Incrementing a Build Pipeline Variable on AzureDevops.

IncrementPipelineVariable.ps1

# In the Phase definition make sure to check "Allow scripts to access the OAuth token" otherwise this will fail.
$VariableName = "ProjectBuildNumber"

$defId = $env:SYSTEM_DEFINITIONID
# Uses built in environment variables to create the token and api call for us
# If you want to hit a release instead just change "build/definitions/" to "release/definitions"
$url = "$env:SYSTEM_TEAMFOUNDATIONCOLLECTIONURI$env:SYSTEM_TEAMPROJECTID/_apis/build/definitions/" + $defId + "?api-version=5.0"
Write-Host "URL: $url"
# Get the build definition
$pipeline = Invoke-RestMethod -Uri $url -Headers @{
    Authorization = "Bearer $env:SYSTEM_ACCESSTOKEN"
} -Method Get

# Increment the variable
$oldValue = $pipeline.variables.($VariableName).value;
$newValue = [int]$oldValue + 1;
$pipeline.variables.($VariableName).value = $newValue;

# Update the object
$updatedPipeline = $pipeline | ConvertTo-Json -Depth 100
$updatedPipeline = [Text.Encoding]::UTF8.GetBytes($updatedPipeline)

Write-Host ("Updating build...")
$pipeline = Invoke-RestMethod -Uri $url -Headers @{
    Authorization = "Bearer $env:SYSTEM_ACCESSTOKEN"
} -Method Put -ContentType "application/json" -Body $updatedPipeline -Verbose -Debug
Write-Host "Successfully incremented $VariableName's value from $oldValue to $newValue"