Rename multiple files' file extensions from the command line with a wildcard

Problem:
I hate not being able to rename multiple files from the command prompt/mac terminal session, ie:

mv *.thisisthewrongextension *.thisiscorrect

or as my example will be for torrent files incorrectly downloaded by a browser as .torrent.html files:

mv *.torrent.html *.torrent

This lack in functionality in bash just drives me crazy.

Solution:
Here are two inline commands, a bash script, a pearl script, and GUI Mac program, that will overcome this shortfall. In these two command examples we will try to rename files that downloaded as myfile.torrent.html to myfile.torrent.

1. find *.torrent.html -exec mv -vn {} `echo {} | cut -f1 -d.`.torrent \;

   find *.torrent.html -exec bash -c 'mv -vn {} $(basename {} .torrent.html).torrent' \;

2. for i in *.torrent.html; do mv -vn "$i" "`basename $i .torrent.html`.torrent"; done

3. bash/pearl script examples located here

4. You could also use this GUI program on a mac, it is VERY, VERY, VERY awesome: A Better Finder Rename

Solution 1 breakdown: In the first example we will use the find command in conjunction with executing another command inline. In the second example we will be using a bash script in a single line. Let's begin. command breakdown:

find *.torrent.html

will output: 

myfile1.torrent.html
myfile2.torrent.html
myfile3.torrent.html

Then we run into the "-exec" parameter of the find command... it may seam a lot to deal wth, but basically it just runs a command each time find returns a result. Moreover it will pass the found file/directory in the form of "{}". for example:

find *.torrent.html -exec echo {} \;

Please note: at the end of an "exec" you must have whitespace(a space in the above example), then a backslash(escape character) followed by a semicolon. If you don't do that you'll get the following error:

find: -exec: no terminating ";" or "+"

anyway, the aforementioned command will output: 

myfile1.torrent.html
myfile2.torrent.html
myfile3.torrent.html

Let's take a break from the "exec" function of the "find" command, and move on to the "mv" command (pun intended). When you move or rename a file you need to specify a source & destination file/directory. Now since we can't move multiple files with a wildcard as the destination with the "mv" command (as is the point of this article) will not work

mv *.torrent.html *.torrent    -   this will not work

However, the following 3 commands will work (and this is exactly what we will be replicating in this first example):

mv myfile1.torrent.html myfile1.torrent
mv myfile2.torrent.html myfile2.torrent
mv myfile3.torrent.html myfile3.torrent

I'm also adding the "-vn" options to the "mv" command so it will be verbose about what it is moving & to prevent "mv" from overwriting an existing file. Now that we know what "mv" will be expecting, let's see how we can get "find" & "-exec" to pass the file names to the "mv" command.

find *.torrent.html -exec mv -vn {} `echo {} | cut -f1 -d.`.torrent \;

find *.torrent.html -exec bash -c 'mv -vn {} $(basename {} .torrent.html).torrent' \;

The {} portion of the above command will be replaced with "myfile1.torrent.html". For the next portion of the command we will look at the bold section in the command above. 

`echo {} | cut -f1 -d.` is translated into:
`echo myfile1.torrent.html | cut -f1 -d.` 

$(basename {} .torrent.html) is translated to:
$(basename myfile1.torrent.html .torrent.html) 

The above example will give the following output (note that anything inside the  ` (tick marks) will be executed first):

myfile1

This output still needs an extension appended to it, so we do the following:

`echo {} | cut -f1 -d.`.torrent

$(basename {} .torrent.html).torrent

And we get the following output:

myfile1.torrent

So now the "mv" command has both a source ("{}" as represented in the example below), and the destination ("`echo {} | cut -f1 -d.`.torrent" "$(basename {} .torrent.html).torrent" also represented below), and we get:

find *.torrent.html -exec mv -vn {} `echo {} | cut -f1 -d.`.torrent \;

find *.torrent.html -exec bash -c 'mv -vn {} $(basename {} .torrent.html).torrent' \;

With the following output:

myfile1.torrent.html -> myfile1.torrent
myfile2.torrent.html -> myfile2.torrent
myfile3.torrent.html -> myfile3.torrent

Phew... all done with the first example.

Solution 2 breakdown... sort of:
Now let's dissect the single line bash script:

for i in *.torrent.html; do mv -vn "$i" "`basename $i .torrent.html`.torrent"; done

Expanded below with added comments it looks like:

for i in *.torrent.html;  #(the for loop begins here)
    do
        mv -vn "$i" "`basename $i .torrent.html`.torrent";  #(here we use the mv command, as we saw above, but we use the "basename" function to split the file name)
    done  #(and then we are done)

I could go into more details, but I think that should be enough for that today. For more details about this function, and to see some other bash function examples go here.

If you need help with a bash script:

1. Feel free to comment/mail me
   
2. Visit a linux forum (www.linuxquestions.org)
3. Take a good long look at the Advanced Bash Scripting Guide
4. Use the "man" pages for further detailed info.

-Brian
=:~)