PowerShell Write-Output not working in try/catch block? - powershell

Question

Below is my Powershell environment (via the Get-Host command):
Name : Windows PowerShell ISE Host
Version : 5.1.15063.608
InstanceId : 46c51405-fc6d-4e36-a2ae-09dbd4069710
UI : System.Management.Automation.Internal.Host.InternalHostUserInterface
CurrentCulture : en-US
CurrentUICulture : en-US
PrivateData : Microsoft.PowerShell.Host.ISE.ISEOptions
DebuggerEnabled : True
IsRunspacePushed : False
Runspace : System.Management.Automation.Runspaces.LocalRunspace
I have a script that makes a REST request to a resource -- everything works fine.
While testing, I changed the URL to be invalid to see if error messages are written to the output file -- this is where I am confused. It appears that the only way to write exceptions to the log is by using Write-Host. When I use Write-Output the exception messages are NEVER written to the log file. Below is the code I am using:
function rest_request( $method, $uri )
{
# Code here create #req dictionary
try {
$json = Invoke-RestMethod #req
} catch {
# If using Write-Output - Nothing is written to output file???
# MUST use Write-Host for error info to be included in output file - WHY?
Write-Host "* REST Request Error:"
Write-Host $error[0] | Format-List -force
$json = $null
}
return $json
}
function do_something()
{
Write-Output "Start..."
# other functions called here but removed for clarity.
# The other functions contain Write-Output statements
# which are showing in the log file.
# Issue REST request with invalid URL to cause exception ...
$json = rest_request "Get" "https://nowhere.com/rest/api"
Write-Output "Done."
}
do_something | Out-File "D:\temp\log_file.txt" -Append
According to this blog post: using Write-Host should be avoided, and yet, it seems that Write-Host is the only way I can get error messages in the output file.
My question is: How do you write a function that makes a REST request and returns the result if successful, but if an error occurs, write the error message to an output file and return $null?
As I am fairly green when it comes to PowerShell, any advice would be appreciated.

Answer 1

Ok - I gave up completely on redirection and instead wrote a function that writes the parameter to the log file, as follows:
function log_write
{
param
(
[Parameter(ValueFromPipeline=$true)] $piped
)
$piped | Out-File $log_file -Append
}
Note: $log_file is set at initialization as I needed a new log file name for each day of the week.
Then, I replaced ALL Write-Output / Write-Host with log_write, i.e.:
log_write "* REST Request Error:"
$s = $error[0] | Format-List -force
log_write $s
Works like a charm and exceptions messages are written to the log file without any of the previous issues.

Answer 2

As far as I know Write-Host only writes to the host, nothing else.
In your example, do_something does not return anything, so it's no wonder you don't get anything in the log file.
Here is how you could do it, but many different approaches exist:
function Send-RESTRequest {
Param($method, $uri, $logFile)
# Code here create #req dictionary
try {
# make sure exception will be caught
$json = Invoke-RestMethod #req -ErrorAction Stop
} catch {
# send exception details to log file
$_ | Out-File $logFile -Append
$json = $null
}
return $json
}
function Do-Something {
# Issue REST request with invalid URL to cause exception ...
$json = Send-RESTRequest -method "Get" -uri "https://nowhere.com/rest/api" -logFile "D:\temp\log_file.txt"
}
Do-Something