How to check if file has valid JSON syntax in Powershell

UPDATE 2021: PowerShell 6 and newer versions

PowerShell 6 brings a brand new Test-Json cmdlet. Here is the reference.

You can simply pass the raw file content directly to the Test-Json cmdlet.

$text = Get-Content .\filename.txt -Raw

if ($text | Test-Json) {
    $powershellRepresentation = ConvertFrom-Json $text -ErrorAction Stop;
    Write-Host "Provided text has been correctly parsed to JSON";
} else {
    Write-Host "Provided text is not a valid JSON string";
}

PowerShell 5 and earlier versions

There is no Test-Json cmdlet in these versions, so the best way is to put your ConvertFrom-Json cmdlet inside a try ... catch block

try {
    $powershellRepresentation = ConvertFrom-Json $text -ErrorAction Stop;
    $validJson = $true;
} catch {
    $validJson = $false;
}

if ($validJson) {
    Write-Host "Provided text has been correctly parsed to JSON";
} else {
    Write-Host "Provided text is not a valid JSON string";
}

If you encounter this question and can use PowerShell 6 or later, there is now a Test-Json cmdlet. It can also not just validate that it's valid JSON, but that it conforms to a particular JSON schema using the -Schema param.

Example

$isValid = Get-Content .\filename.txt -Raw | Test-Json 

if($isValid){
 Write-Host "not JSON"
}
else{
 Write-Host "True"
}

ARM Template Warning

A note for users looking to validate an ARM template via -Schema (I can't imagine a more perfect use case). At the time of writing, there are one or more bugs in the underlying library Test-Json uses for validation, NJsonSchema, and it is not possible to validate an ARM template.

GitHub Issues

• PowerShell Issue #9560
• NJsonSchema Issue #588