In PowerShell, this is as easy as:

PS C:\> 'a,,b,c,d,' -replace ','
abcd

To remove multiple unwanted characters, simply chain more -replace operators on the end (note that some characters must be escaped, as we are using a regex the escape character is a backslash):

PS C:\> '.a,a\b,b\c,b\d\,d.\' -replace ',' -replace '\\' -replace '\.'
aabbcbdd

Function get-Unixdate ($UnixDate) {
    [timezone]::CurrentTimeZone.ToLocalTime(([datetime]'1/1/1970').`
    AddSeconds($UnixDate))
}

It returns a PowerShell DateTime object, so can then be manipulated using the standard DateTime methods, eg

PS> $logtime = Get-UnixDate 1269313872.893866062
PS> $logtime

Tuesday, 23 March 2010 4:11:12 p.m.

PS> $logtime.IsDaylightSavingTime()
True
PS> $logtime.dayofweek
Tuesday
PS> $logtime.minute
11
PS> $logtime.tostring("g")
23/03/2010 4:11 p.m.
PS> $logtime.DayOfYear
82
PS> $logtime.AddDays(-30)

Sunday, 21 February 2010 4:11:12 p.m.

PS> ($logtime.adddays(14)).isdaylightsavingtime()
False

etc.

I have added the function to my profile, it comes in useful quite often.

Here’s some tips on working with PowerShell DateTime objects and the MSDN documentation on formatting DateTime output