Ok, so after looking at this for a while, I decided there had to be a way to do this with a one liner. Here it is:
(gc "c:\myfile.txt") | % -Begin {$test = (gc "c:\myfile.txt" | select -first 1 -last 1)} -Process {if ( $_ -eq $test[0] -or $_ -eq $test[-1] ) { $_ -replace "-" } else { $_ }} | Set-Content "c:\myfile.txt"
Here is a breakdown of what this is doing:
First, the aliases for those now familiar. I only put them in because the command is long enough as it is, so this helps keep things manageable:
gc means Get-Content% means Foreach$_ is for the current pipeline value (this isn't an alias, but I thought I would define it since you said you were new)
Ok, now here is what is happening in this:
(gc "c:\myfile.txt") | --> Gets the content of c:\myfile.txt and sends it down the line% --> Does a foreach loop (goes through each item in the pipeline individually)-Begin {$test = (gc "c:\myfile.txt" | select -first 1 -last 1)} --> This is a begin block, it runs everything here before it goes onto the pipeline stuff. It is loading the first and last line of c:\myfile.txt into an array so we can check for first and last items-Process {if ( $_ -eq $test[0] -or $_ -eq $test[-1] ) --> This runs a check on each item in the pipeline, checking if it's the first or the last item in the file{ $_ -replace "-" } else { $_ } --> if it's the first or last, it does the replacement, if it's not, it just leaves it alone| Set-Content "c:\myfile.txt" --> This puts the new values back into the file.
Please see the following sites for more information on each of these items:
Get-Content uses
Get-Content definition
Foreach
The Pipeline
Begin and Process part of the Foreach (this are usually for custom function, but they work in the foreach loop as well)
If ... else statements
Set-Content
So I was thinking about what if you wanted to do this to many files, or wanted to do this often. I decided to make a function that does what you are asking. Here is the function:
function Replace-FirstLast {
[CmdletBinding()]
param(
[Parameter( `
Position=0, `
Mandatory=$true)]
[String]$File,
[Parameter( `
Position=1, `
Mandatory=$true)]
[ValidateNotNull()]
[regex]$Regex,
[Parameter( `
position=2, `
Mandatory=$false)]
[string]$ReplaceWith=""
)
Begin {
$lines = Get-Content $File
} #end begin
Process {
foreach ($line in $lines) {
if ( $line -eq $lines[0] ) {
$lines[0] = $line -replace $Regex,$ReplaceWith
} #end if
if ( $line -eq $lines[-1] ) {
$lines[-1] = $line -replace $Regex,$ReplaceWith
}
} #end foreach
}#End process
end {
$lines | Set-Content $File
}#end end
} #end function
This will create a command called Replace-FirstLast. It would be called like this:
Replace-FirstLast -File "C:\myfiles.txt" -Regex "-" -ReplaceWith "NewText"
The -Replacewith is optional, if it is blank it will just remove (default value of ""). The -Regex is looking for a regular expression to match your command. For information on placing this into your profile check this article
Please note: If you file is very large (several GBs), this isn't the best solution. This would cause the whole file to live in memory, which could potentially cause other issues.
