Test if file exists in PowerShell

Question

I can use Test-Path to check whether the file name entered exists, but I'd like to avoid producing a system error if the user hits RETURN and the input string is blank. I thought the -ErrorAction Common Parameter would do the trick, but this:

$configFile = Read-Host "Please specify a config. file: "
$checkfile = Test-Path $configFile -ErrorAction SilentlyContinue

still produces:

Test-Path : Cannot bind argument to parameter 'Path' because it is an empty string.
At C:\Scripts\testparm2.ps1:19 char:31
+         $checkfile = Test-Path <<<<  $configFile -ErrorAction SilentlyContinue
    + CategoryInfo          : InvalidData: (:) [Test-Path], ParameterBindingValidationException
    + FullyQualifiedErrorId : ParameterArgumentValidationErrorEmptyStringNotAllowed,Microsoft.PowerShell.Commands.TestPathCommand

Do I have to check that the string isn't blank or NULL explicitly?

I'm using PowerShell v2.0

if you check Ansgar Wiechers answer... your code returns $true when you pass a 0 — Oscar Foley
– Oscar Foley, Commented Jul 14, 2015 at 13:54
Just to avoid confusing innocent bystanders: 0 and null are different values with different meanings. — Ansgar Wiechers
– Ansgar Wiechers, Commented Jul 14, 2015 at 14:04

Ansgar Wiechers · Accepted Answer · 2015-07-14 15:04:35Z

2

You can do something like this:

$checkfile = if ("$configFile") {
               Test-Path -LiteralPath $configFile
             } else {
               $false
             }

The double quotes prevent false negatives, e.g. in case you want to test for the existence of a folder named 0.

Another option would be to set $ErrorActionPreference. However, in that case you need to cast the result of Test-Path to a boolean value, because although the exception is suppressed the cmdlet still doesn't return a result. Casting that $null "return value" to bool produces $false.

$oldEAP = $ErrorActionPreference
$ErrorActionPreference = 'SilentlyContinue'

$checkfile = [bool](Test-Path -LiteralPath $configFile)

$ErrorActionPreference = $oldEAP

edited Jul 14, 2015 at 15:04

answered Jul 14, 2015 at 13:39

Ansgar Wiechers

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5 Comments

Oscar Foley Over a year ago

Interesting "false negative" when passing 0 as parameter

Ansgar Wiechers Over a year ago

@cad A numeric value of 0 would be interpreted as $false whereas a string "0" wouldn't (because it's not empty). See here for details.

rojomoke Over a year ago

@cad @Ansgar Wiechers, Thanks to both of you. Can you explain why the -ErrorAction clause didn't work?

Oscar Foley Over a year ago

It should... I don't know why.

Ansgar Wiechers Over a year ago

@rojomoke I would suspect that the parameter binding errors are not covered by the error action (since -ErrorAction is a parameter too). You can set $ErrorActionPreference = 'SilentlyContinue' instead.

Oscar Foley · Accepted Answer · 2015-07-14 13:47:32Z

2

Yes, you have to check explicitly for string null or empty:

$configFile = Read-Host "Please specify a config. file: "
if ([string]::IsNullOrEmpty($configFile))
{
    $checkfile = $false
}
else 
{
    $checkfile = Test-Path $configFile -ErrorAction SilentlyContinue
}

Or use try/catch:

$configFile = Read-Host "Please specify a config. file: "
if ( $(Try { Test-Path $configFile.trim() } Catch { $false }) ) 
{
   $checkfile = $true
}
else 
{
   $checkfile = $false
}

edited Jul 14, 2015 at 13:47

answered Jul 14, 2015 at 13:37

Oscar Foley

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