There are a number of different ways to output messages. What is the effective difference between outputting something via Write-Host, Write-Output, or [console]::WriteLine?
I also notice that if I use:
Write-Host "count=" + $count
The + gets included in the output. Why's that? Shouldn't the expression be evaluated to produce a single concatenated string before it gets written out?
Write-Output should be used when you want to send data on in the pipeline, but not necessarily want to display it on screen. The pipeline will eventually write it to Out-Default if nothing else uses it first.
Write-Host should be used when you want to do the opposite.
[console]::WriteLine is essentially what Write-Host is doing behind the scenes.
Run this demonstration code and examine the result.
function Test-Output {
Write-Output "Hello, World!"
}
function Test-Output2 {
Write-Host "Hello, World!" -foreground Green
}
function Receive-Output {
process { Write-Host $_ -foreground Yellow }
}
# Output is piped to another function, not displayed in the first.
Test-Output | Receive-Output
# Output us not piped to the 2nd function, only displayed in the first.
Test-Output2 | Receive-Output
# The pipeline sends to Out-Default at the end.
Test-Output
You'll need to enclose the concatenation operation in parentheses, so that PowerShell processes the concatenation before tokenizing the parameter list for Write-Host, or use string interpolation
Write-Host ("count=" + $count)
# or
Write-Host "count=$count"
BTW - Watch this video of Jeffrey Snover explaining how the pipeline works. Back when I started learning PowerShell, I found this to be the most useful explanation of how the pipeline works.
Write-Output is the default e.g. if you have a PsObject and you just spit it out to the screen by doing this $object it will actually do the same thing as this Write-Output $object. Might be worth mentioning
Write-Output when possible. See github.com/PoshCode/PowerShellPracticeAndStyle/issues/… > Use return only for ending execution. > Avoid Write-Output (...). Instead, when you want to make output clearer, just assign output to a relevantly named variable. and put that a variable on a line by itself to signal explicit output. > Write-Output -NoEnumerate is broken on PowerShell 6 and has been for 16 months or longer -- there's no plan to fix it. In summary: > DO NOT USE Write-Output -- EVER.
Write-Host no longer writes directly to the console. Instead, it writes to the new information stream, which may end up in the console, if not redirected (e.g. Write-Host 'foo' 6>&1 | Set-Content file.txt).Apart from what Andy mentioned, there is another difference which could be important: Write-Host directly writes to the host and returns nothing, meaning that you can't redirect the output, e.g., to a file.
---- script a.ps1 ----
Write-Host "hello"
Now run in PowerShell:
PS> .\a.ps1 > someFile.txt
hello
PS> type someFile.txt
PS>
As seen, you can't redirect them into a file. This maybe surprising for someone who are not careful.
But if switched to use Write-Output instead, you'll get redirection working as expected.
Here's another way to accomplish the equivalent of Write-Output. Just put your string in quotes:
"count=$count"
You can make sure this works the same as Write-Output by running this experiment:
"blah blah" > out.txt
Write-Output "blah blah" > out.txt
Write-Host "blah blah" > out.txt
The first two will output "blah blah" to out.txt, but the third one won't.
"help Write-Output" gives a hint of this behavior:
This cmdlet is typically used in scripts to display strings and other objects on the console. However, because the default behavior is to display the objects at the end of a pipeline, it is generally not necessary to use the cmdlet.
In this case, the string itself "count=$count" is the object at the end of a pipeline, and is displayed.
For usages of Write-Host, PSScriptAnalyzer produces the following diagnostic:
Avoid using
Write-Hostbecause it might not work in all hosts, does not work when there is no host, and (prior to PS 5.0) cannot be suppressed, captured, or redirected. Instead, useWrite-Output,Write-Verbose, orWrite-Information.
See the documentation behind that rule for more information. Excerpts for posterity:
The use of
Write-Hostis greatly discouraged unless in the use of commands with theShowverb. TheShowverb explicitly means "show on the screen, with no other possibilities".Commands with the
Showverb do not have this check applied.
Jeffrey Snover has a blog post, Write-Host Considered Harmful, in which he claims Write-Host is almost always the wrong thing to do, because it interferes with automation and provides more explanation behind the diagnostic. However, the above is a good summary.
From my testing, Write-Output and [Console]::WriteLine() perform much better than Write-Host.
Depending on how much text you need to write out, this may be important.
Below is the result of five tests each for Write-Host, Write-Output and [Console]::WriteLine().
In my limited experience, I've found when working with any sort of real world data I need to abandon the cmdlets and go straight for the lower level commands to get any decent performance out of my scripts.
measure-command {$count = 0; while ($count -lt 1000) { Write-Host "hello"; $count++ }}
1312 ms
1651 ms
1909 ms
1685 ms
1788 ms
measure-command { $count = 0; while ($count -lt 1000) { Write-Output "hello"; $count++ }}
97 ms
105 ms
94 ms
105 ms
98 ms
measure-command { $count = 0; while ($count -lt 1000) { [console]::WriteLine("hello"); $count++ }}
158 ms
105 ms
124 ms
99 ms
95 ms
Write-Output and Write-Host serve different purposes, so there is no point in comparing their performance. Yes, direct use of .NET types and their methods is faster, but you miss out on the higher-level features that PowerShell cmdlets can provide. [Console]::WriteLine() is akin to Write-Host in the case at hand, but won't work in all circumstances, because not all PowerShell hosts are consoles.Regarding [Console]::WriteLine(), you should use it if you are going to use pipelines in CMD (not in PowerShell). Say you want your ps1 to stream a lot of data to standard output, and some other utility to consume/transform it. If you use Write-Host in the script, it will be much slower.
Write-Outputwhen you're emitting results.Write-Hostwhen you're emitting logging information. Never use[console]::writeline().[console]::writeline("hello world")that you can't do withWrite-Host "hello world". Another, better, more recently applicable answer is thatwrite-hostwrapswrite-informationso its data gets put on a stream likewrite-errorso you can capture it and use it elsewhere.[console]::writeline()does not do that