echo statement will not be called"
fi
echo "This statement is outside the comment block"

Added a false condition in the IF block so that it won't get executed.

Method 3

Using vi editor you can easily comment/uncomment from line n to m

Comment

<ESC>
:10,100s/^/#/

Above command says from line 10 to 100 substitute line start (^) with a # sign

Uncomment

<ESC>
:10,100s/^#//

Above command says from line 10 to 100 substitute line start (^) followed by # with noting //

Linux : Base64 Encode & Decode with OpenSSL

Base64 is a group of similar binary-to-text encoding schemes that represent binary data in an ASCII string format by translating it into a radix-64 representation. The term Base64 originates from a specific MIME content transfer encoding.

Base64 encoding schemes are commonly used when there is a need to encode binary data that needs to be stored and transferred over media that is designed to deal with textual data. This is to ensure that the data remains intact without modification during transport. Base64 is commonly used in a number of applications including email via MIME, and storing complex data in XML.

In Linux you can achieve Base64 Encoding and Decoding using pre-installed OpenSSL package.

Encode

Below command will encode any text contents within the quotes and display the encoded contents on a new line.

echo 'text content' | openssl base64

You can also encode multiple lines using below command

echo 'text content' | openssl base64 && echo 'another text content' | openssl base64

Decode

Similarly you can decode using '-d' flag as shown below

echo 'dGV4dCBjb250ZW50Cg==' | openssl base64 -d


Using the OpenSSL package, you can also encode or decode a specific file, as shown below:

encode file
openssl base64 -in 'file.txt' -out 'encodedFile.txt'

decode file
openssl base64 -d -in 'encodedFile.txt' -out 'file.txt'

Unix/Linux - Shell script to find and replace a text from the list of files

Below is the automated shell script which will find a text and replace it with the given string. It will also keep a backup of existing file. It can also be used for global search and replace string from a text file. Please see the self-explanatory shell script below -

replace.sh

#!/bin/bash

#find string
searchString="text1"

#replace with
replaceWith="text2"

echo "searching "$searchString" ..."
grep -l $searchString * >foundIn.txt
chmod 777 foundIn.txt

echo "replacing "$replaceWith" ..."
count=0
while read x
do
  if [ $x != replace.sh ]
  then
          (cat  $x) >$x"_bkp"
          sed "s/${searchString}/${replaceWith}/g" $x"_bkp" >  $x
          count=$(($count+1))
          echo "Found in... "$x
  fi
done <foundIn.txt

echo $count "occurences have been replaced."

Linux/Unix - Shell script to read a text file

Method 1 - By iterating each line using while loop

#!/bin/bash

#Name of the text file
txtFile=myFile.txt

echo "Reading text file "$txtFile" ..."

while read x
do
echo $x
done <$txtFile


Method 2 - By using cat (concatenate and display files) command

#!/bin/bash

#Name of the text file
txtFile=myFile.txt

echo "Reading text file "$txtFile" ..."

cat $txtFile

How to use scp command without prompting for password

You may need to write a shell script in which you want to copy a file from one server to another. OfCouse you can easily achieve it with SCP command however when you automate the shell script using crontab it will get stuck because of password prompt. To avoid this you want SCP command to work without prompting for password.

Secure Copy (SCP) allows files to be copied to, from, or between different hosts

Let’s assume you want to copy a file from host server (HOST) to destination server (DEST).

1. On HOST server run below command

$ ssh-keygen -t rsa

First it will prompt for "Enter file in which to save the key" just keep it blank by pressing enter. Then it will prompt for passphrase, again keep it blank by pressing enter. It will generate a public key (id_rsa.pub) and a private key (id_rsa) in the folder <your_home_dir>/.ssh

Output -

$ ssh-keygen -t rsa
Generating public/private rsa key pair.
Enter file in which to save the key (/home/.ssh/id_rsa): 
Enter passphrase (empty for no passphrase): 
Enter same passphrase again: 
Your identification has been saved in /home/.ssh/id_rsa.
Your public key has been saved in /home/.ssh/id_rsa.pub.
The key fingerprint is:
1a:7b:3b:d7:5c:f0:6d:f1:01:75:0f:b1:71:6a:15:be user@HOST

2. Share the public key (id_rsa.pub) with DEST server

Copy the file id_rsa.pub from HOST to <your_home_dir>/.ssh folder of DEST. You can use scp or ftp anyway you are comfortable with.

3. Copy the contents of id_rsa.pub to <your_home_dir>/.ssh/authorized_keys2 in DEST server

Below command will append the content of id_rsa.pub to authorized_keys2. Note - you may even copy to authorized_keys whichever file exits on <your_home_dir>/.ssh in DEST server

$ cat id_rsa.pub >> authorized_keys2

Now if you use SCP command to transfer a file from HOST to DEST it won’t prompt for password. I hope it helped you.