It is quite common when writing PowerShell script that you need to check if a variable, which is supposed to be a string, is not null or empty before processing it. I see many folks use [string]::IsNullOrEmpty($str) including myself in the past. But it appears that this approach is unnecessary and requires more typing. In PowerShell you can easily test for a non-null or empty string like so:
1> function IsNullOrEmpty($str) {if ($str) {"String is not empty"} else {"String is null or empty"}}
Now let’s double check that this is working as expected:
2> $str = $null
3> IsNullOrEmpty $str
String is null or empty
3> IsNullOrEmpty $str
String is null or empty
4> $str = ”
5> IsNullOrEmpty $str
String is null or empty
5> IsNullOrEmpty $str
String is null or empty
6> $str = ‘hi’
7> IsNullOrEmpty $str
String is not empty
7> IsNullOrEmpty $str
String is not empty
So as we can see. It is enough to do a simple check "if ($str)" on a string to make sure it is not null nor empty.
Update 4/25/2008: Bill points out in a comment that you do need to still explicitly test for [DBNull]::Value if you work with data from a database.
Keith Hill's Blog 
I read your post and thought.. Cool. that makes a lot of sense.. and quickly threw away my IsNullOrEmpty function.
But, if($str) didn\’t seem to work in all cases.
After some checking, remembered that objects from the database are not $Null, they are [DBNull]::Value.
So I re-built my function and added. if ($str -eq [DBNull]::Value){ return $True;}
Not technically a string, but keeps me from having to check the value twice.
Bill
Great article and great comment by Bill
Bill, good point. I don\’t do much with databases so the DBNull issue didn\’t occur to me. Thanks for pointing that out.
Thanks Bill,
When loading a CSV and testing a column for existance, I was getting back unexpected results (empty columns being returned). I found your DBNULL test worked…
EG:
$csv = import-csv “.\links.csv”
foreach ($line in $csv)
if ($($line.Column1) -ne [DBNull]::Value)
{
$link = $($line.Column1)
write-host “$link”
}
Thanks,
SebastianRizzo
New to powershell, but figured I’d share my findings. This (!$var) also works for an empty value in a CSV. Kinda a long example, but it’s what I was working on.
====CSV FILE====
id,firstname,lastname
user-a,,lastname-a
user-b,firstname-b,lastname-b
user-c,firstname-c,
====CODE====
# CSV file location
$CSVFile = Import-Csv -path “c:\scripts\logs\ad_update.txt”
foreach ($account in $CSVFile)
{
Write-host “$($account.id),$($account.firstname),$($account.lastname)”
if (($account.firstname) -and ($account.lastname))
{
Write-Host “First and Last Name Change for account: $($account.id)”
}
if (($account.firstname) -and (!$account.lastname))
{
Write-Host “First Name Change for account: $($account.id)”
}
if ((!$account.firstname) -and ($account.lastname))
{
Write-Host “Last Name Change for account: $($account.id)”
}
Write-Host
}
====OUTPUT====
user-a,,lastname-a
Last Name Change for account: user-a
user-b,firstname-b,lastname-b
First and Last Name Change for account: user-b
user-c,firstname-c,
First Name Change for account: user-c
I just think it is ok only for strings not bools
Not sure I follow you. Booleans work even better because no coercion is involved.
$abool = $true
if ($abool) { “This should be output” }
Works just as well as:
if ($abool -eq $true) { “This should be output” }
You check if a variable is NULL, EMPTY, or having whitespaces by using system.string dotnet class. Check http://techibee.com/powershell/check-if-a-string-is-null-or-empty-using-powershell/1889 for examples.
Yes, if you also need to check for a string containing only whitespace then [string]::IsNullOrWhitespace($str) is the way to go. However, in my experience with PowerShell script, I’ve rarely needed the extra “whitespace” test.
Had good success with trim() and trimEnd() methods on a string.
Saw unexpected errors looping thru a list of PC names due to trailing space characters. I introduced the trailing spaces by “cleaning up” my PC list with a programming editor. Now I don’t trust “human generated” input before I use trim() on it.
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