BASH : Using while loop to ssh to multiple servers and run multiple commands

So today I got a requirement to perform some regular tasks to be executed on 100 Hosts.

These are the two task that i have to perform on 100 servers from SSH remote execution.

Create a Symbolic Link for a directory into User's directory
Give Ownership of the same Symbolic link.

Commands involved.
ln -s /home/BackUP_Data/  /home/user_directory
chown -R userid.gid  /home/user_directory/BackUP_Data

So it seems to be pretty simply , but here are two variables that I'm supposed to fetch out from two files.

/tmp/xyz_host.txt contains all 100 hostname
/tmp/xyz_user.txt conatins all 100 Users with respect to the same hostname in parralel to it.

Hence upon addition of variables from my files we have to execute something like below.

ssh root@$x " ln -s /home/BackUP_Data/  /home/$y  && chown -R userid.gid  /home/user_directory/BackUP_Data "
"

Task looks simple and i tried to use my old WHILE loop snippet to perform the same activity.

While Loop with Multiple Variables from multiple file as input

But somehow it didn't went well just after executing the commands into the first host , the loop stucks at all.

Somehow our common friend google is here and STACKexchnage is my best friend during scripting.

It somehow fixed me , just by adding  an extra   < /dev/null;


So overall the while loop will look like

while read -r x && read -r y <&3; do ssh root@$x " ln -s /home/BackUP_Data/  /home/$y  && chown -R userid.gid  /home/user_directory/BackUP_Data
" < /dev/null; done</tmp/xyz_host.txt 3</tmp/xyz_user.txt

The same has been briefly explained here as

ssh is reading the rest of your standard input.

while read HOST ; do … ; done < servers.txt

read reads from stdin. The < redirects stdin from a file.

Unfortunately, the command you're trying to run also reads stdin, so it winds up eating the rest of your file. You can see it clearly with

Explained well as Workaround / Solution for it by using SECOND Redirect

while read HOST ; do ssh $HOST "uname -a" < /dev/null; done < servers.txt

A detailed explanation can be found here ...  Stackexchange
Reference : Stackexchange

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BASH : While Loop with Multiple Variables from multiple file as input

This blog post will help you to use the WHILE loop with multiple variables fetched out from multiple files.
If you have two files and some content within it which you want in parallel to work together.

Lets assume , i have a two file named as  hostname.txt and serial_no.txt and here are the contents for it.

cybeerkeeda@Linux-Maniac:~ cat hostname.txt 

Myhost01

Myhost02

Myhost03

cybeerkeeda@Linux-Maniac:~ cat serial_no.txt 

SGH120X

SGH345U

SGH6YUI

Now what I want my output should look like 

Myhost01    SGH120X

Myhost02    SGH345U

Myhost03    SHH6YUI

We can do these in numbers of way but for now we will simply demonstarate, how we can use WHILE loop to achieve it.

while read -r x && read -r y <&3; do echo " $x  $y " ; done<hostname.txt 3<serial_no.txt

We can even write it to a file too

while read -r x && read -r y <&3; do echo " $x  $y " ; done<hostname.txt 3<serial_no.txt  >> /tmp/my_inventory.txt

Depending upon your need you can modify the commands , just one more example , i have used the same loop to modify some data with SED.

while read -r x && read -r y <&3; do   echo "sed -n '/6\/$x\/2016/,/6\/$y\/2016/p'"; done <old_date.txt 3<new_date.txt > sed_date.txt

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