Strip lines from text file based on content - powershell

Question

I like to use one of the packaged HOSTS (MVPS,) files to protect myself from some of the nastier domains. Unfortunately, sometimes these files are a bit overzealous for me (blocking googleadsservices is a pain sometimes). I want an easy way to strip certain lines out of these files. In Linux I use:
cat hosts |grep -v <pattern> >hosts.new
And the file is rewritten minus the lines referencing the pattern I specified in the grep. So I just set it up to replace hosts with hosts.new on reboot and I'm done.
Is there an easy way to do this in PowerShell?

Answer 1

In PowerShell you'd do
(Get-Content hosts) -notmatch $pattern | Out-File hosts.new
or
(cat hosts) -notmatch $pattern > hosts.new
for short.
Of course, since Out-File (and with it the redirection operator) default to Unicode format, you may actually want to use Set-Content instead of Out-File:
(Get-Content hosts) -notmatch $pattern | Set-Content hosts.new
or
(gc hosts) -notmatch $pattern | sc hosts.new
And since the input file is read in a grouping expression (the parentheses around Get-Content hosts) you could actually write the output back to the source file:
(Get-Content hosts) -notmatch $pattern | Set-Content hosts

Answer 2

To complement Ansgar Wiechers' helpful answer (which offers pragmatic and concise solutions based on reading the entire input file into memory up-front):
PowerShell's grep equivalent is the Select-String cmdlet and, just like grep, it directly accepts a filename argument (PSv3+ syntax):
Select-String -NotMatch <pattern> hosts | ForEach-Object Line | Set-Content hosts.new
Select-String -NotMatch <pattern> hosts is short for
Select-String -NotMatch -Pattern <pattern> -LiteralPath hosts and is the virtual equivalent of
grep -v <pattern> hosts
However, Select-String doesn't output strings, it outputs [Microsoft.PowerShell.Commands.MatchInfo] instances that wrap matching lines (stored in property .Line) along with metadata about the match.
ForEach-Object Line extracts just the matching lines (the value of property .Line) from these objects.
Set-Content hosts.new writes the matching lines to file hosts.new, using "ANSI" encoding in Windows PowerShell - i.e., it uses the legacy code page implied by the active system locale, typically a supranational 8-bit superset of ASCII - and UTF-8 encoding (without BOM) in PowerShell Core.
Use the -Encoding parameter to specify a different encoding.
>, by contrast (an effective alias of the Out-File cmdlet), creates:
UTF16-LE ("Unicode") files by default in Windows PowerShell.
UTF-8 files (without BOM) in PowerShell Core - in other words: in PowerShell Core, using
> hosts.new in lieu of | Set-Content hosts.new will do.
Note: While both > / Out-File and Set-Content are suitable for sending string inputs to an output file, they are not generally suitable for sending other data types to a file for programmatic processing: > / Out-File output objects the way they would print to the console / terminal, which is pretty format for display, whereas Set-Content stringifies (simply put: calls .ToString() on) the input objects, which often results in loss of information.
For non-string data, consider a (more) structured data format such as XML (Export-CliXml), JSON (ConvertTo-Json) or CSV (Export-Csv).