Wednesday, April 30, 2014

Episode #177: There and Back Again

Hal finds some old mail

Way, way back after Episode #170 Tony Reusser sent us a follow-up query. If you recall, Episode #170 showed how to change files named "fileaa", "fileab", "fileac", etc to files named "file.001", "file.002", "file.003". Tony's question was how to go back the other way-- from "file.001" to "fileaa", "file.002" to "fileab", and so on.

Why would we want to do this? Heck, I don't know! Ask Tony. Maybe he just wants to torture us to build character. Well we here at Command Line Kung Fu fear no characters, though we may occasionally lose reader emails behind the refrigerator for several months.

The solution is a little scripty, but I did actually type it in on the command line:

    for l1 in {a..z}; do 
        for l2 in {a..z}; do 
            printf -v ext %03d $(( c++ ))
            [[ -f file.$ext ]] && mv file.$ext file$l1$l2 || break 2

There are two nested loops here which work together to create our alphabetic file extensions. $l1 represents the first of the two letters, ranging from 'a' to 'z'. $l2 is the second letter, also ranging from 'a' to 'z'. Put them next to each other and you get "aa", "ab", "ac", etc.

Like Episode #170, I'm using a counter variable named $c to track the numeric file extension. So, for all you computer science nerds, this is a rather weird looping construct because I'm using three different loop control variables. And weird is how I roll.

Inside the loop, I re-use the code from Episode #170 to format $c as our three-digit file extension (saved in variable $ext) and auto-increment $c in the same expression. Then I check to see if "file.$ext" exists. If we have a "file.$ext", then we rename it to "file$l1$l2" ("fileaa", "fileab", etc). If "file.$ext" does not exist, then we've run out of "" pieces and we can stop looping. "break 2" breaks out of both enclosing loops and terminates our command line.

And there you go. I hope Tim has as much fun as I did with this. I bet he'd have even more fun if I made him do it in CMD.EXE. Frankly all that PowerShell has made him a bit sloppy...

Tim mails this one in just in time from Abu Dhabi

Wow, CMD huh? That trip to Scriptistan must have made him crazy. I think a CMD version of this would violate the laws of physics.

I'm on the other side of the world looking for Nermal in Abu Dhabi. Technically, it is May 1 here, but since the publication date shows April I think this counts as our April post. At least, that is the story I'm sticking to.

Sadly, I too will have to enter Scriptistan. I have a visa for a long stay on this one. I'll start by using a function to convert a number to Base26. The basis of this function is taken from here. I modified the function so we could add leading A's to adjust the width.

function Convert-ToLetters ([parameter(Mandatory=$true,ValueFromPipeline=$true)][int] $Value, [int]$MinWidth=0) {
    $currVal = $Value
    if ($LeadingDigits -gt 0) { $currVal = $currVal + [int][Math]::Pow(26, $LeadingDigits) }
    $returnVal = '';
    while ($currVal -ge 26) {
        $returnVal = [char](($currVal) % 26 + 97) + $returnVal;
        $currVal =  [int][math]::Floor($currVal / 26)
    $returnVal = [char](($currVal) + 64) + $returnVal;
    return $returnVal

This allows me to cheat greatly simplify the renaming process.

PS C:\> ls file.* | % { move $_ "file.$(Convert-ToLetters [int]$_.Extension.Substring(1) -MinWidth 3 )" }

This command will read the files starting with "file.", translate the extension in to Base26 (letters), and rename the file. The minimum width is configurable as well, so file.001 could be file.a, file.aa,, etc. Also, this version will support more than 26^2 files.