One-line way to remove last n characters from string - bash

Question

I have a bunch of files in the format photo.jpg.png in a folder, and for every photo in this folder, I want to replace the .jpg.png with .png. How can I do this from Terminal?
I have a basic Python and bash background, so I know I'd want to do something like this:
$ for i in *.png; do mv $i $i[:-8]; done
$ for i in *; do mv $i $i.png; done
But what would I replace the Pythonic [:-8] with in order to remove the last 8 characters of each filename?
EDIT I now realize that a substring that counts from the end of the string would be superior. Is there a way to do this as well?

Answer 1

You can use pattern expansion:
for f in *.jpg.png; do mv -v "$f" "${f/.jpg.png/.png}"; done
Though you might still have problems with a filename like foo.jpg.png.gif.
If you really want to strip the last 7 or 8 characters, you can use a substring expansion:
for f in *.jpg.png; do mv -v "$f" "${f:0:-7}png"; done
Note that use of negative numbers in substring length requires bash version 4 or higher.

Answer 2

With Perl's standalone rename command:
rename -n 's/jpg\.png$/png/' *.jpg.png
or
rename -n 's/.......$/png/' *.jpg.png
Output:
rename(photo.jpg.png, photo.jpg)
If everything looks okay, remove `-n'.

Answer 3

rename is designed for this kinda thing;
$ rename .jpg.png .png *.jpg.png
For MacOS, I realized that rename may not be available by default, you can install it using brew.
$ brew install rename
and then use -s option for rename;
$ rename -s .jpg.png .png *.jpg.png

Answer 4

Removing the last 8 characters using perl's rename :
$ rename -n 's/.{8}$//' *.png
(remove -n switch when your tests are OK)
or with bash :
for i in *.png; do
echo mv "$i" "${i:0:-8}"
done
(remove echo when your tests are OK)
There are other tools with the same name which may or may not be able to do this, so be careful.
If you run the following command (GNU)
$ file "$(readlink -f "$(type -p rename)")"
and you have a result like
.../rename: Perl script, ASCII text executable
and not containing:
ELF
then this seems to be the right tool =)
If not, to make it the default (usually already the case) on Debian and derivative like Ubuntu :
$ sudo update-alternatives --set rename /path/to/rename
(replace /path/to/rename to the path of your perl's rename command.
If you don't have this command, search your package manager to install it or do it manually
Last but not least, this tool was originally written by Larry Wall, the Perl's dad.

Answer 5

Something like that:
$ v=test.jpg.png
$ echo ${v:0:-8}
test

Answer 6

This should work:
for file in *.jpg.png; do mv $file ${file//jpg.png/jpg} ; done