PowerShell if-statement with a condition in a string

Question

I have a condition stored in a string variable and I want to use this in an if-statement.

In this example there is always a match, as the variable is not zero or null, etc. It's not "executing" the variable as condition.

My wish is to "execute" the condition what is stored in the variable.

$myCheckTest = '$test -eq 4'
$test = 5

if ($myCheckTest) {
    write-host "TEST MATCH"
} else {
    write-host "TEST NO MATCH"
}

This works without the quotes like: $myCheckTest = $test -eq 4 but the condition is externally stored in a json file.

I also tried if ( & {$myCheckTest} ) { as I think that this is comparable with JavaScript if ( eval(condition) ) but this is still not giving me the proper answer "TEST NO MATCH" that I expect.

I am aware of powershell IF condition in a Variable but here the condition is not in a string-variable like I have.

The variable $test is just a string inside single quotes and doesn't get substituted. Use double quotes instead. — jdweng
– jdweng, Commented Dec 14, 2022 at 11:57
Thank you jdweng, but substitution is not what I want. The string in variable $myCheckTest comes from a json file, so substitution will not happen... I switched the first 2 lines in the example to make it more clear. — Maurice van 't Loo
– Maurice van 't Loo, Commented Dec 14, 2022 at 12:01
WHAT???? You if statement is always doing '$test -eq 4' $test is always the string "$test". So "$test" never will equal "4". — jdweng
– jdweng, Commented Dec 14, 2022 at 12:07
My wish is that '$test -eq 4' will be executed in the if-statement and that within the if-statement the $test will be replaced by 5, not before... — Maurice van 't Loo
– Maurice van 't Loo, Commented Dec 14, 2022 at 12:12

iRon · Accepted Answer · 2022-12-14 12:46:32Z

1

You might use the $ExecutionContext.InvokeCommand.ExpandString method to evaluate the string expression:

$test = 5
$myCheckTest = '$test -eq 4'
$ExecutionContext.InvokeCommand.ExpandString("`$($myCheckTest)")
False
$myCheckTest = '$test -eq 5'
$ExecutionContext.InvokeCommand.ExpandString("`$($myCheckTest)")
True

Note that the backtick (`) prevents the first dollar to be substituted so that you actual string expression will be something like: $($test -eq 4)
Or in your function:

if ($ExecutionContext.InvokeCommand.ExpandString("`$($myCheckTest)")) {
    write-host "TEST MATCH"
} else {
    write-host "TEST NO MATCH"
}

A more common way to do thing like this is, is to use a scriptblock:

$myCheckTest = {$test -eq 4}
$test = 5

if (&$myCheckTest) {
    write-host "TEST MATCH"
} else {
    write-host "TEST NO MATCH"
}

edited Dec 14, 2022 at 12:46

answered Dec 14, 2022 at 12:35

iRon

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5 Comments

Maurice van 't Loo Over a year ago

Thank you iRon for such clear and complete response, I tested this in my actual script and this works as well. I shall read about both $ExecutionContext.InvokeCommand.ExpandString as about Invoke-Expression to see the differences.

RonJohn Over a year ago

Can we use the test as a parameter to a formatted string? For example: "{0}" -f &{ if (5 -eq 5) { "yes" } else { "No" } }. It doesn't work, but maybe I'm missing something.

iRon Over a year ago

@RonJohn, that expression doesn't work by itself and looks quiet cumbersome for something that could be done like: $ExecutionContext.InvokeCommand.ExpandString('$(if (5 -eq 5) { "yes" } else { "No" })') or: $ExecutionContext.InvokeCommand.ExpandString('$(@("No", "Yes")[5 -eq 5])')

RonJohn Over a year ago

I just gave a trivial example. The real formatted string would be much more complicated.

iRon Over a year ago

@RonJohn, I see. If I didn't answer your comment, I recommend you to post a formal question with a more specific example.

Maurice van 't Loo · Accepted Answer · 2022-12-14 12:44:39Z

0

https://learn.microsoft.com/en-us/powershell/module/microsoft.powershell.utility/invoke-expression

A collegae pointed me to the Invoke-Expression command:

if (Invoke-Expression $myCheckTest) {
    write-host "TEST MATCH"
} else {
    write-host "TEST NO MATCH"
}

This works within my actual script.

answered Dec 14, 2022 at 12:44

Maurice van 't Loo

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Comments

Santiago Squarzon · Accepted Answer · 2022-12-14 13:25:23Z

0

You can use ScriptBlock.InvokeWithContext for this:

$myCheckTest = '$test -eq 4'

# Convert the string to ScriptBlock
$sb  = [scriptblock]::Create($myCheckTest)
# Argument to feed to the ScriptBlock
$arg = [psvariable]::new('test', 5)

if ($sb.InvokeWithContext(@{}, $arg)) {
    write-host "TEST MATCH"
}
else {
    write-host "TEST NO MATCH"
}

answered Dec 14, 2022 at 13:25

Santiago Squarzon

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2 Comments

Maurice van 't Loo Over a year ago

Thank you Santiago, nice to see extra options :-) but I have to say that this is a less readable then if (Invoke-Expression $myCheckTest) Still it can be very useful in certain cases....

Santiago Squarzon Over a year ago

@Mauricevan'tLoo yup; agreed. The only problem with iex is no way of handling possible arbitrary code

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