Add eighth and ninth lines to all *.txt files - powershell

Question

i have more than 100 txt files in C:\myfolder*.txt
when i run this script from "C:\myfolder" i can add eighth and ninth lines to somename.txt
#echo off
powershell "$f=(Get-Content somename.txt);$f[8]='heretext1';$f | set-content somename.txt"
powershell "$f=(Get-Content somename.txt);$f[9]='heretext2';$f | set-content somename.txt"
but how can i add eighth and ninth lines to all *.txt files located in path C:\myfolder*.txt
Can someone explain me how to do it please...
Sorry for my English and Sorry if i didn't explaned my problem. i will try now:
I uses "*.uci" files, instead of *.txt files. i wrote txt because uci extensions are unknown for most of the people. These *.uci files are settings for chess engines with uci protocol.
So when you use chessbase program you have a lot of chess engines and each engine creates their "enginename.uci" file.
If you want to change the numbers of core used on your PC from 1 to 16 you need to do it manually by adding following information in *.uci file like this:
[OPTIONS]
Threads=1
That's why is better to make small batch or ps1 to change settings to all engines by adding these two lines with one click

Answer 1

Perhaps something like this PowerShell script would suit your task:
Get-ChildItem -Path 'C:\myfolder' -Filter '*.txt' | ForEach-Object {
$LineIndex = 0
$FileContent = Switch -File $_.FullName {Default {
$LineIndex++
If ($LineIndex -Eq 8) {#'
heretext1
heretext2
'#}
$_}}
Set-Content -Path $_.FullName -Value $FileContent}

Answer 2

Note:
Your code isn't adding lines, it is modifying existing lines. The solution below does the same.
Indices [8] and [9] access the 9th and 10th lines, not the 8th and 9th, given that array indexing is 0-based.
You need to call Get-ChildItem with your file-name pattern, C:\myfolder\*.txt, and process each matching file via ForEach-Object:
#echo off
powershell "Get-ChildItem C:\myfolder\*.txt | ForEach-Object { $f=$_ | Get-Content -ReadCount 0; $f[8]='heretext1'; $f[9]='heretext2'; Set-Content $_.FullName $f }"
Due to calling from a batch file (cmd.exe), the PowerShell command is specified on a single line; here's the readable version:
Get-ChildItem C:\myfolder\*.txt | # get all matching files
ForEach-Object { # process each
$f = $_ | Get-Content -ReadCount 0 # read all lines
$f[8] = 'heretext1'; $f[9] = 'heretext2' # update the 9th and 10th line
Set-Content $_.FullName $f # save result back to input file
}
Note:
Consider adding -noprofile after powershell, so as to suppress potentially unnecessary loading of profile files - see the documentation of the Windows PowerShell CLI, powershell.exe.
Using -ReadCount 0 with Get-Content greatly speeds up processing, because all lines are then read into a single array, instead of streaming the lines one by one, which requires collecting them in an array, which is much slower.
Note: If a given file has fewer than 10 lines, the above solution won't work, because you can only assign to existing elements of an array (an array is a fixed-size data structure). If you need to deal wit this case, insert the following after the $f = $_ | Get-Content -ReadCount 0 line, which inserts empty lines as needed to ensure that at least 10 lines are present:
if ($f.Count -lt 10) { $f += #('') * (10 - $f.Count) }

Answer 3

Easiest solution I can think of is using the -Index parameter provided in Select-Object for that.
Get-ChildItem -Path .\Desktop\*.txt | % { Get-Content $_.FullName | Select-Object -Index 7,8 } |
Out-File -FilePath .\Desktop\index.txt
Edit: based on your post.