Comparing two text files and output the differences in Powershell - powershell

Question

So I'm new to the Powershell scripting world and I'm trying to compare a list of IPs in text file against a database of IP list. If an IP from (file) does not exist in the (database) file put it in a new file, let's call it compared.txt. When I tried to run the script, I didn't get any result. What am I missing here?
$file = Get-Content "C:\Users\zack\Desktop\file.txt"
$database = Get-Content "C:\Users\zack\Desktop\database.txt"
foreach($line1 in $file){
$check = 0
foreach($line2 in $database)
{
if($line1 != $line2)
{
$check = 1
}
else
{
$check = 0
break
}
}
if ($check == 1 )
{
$line2 | Out-File "C:\Users\zack\Desktop\compared.txt"
}
}

Answer 1

There is a problem with your use of PowerShell comparison operators unlike in C#, equality and inequality are -eq and -ne, and since PowerShell is a case insensitive language, there is also -ceq and -cne.
There is also a problem with your code's logic, a simple working version of it would be:
$database = Get-Content "C:\Users\zack\Desktop\database.txt"
# iterate each line in `file.txt`
$result = foreach($line1 in Get-Content "C:\Users\zack\Desktop\file.txt") {
# iterate each line in `database.txt`
# this happens on each iteration of the outer loop
$check = foreach($line2 in $database) {
# if this line of `file.txt` is the same as this line of `database.txt`
if($line1 -eq $line2) {
# we don't need to keep checking, output this boolean
$true
# and break the inner loop
break
}
}
# if above condition was NOT true
if(-not $check) {
# output this line, can be `$line1` or `$line2` (same thing here)
$line1
}
}
$result | Set-Content path\to\comparisonresult.txt
However, there are even more simplified ways you could achieve the same results:
Using containment operators:
$database = Get-Content "C:\Users\zack\Desktop\database.txt"
$result = foreach($line1 in Get-Content "C:\Users\zack\Desktop\file.txt") {
if($line1 -notin $database) {
$line1
}
}
$result | Set-Content path\to\comparisonresult.txt
Using Where-Object:
$database = Get-Content "C:\Users\zack\Desktop\database.txt"
Get-Content "C:\Users\zack\Desktop\file.txt" | Where-Object { $_ -notin $database } |
Set-Content path\to\comparisonresult.txt
Using a HashSet<T> and it's ExceptWith method (Note, this will also get rid of duplicates in your file.txt):
$file = [System.Collections.Generic.HashSet[string]]#(
Get-Content "C:\Users\zack\Desktop\file.txt"
)
$database = [string[]]#(Get-Content "C:\Users\zack\Desktop\database.txt")
$file.ExceptWith($database)
$file | Set-Content path\to\comparisonresult.txt