重定向到文件时，Write-Host 会导致空行

Question

The snippet

powershell { Write-Host "a"; Write-Host "b" } > test.txt; Write-Host "File contents:"; cat test.txt; rm test.txt

prints

a
b
File contents:
a


b

Why are there 2 blank lines after every Write-Host call in the text file?

Even more confusing is the behavior when we redirect all streams to a file:

powershell { Write-Host "a"; Write-Host "b" } *> test.txt; Write-Host "File contents:"; cat test.txt; rm test.txt

prints

File contents:
a


b


a
b

Now the file contains everything twice, first with 2 blank lines and then normally. Why does the file contain everything twice now?

Answer 1

Your script does the following:

Write-Host "a" Writes a[CR][LF] to stdout . Then powershell for some reason* adds [LF] to it and stores it as an element in return list. When it prints return list, it prints each element as [$value.toString()][CR][LF] .

This gives you result that Write-Host "a" becomes a[CR][LF][LF][CR][LF] in output.

* Needs investigation. Possibly it's if value from stdout and ends with [CR][LF] . Again, stdout is not supposed to work as pipeline in powershell, because powershell has it's own pipeline of objects.

---

Powershell's Write-Host is not designed to be catched to output. Instead, there is Write-Output command, or return .

There is no good built-in command to output data both to file and to stdout except Tee-Object , but this does some different thing.

You should avoid using powershell's stdout output as stdin for other commands, as it can cantain better-look enchancements , truncated parts or line wraps that you can not expect. If you can not avoid it, use Write-Output and void any possible output with [void] or .. | Out-Null .. | Out-Null . All unhandled returns from any operations (like, new-item ) will be passed to stdout

---

powershell {Write-Output "a" ;Write-Output "b"} > test.txt; Write-Host "File contents:"; cat test.txt; 
powershell {Write-Output "a" ;Write-Output "b"} | Out-File 'test.txt' ; Write-Host "File contents:"; cat test.txt; 
powershell { @('A';'B') | Out-File 'test.txt' } ; Write-Host "File contents:"; cat test.txt; 
powershell { @('A';'B') | Tee-Object -FilePath 'test.txt' | % { Write-Host "+ $_" } } | Out-Null ; Write-Host "File contents:"; cat test.txt; 
powershell {Write-Host "a" -NoNewline;Write-Host "b" -NoNewline} > test.txt; Write-Host "File contents:"; cat test.txt;