I am writing Powershell script, which will essentially be a menu driven utility for doing random tasks. One of the feature includes, to display java version. I am using REPLto have it in continues loop. My issue is that the Java version is just not being displayed on the screen. Can anyone please help me with the reason for this or a work around ? Below is the code:

 enum Outcome
{
    Continue
    Quit
}
 
 Class Opatch_Manager {
     
     REPL() {

        while($this.ShowMenu() -eq 'Continue') {
            $this.Print()
        }
    }

    Print() {
        Write-Host "Test"
    }
    
   [String] ShowJava() {
     java -version
    }
   
    [Outcome] ShowMenu() {
        $this.ShowJava() 
        return [Outcome]::Quit
     }
     
 }
 
 $obj = New-Object Opatch_Manager
 $obj.REPL()
 

CodePudding user response：

Unlike direct use of commands in scripts and functions, methods in PowerShell custom classes must use return in order to produce output (and the method must declare a suitable return type).

Similarly, stderr output - which is what java -version produces - isn't automatically passed through.

Note:

• Using java --version instead, i.e two leading -, produces stdout instead, so that
  return java --version would produce output, albeit probably not in the expected format - see below, which discusses the formatting aspect as well as the general case when stderr output must be captured.

Thus, combine return with a 2>&1 redirection (simplified example):

class Foo {

  [String] ShowJava() {
    return (java -version 2>&1) -join "`n"
   }

}  

[Foo]::new().ShowJava() 

• 2>&1 redirects stderr (2) output to PowerShell's success output stream (the analog to stdout) so that return can return it.

• -join "`n" is used to return the output lines as a single, multi-line string.

  • This is necessary, because your method is declared as [string], whereas output from an external program is reported line by line by PowerShell, which becomes an array when collected, and PowerShell stringifies arrays (typically) as a single-line string, namely by concatenating the (stringified) elements with spaces.

    • Note: The simpler java -version 2>&1 | Out-String works in principle, but Out-String unfortunately invariably adds a trailing newline to the resulting string - see GitHub issue #14444 for a discussion.

  • The alternative is to declare your method as [string[]] - then
    return java -version 2>&1 will do (and an array of lines is returned)