I'm looking for a way to automate renaming all images with a wrong filename extension. So far I at least found out how to get the list of all these files:
find /media/folder/ -name *.jpg -exec file {} \; | grep 'PNG\|GIF' > foobar.txt
find /media/folder/ -name *.png -exec file {} \; | grep 'JPEG\|GIF' >> foobar.txt
find /media/folder/ -name *.gif -exec file {} \; | grep 'JPEG\|PNG' >> foobar.txt
However, I would also like to automate the renaming. I tried things like
find /media/folder/ -name *.jpg -exec file {} \; | grep -l PNG | rename s/.jpg/.png/
but in this case grep -l or grep -lH don't list only filenames like I thought they would.
The -l and -H flags of grep are not useful in your example. These flags have no effect when used with the standard input, like in your example coming from the pipe. These flags only work if you specify files (or directories and the -r flag for recursion), for example:
grep -rl PNG path/to/dir1 file2 file3
In your example the -l has no effect, so the output is the complete lines that matched PNG, which in your example probably look something like this:
icon.png: PNG image, 512 x 512, 8-bit/color RGBA, non-interlaced
To get only the filename, maybe you can cut off everything after the colon like this:
find /media/folder/ -name *.jpg -exec file {} \; | grep PNG | sed -e s/:.*// | rename s/.jpg/.png/