Exercise - Error handling

3 minutes

In this exercise, you'll use a Try/Catch block to ensure the script stops responding early if a certain condition isn't met. You'll again work with your backup script.

Say you've noticed that you sometimes specify an erroneous path, which causes backup of files that shouldn't be backed up. You decide to add some error management.

Note

This exercise is optional. If you want to complete this exercise, you'll need to create an Azure subscription before you begin. If you don't have an Azure account or you don't want to create one at this time, you can read through the instructions so you understand the information that's being presented.

Note

In this unit, you use Azure Cloud Shell as a terminal. You can access Cloud Shell through the Azure portal or the Cloud Shell sign-in. You don't have to install anything on your PC or laptop to use it.

Note

Run the following commands only if you haven't completed any of the previous exercises in this module. We're assuming you've completed the previous exercises. If you haven't done so, you need a few files.

If you haven't completed the previous exercises in this module, run the following bash commands in a terminal:
```
mkdir webapp
cd webapp
touch index.html app.js
cd ..
```
These commands will create a directory that contains files typically associated with web development.

You also need a file named Backup.ps1. Run these commands:

touch Backup.ps1
code Backup.ps1

Now that you have an editor running, add the required code. Paste this code into the editor and save the file:

Param(
     [string]$Path = './app',
     [string]$DestinationPath = './'
   )

If(-Not (Test-Path $Path)) 
{
   Throw "The source directory $Path does not exist, please specify an existing directory"
}

$date = Get-Date -format "yyyy-MM-dd"

$DestinationFile = "$($DestinationPath + 'backup-' + $date + '.zip')"
If (-Not (Test-Path $DestinationFile)) 
{
  Compress-Archive -Path $Path -CompressionLevel 'Fastest' -DestinationPath "$($DestinationPath + 'backup-' + $date)"
  Write-Host "Created backup at $($DestinationPath + 'backup-' + $date + '.zip')"
} Else {
  Write-Error "Today's backup already exists"
}

Implement a business requirement by using `Try/Catch`

Assume your company mostly builds web apps. These apps consist of HTML, CSS, and JavaScript files. You decide to optimize the script to recognize web apps.

Use an existing PowerShell shell, if you have one running. Otherwise, start one by typing pwsh in a terminal:
```
pwsh
```
Open Backup.ps1. In the Param section, add a comma after the last parameter, and then add the following parameter:
```
[switch]$PathIsWebApp
```
You've added a switch parameter. If this parameter is present when the script is invoked, you perform the check on the content. After that, you can determine if a backup file should be created.
Under the Param section, add this code, then save the file:
```
If ($PathIsWebApp -eq $True) {
   Try 
   {
     $ContainsApplicationFiles = "$((Get-ChildItem $Path).Extension | Sort-Object -Unique)" -match  '\.js|\.html|\.css'

     If ( -Not $ContainsApplicationFiles) {
       Throw "Not a web app"
     } Else {
       Write-Host "Source files look good, continuing"
     }
   } Catch {
    Throw "No backup created due to: $($_.Exception.Message)"
   }
}
```
The preceding code first checks if the parameter $PathIsWebApp is provided at runtime. If it is, the code continues to get a list of file extensions from the directory specified by $Path. In our case, if you run that part of the code on the webapp directory, the following code will print a list of items:
```
(Get-ChildItem $Path).Extension | Sort-Object -Unique
```
Here's the output:
```
.html
.js
```
In the full statement, we're using the -match operator. The -match operator expects a regular expression pattern. In this case, the expression states "do any of the file extensions match .html, .js, or .css?" The result of the statement is saved to the variable $ContainsApplicationFiles.

Then the If block checks whether the $ContainsApplicationFiles variable is True or False. At this point, the code can take two paths:
- If the source directory is for a web app, the script writes out "Source files look good, continuing."
- If the source directory isn't for a web app, the script throws an error that states "Not a web app." The error is caught in a Catch block. The script stops, and you rethrow the error with an improved error message.
Test the script by providing the switch $PathIsWebApp:

Note

Before you run the script, make sure there are no .zip files present. They might have been created when you completed previous exercises in this module. Use Remove-Item *zip to remove them.
```
./Backup.ps1 -PathIsWebApp -Path './webapp'
```
The script should print output that looks similar to this text:
```
Source files looks good, continuing
Created backup at ./backup-2021-12-30.zip
```
Using your terminal, create a directory named python-app. In the new directory, create a file called script.py:
```
mkdir python-app
cd python-app
touch script.py
cd ..
```
Your directory should now look like this:
```
-| webapp/
---| app.js
---| index.html
-| python-app/
---| script.py
-| Backup.ps1
```
In the PowerShell shell, run the script again, but this time change the -Path value to point to ./python-app:
```
./Backup.ps1 -PathIsWebApp -Path './python-app'
```
Your script should now print this text:
```
No backup created due to: Not a web app
```
The output indicates that the check failed. It should have, because there are no files in the directory that have an .html, .js, or .css extension. Your code raised an exception that was caught by your Catch block, and the script stopped early.

Congratulations! You've implemented a business requirement.

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Exercise - Error handling

Implement a business requirement by using Try/Catch

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Implement a business requirement by using `Try/Catch`