How does powershell handle default-value-if-null in a variable declaration?

Question

For example, in BASH I can write:

# if foo is empty, it will equal 'bar'
foo=${foo:='bar'}

How would I do that in powershell? Ideally, I can set a parameter's default value to an environment variable that defaults to a literal, written for communication purposes in pseudo code:

param(
  $foo=${env:foo:='bar'} # in pseudo code
)

How does powershell handle this situation?

Does this answer your question? Set PowerShell variable with environment variable or a default if not present — user459872
– user459872, Commented Oct 16, 2023 at 17:03
if you want to handle the condition inside the param block, you would need to use $(...) i.e.: param($foo = $( <# condition inside #> ).... personally wouldn't recommend that, much better to handle this in the blocks. I believe Abdul's link answers this otherwise — Santiago Squarzon
– Santiago Squarzon, Commented Oct 16, 2023 at 17:11

mklement0 · Accepted Answer · 2023-10-17 22:49:52Z

2

In Windows PowerShell:

param(
  $foo = ($env:foo, 'foo')[$null -eq $env:foo]
)

Note:

This takes advantage of the fact that when a Boolean expression such as $null -eq $env:foo is used as an array index, $true is coerced to 1, and $false to 0, so that the appropriate array element is returned.
- Note: It is not a concern here, but due to use of an array literal, all array elements are evaluated up front, before the element of interest is extracted.
The conceptually clearer (and short-circuiting), but more verbose alternative is to use a regular if statement, which, however, requires enclosure in $(...), the subexpression operator:
```
param(
  $foo = $(if ($null -eq $env:foo) { 'foo' } else { $env:foo })
)
```

In PowerShell (Core) 7+, you can use ??, the null-coalescing operator:

param(
  $foo = $env:foo ?? 'foo'
)

edited Oct 17, 2023 at 22:49

answered Oct 16, 2023 at 20:22

mklement0

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