To complement Theo's helpful answer with a performance optimization based on the little-used -ReadCount parameter:
Set-Content -NoNewLine -Encoding ascii .\outfile.txt -Value (
(Get-Content -First 100 -ReadCount 100 .\file.txt) -join "`n") + "`n"
)
-First 100 instructs Get-Content to read (at most) 100 lines.
-ReadCount 100 causes these 100 lines to be read and emitted together, as an array, which speeds up reading and subsequent processing.
- Note: In PowerShell [Core] v7.0+ you can use shorthand
-ReadCount 0 in combination with -First <n> to mean: read the requested <n> lines as a single array; due to a bug in earlier versions, including Windows PowerShell, -ReadCount 0 always reads the entire file, even in the presence of -First (aka -TotalCount aka -Head).
Also, even as of PowerShell [Core] 7.0.0-rc.2 (current as of this writing), combining -ReadCount 0 with -Last <n> (aka -Tail) should be avoided (for now): while output produced is correct, behind the scenes it is again the whole file that is read; see this GitHub issue.
Note the + "`n", which ensures that the output file will have a trailing newline as well (which text files in the Unix world are expected to have).
While the above also works with -Last <n> (-Tail <n>) to extract from the end of the file, Theo's (slower) Select-Object solution offers more flexibility with respect to extracting arbitrary ranges of lines, thanks to available parameters -Skip, -SkipLast, and -Index; however, offering these parameters also directly on Get-Content for superior performance is being proposed in this GitHub feature request.
Also note that I've used Set-Content instead of Out-File.
If you know you're writing text, Set-Content is sufficient and generally faster (though in this case this won't matter, given that the data to write is passed as a single value).
For a comprehensive overview of the differences between Set-Content and Out-File / >, see this answer.
Set-Content vs. Out-File benchmark:
Note: This benchmark compares the two cmdlets with respect to writing many input strings received via the pipeline to a file.
# Sample array of 100,000 lines.
$arr = (, 'foooooooooooooooooooooo') * 1e5
# Time writing the array lines to a file, first with Set-Content, then
# with Out-File.
$file = [IO.Path]::GetTempFileName()
{ $arr | Set-Content -Encoding Ascii $file },
{ $arr | Out-File -Encoding Ascii $file } | % { (Measure-Command $_).TotalSeconds }
Remove-Item $file
Sample timing in seconds from my Windows 10 VM with Windows PowerShell v5.1:
2.6637108 # Set-Content
5.1850954 # Out-File; took almost twice as long.
