HOW TO USE FOR LOOP IN LINUX COMMAND LINE

Lesson:
In this lesson you'll learn the following:
1. How to run distributed for loop from one Linux server to check things on multiple other Linux servers
2. The above is done using public key authentication on the management server

Management server:
Steps:
1. ssh-keygen
2. copy contents of id_rsa.pub into .ssh/authorized_keys on the other nodes

webserver1, webserver2 and database servers:
Steps:
1. useradd admin -m -G wheel -c "Admin User"
2. sudo su - admin
3. mkdir .ssh
4. chmod 700 .ssh
5. vi .ssh/authorized_keys and add contents of id_rsa.pub and save the file
6. chmod 400 .ssh/authorized_keys

Directory and file permission should resemble the following on the other nodes:
drwx------. 2 admin admin 61 Jan 6 20:21 .ssh/
-r--------. 1 admin admin 398 Jan 6 20:21 .ssh/authorized_keys

Example scenarios:
1. Check if a particular service is running:

for i in webserver1 webserver2 database ; do ssh $i "sudo /bin/hostname; sudo date; sudo systemctl status chronyd"; done
for i in webserver1 webserver2 database ; do ssh $i "sudo /bin/hostname; sudo date; sudo systemctl status chronyd| egrep 'Active|Loaded'"; done

(ensure web/database server names are resolvable using DNS or the names exist in /etc/hosts in management server)

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