Passing $args[x] in an if statement in .ps1 (PowerShell) does not seem to work when it does not exist

Question

Suppose I have the following testArgsX.ps1 file:

$ARG_X_PARAMETER=1
If ($args[0] -ne "") {
    echo ARGS[X]="`"$args[0]`""
    $ARG_X_PARAMETER=$args[0]
}
echo ARG_X_PARAMETER=$ARG_X_PARAMETER

When I execute it either in a command prompt (C:\Test> powershell .\testArgsX.ps1) or in PowerShell prompt (PS C:\Test> .\testArgsX.ps1), it outputs the following:

ARGS[X]="[0]"
ARG_X_PARAMETER=

It seems that $args[0], in the if-condition, is not interpreted as a scalar value. Although I understand there are 2 ways to circumvent this problem (shown below). I want to know why it does not interpret it as a scalar value and if there is a way to fix it so that it does.

Take out the entire if statement (If ( ... ) { ... }) and replace it with Param ( [Int]$RUN_JUST_ONCE = 1 )
Change the if condition from If ($args[0] -ne "") { to If ($args.Length -gt 0) {

mklement0 · Accepted Answer · 2020-12-02T04:06:16.580

$args[0] is a scalar, but accessing a non-existent array element in PowerShell returns $null^[1], and $null is indeed not equal to the empty string ("" or '').

Therefore, you should test for $null.
- Note that testing for $null is best performed by placing $null on the LHS ($null -ne $args[0]), because on the RHS it would act as a filter instead of returning a Boolean in case the other operand happens to be an array.
Also, note that in order to use an expression such as $args[0] inside "...", an expandable (interpolating) string, you must use $(), the subexpression operator; without it, $args as a whole is expanded, and [0] is used verbatim - see this answer
echo is a built-in alias for the Write-Output cmdlet, whose explicit use is rarely needed; simply using a command or expression whose output is neither captured nor redirected is implicitly output.

To put it all together:

$ARG_X_PARAMETER=1
If ($null -ne $args[0]) {
    "ARGS[X]=`"$($args[0])`""
    $ARG_X_PARAMETER=$args[0]
}

"ARG_X_PARAMETER=$ARG_X_PARAMETER"

Alternatively, declare parameters for your script, which also makes it easy to assign default values:

param(
  # This implicitly becomes the 1st positional parameter.
  $ARG_X_PARAMETER = 1
)

"ARG_X_PARAMETER=$ARG_X_PARAMETER"

By default, parameters are positional, which means they can be used unnamed (e.g. testArgsX.ps1 42), though using the name explicitly is always an option (e.g.
testArgsX.ps1 -ARG_X_PARAMETER 42) - though a better name is the probably called for.

PowerShell optionally allows you to assign a data type to parameters and use a variety of attributes, notably ones for argument validation.

See about_Functions and about_Functions_Advanced_Parameters.

^{[1] With Set-StrictMode -Version 3 or higher in effect, accessing a non-existent element causes a statement-terminating error instead.}

Passing $args[x] in an if statement in .ps1 (PowerShell) does not seem to work when it does not exist

1 Answers1