Using Bash How Do You Replace a Line in a File by Its Line Number?

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Problem scenario
On your Linux server you know one line in a file needs to be replaced.  You can identify the file by its line number.  You have a string or pattern you want to be inserted where this line is. How do you do this with Bash?

Solution
To replace all the content of line 15 of foobar.txt with “Hello World” run this command:

perl -i -pe ‘s/.*/Hello World/ if $.==15’ foobar.txt

For something more advanced,

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How Do You Deploy Puppet Agent to an Ubuntu 16 Linux Server in AWS?

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Updated on 4/15/18

Problem scenario
You installed Puppet Master on an AWS instance of Linux.  (See this link if you want to install Puppet Master on Ubuntu.  If you want to install Puppet Master on a Red Hat Enterprise Linux server, see this link). You now want an AWS instance of Ubuntu to be configured as a Puppet Agent node with open source Puppet version 5.

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How Do You Have PHP Return an Informative Error Message When a SQL Command Returns No Results?

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Problem scenario
You have a PHP web page that integrates with Postgres behind the scenes.  You find that when a SQL command returns nothing (e.g., when a table name is used and it is known to not exist in the underlying database), the PHP web page is blank.  You want the result to be more clear about what happened or potentially happened.  How do you handle an exception or commonly occurring SQL query with no results using PHP?  

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How Do You Install Puppet Master on an AWS Instance of Ubuntu 16?

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Updated 12/1/17

Problem scenario
You need to install open source Puppet version 5 on a server to become a Puppet Master.  You have an AWS Ubuntu Linux server.  You only have five minutes to do it.  How do you install Puppet Master immediately?

Solution
1.  Create puppetinstaller.sh in the /tmp directory.  It should have the following lines (stop before you get to step #2):

#!/bin/bash
# Written by continualintegration.com

apt-get update
apt-get -y install ntp
echo ‘
server 0.us.pool.ntp.org
server 1.us.pool.ntp.org
server 2.us.pool.ntp.org
server 3.us.pool.ntp.org’ >>

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How Do You Create a Table inside a Specific Postgres Database?

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Problem scenario
You want to create a table in a specific Postgres database.  You want to use a command line interface (not a GUI) for this task.  (The GUI version of these directions can be found here.)  What are the SQL commands to do this?

Solution
This assumes you have a role (a username and password) to connect to the Postgres database with permissions to create tables.

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How Do You List the Tables of a Specific Postgres Database?

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Problem scenario
You want to see the names of the tables of a specific Postgres database.  You know the Postgres database name.  How do you do list its tables?

Solution
This assumes you have a role (a username and password) to connect to the Postgres database.

1.  Connect to the Postgres database (e.g., either through a web front-end or the command line interface). 

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How Do You Change a Firewall Rule in Google Cloud Platform?

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Problem scenario
You want to create an exception to your GCP firewall to allow connectivity from a different workstation or server.  What should you do?

Solution
1.  In the upper right hand corner of the console, click the icon with three horizontal bars.  This is the “Products and Services” button.  Then go to “VPC Network” -“Firewall Rules.”

2.  Click “Create Firewall Rule.”

3. 

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How Do You Install Npm and/or node.js on a AWS Instance of RHEL?

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Problem scenario
Using a RHEL server in AWS, you try this command “npm -v”, and it shows that npm is not installed.  You want to use the npm utility or you want Node.js to be installed on your RedHat version 7 Linux server in AWS.  You may have tried various yum commands but received errors about dependencies.  Various other scripts fail to get npm to be a recognized command.  How do you install npm and/or Node.js without changing yum repositories and without using the wget command?

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How Do You Find You out the AWS Instance ID from inside the OS of a Server?

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Problem scenario
You are using AWS via web browser while using multiple Putty sessions are you logged into several AWS servers (aka AWS instances).  You can find the host names and internal IP addresses of the servers when you are logged into via the command line of the OS itself.  From the AWS console you can find external IP addresses and external FQDNs.  You do not know how to correlate these instances (as the information readily available in Putty at the OS level is independent of the information available via the AWS console).

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How Do You Fix the Ansible Problem “Failed to connect to the host via ssh: percent_expand: unknown key %C\r\n”, “unreachable”: true”?

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Problem scenario 
When you run an Ansible playbook on the control node that is not running on a Mac (e.g., you are running Ansible on RedHat, Ubuntu or SUSE), you get this error: “Failed to connect to the host via ssh: percent_expand: unknown key %C\r\n”, “unreachable”: true”.   How do you debug your playbook to get it to run?

Root cause
​The problem may not be in the playbook.  

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