Note: The issue isn't specific to Unix, as originally stated - see #15329

... | Select-Object -First 1 stops the upstream command after receiving the 1st input object, but the same is not true when the sink is an external program:

Steps to reproduce

1..10 | Write-Output -ov result | head -n 1; $result.Count

Expected behavior

1
1

Actual behavior

1
10

That is, Write-Output unexpectedly ran to completion, even though head exited after receiving the first input line, making it pointless to send additional objects to the pipeline.

Environment data

PowerShell Core v6.2.0-preview.2 on macOS 10.14.1
PowerShell Core v6.2.0-preview.2 on Ubuntu 18.04.1 LTS