Messages

You can use the cut command which will select the specified column.
$cat /etc/passwd | cut -d : -f 1
Reference: https://www.networking-forums.com/programming-goodies-and-software-defined-networking/list-only-usernames

You can use mysql command like below. For more information look https://www.networking-forums.com/programming-goodies-and-software-defined-networking/how-to-list-databases-from-linux-command-list

It is easier than you think. Use the usermod command with root privileges. Following example will remove the user george from the root group.
$ sudo usermod -G root george

Referance: https://www.networking-forums.com/programming-goodies-and-software-defined-networking/remove-a-user-from-the-specific-group-in-linux

It is very easy just put the script file path after the ssh command like below.
ssh ismail@192.168.142.144 'bash -s' < cat myscript.sh
Look: https://www.networking-forums.com/programming-goodies-and-software-defined-networking/run-local-script-on-remote-system-via-ssh

There are different functions and ways to match a word in PHP. Best way is using preg_match() function. In the following example we will search the forum word in the $bigtext content.
preg_match('/forum/',$bigtext)
Look: https://www.networking-forums.com/programming-goodies-and-software-defined-networking/match-for-a-word-in-a-text-with-the-php-script

No. You shouldn't write a bash script. Just use the cp command bulk copy feature and provide all files you want to copy and add the path which is the destination. 
cp file1 file2 file3 /mnt/backup 
Referece: https://www.networking-forums.com/programming-goodies-and-software-defined-networking/copy-bulk-files-in-linux

There are different ways. You can also use the systemd but as you are asking for init.d use the following command.
sudo /etc/init.d/networking start
Reference: https://www.networking-forums.com/programming-goodies-and-software-defined-networking/stop-network-service-with-init-d-for-ubuntu

You can use the Python command to get the Python interpreter version. For the default Python installation use
python -v         
OR for Python2
  python2 -v
Or Python3
python3 -v           

Reference: https://www.networking-forums.com/programming-goodies-and-software-defined-networking/check-python-interpreter-version

Python provides exists() function which can be used like below. Look https://www.networking-forums.com/programming-goodies-and-software-defined-networking/check-file-existing-with-python-script for more information. 

import os.path 

if(os.path.exists("/etc/passwd")): 

   print("/etc/passwd exists")