what is var=${var:-word} bash equivalent in powershell - bash

Question

Given this bash code:
HELLO =${HELLO:-hello}
the variable HELLO takes a value from the HELLO environment variable if it exists. Otherwise it sets the value to be hello.
What is the Powershell equivalent?

Answer 1

PowerShell, as of Windows PowerShell v5.1 / PowerShell Core 6.1.0, has no equivalent functionality to Bash's parameter expansion feature, of which ${HELLO:-hello} is an instance[1].
Note:
In Bash, environment variables and Bash's own shell variables share the same namespace, and environment variables are automatically exposed as shell variables.
In PowerShell, only PowerShell's own variables can be referenced directly - e.g., $myVar - whereas referencing environment variables requires explicit use of the env: namespace - e.g., $env:PATH
The solutions below focus mostly on PowerShell's own variables, but the techniques can analogously be applied to environment variables.
Note that while environment variables are always strings, PowerShell variables can be of any (.NET) type.
To emulate HELLO=${HELLO:-hello} in PowerShell, use:
# To target an *environment* variable, use $env:HELLO instead.
$HELLO = if ("$HELLO") { $HELLO } else { 'hello' }
Note the "..." around $HELLO in the conditional, which ensures that the value is converted to a string before coercing it to a Boolean[2]: that way, both the case of $HELLO not having been defined (or explicitly containing $null) and the variable containing the empty string evaluate to $False, which parallels Bash's behavior.
Without the stringification with "...", non-string values such as 0 or $False too would trigger the else branch.
However, if you only ever expect $HELLO to contain a string value, if any, you can omit the "...".
Similarly, the above also works with environment variables, but since they are always strings, you don't strictly need the enclosing "..." in that case:
$env:HELLO = if ($env:HELLO) { $env:HELLO } else { 'hello' }
In the simple case of leaving any preexisting value of $HELLO alone and only assigning a default value in the in the absence of the former:
if (-not "$HELLO") { $HELLO = 'hello' }
# As an environment variable
if (-not $env:HELLO) { $env:HELLO = 'hello' }
To emulate HELLO=${HELLO-hello} - note the absence of the : -, use:
$HELLO = if ($null -eq $HELLO) { 'hello' } else { $HELLO }
# Simplified
if ($null -eq $HELLO) { $HELLO = 'hello' }
This covers only the case of $HELLO not being defined (and also it explicitly containing $null, but that isn't common).
Note that the $null is deliberately used as the LHS, which is a good habit to form in PowerShell to avoid surprises if the LHS happens to be an array, in which case -eq acts an array filter rather than returning a Boolean.
[1] While Bash's parameter expansion will likely never be implemented in PowerShell as such, simply because it is not a good syntactic fit for the language, providing concise, PowerShell-idiomatic analogs to Bash's ${HELLO-hello} and ${HELLO=hello} is being discussed, as $HELLO ?? 'hello' and $HELLO ?= 'hello' - see this GitHub issue.
[2] PowerShell coerces any string to a Boolean with this simple rule: if the string is empty, it evaluates to $False; if it is non-empty - whatever its contents - it evaluates to $True.

Answer 2

The most straight forward way is :
$Hello = If($env:hello -eq $null){"WORLD"}else{$env:hello}
Or you can make a alias if you plan on using it a lot
function IfNull($If, $Else){
if($If -eq $Null){
$Else
}else{
$If
}
}
Add-Alias "??" IfNull
$Hello = ?? $env:hello "World2"