Rename multiple files with bash for loop, mv, and sed

Question

My goal is to rename a folder of files of the form 'img_MM-DD-YY_XX.jpg' to the form 'newyears_YYYY-MM-DD_XXX.jpg' by iterating through each filename and using sed to perform substitutions based on character positions. Unfortunately I cannot seem to get the position-based swaps to work.

e.g. s/.\{4\}[0-9][0-9]/.\{10\}[0-9][0-9]/ attempts to replace MM with YY

Here is my attempt (neglecting for now the _XX part):

for filename in images/*
do
newname=$(echo $filename | sed 's/.\{4\}[0-9][0-9]/.\{10\}[0-9][0-9]/;
            s/.\{7\}[0-9][0-9]/.\{4\}[0-9][0-9]/;
            s/.\{10\}[0-9][0-9]/.\{7\}[0-9][0-9]/;
            s/img_/newyears_20/')
mv $filename $newname
done

Any ideas how I can fix this?

Answer 1

$ echo 'img_11-22-14_XX.jpg' | sed -r 's/[^_]*_([0-9]{2})-([0-9]{2})-([0-9]{2})/newyears_20\3-\1-\2/'
newyears_2014-11-22_XX.jpg

The above looks for anything up to and including the first underline followed by a 6-digit date. It replaces the initial part with newyears_ and reformats the date from mm-dd-yy to 20yy-mm-dd.

The two-digit mm, dd, or yy values are matched with ([0-9]{2}). The parentheses indicate that sed should capture the value for later use. The output side of the substitution is _20\3-\1-\2. This restores the underline and adds a 20 to the front of the year. The year was the third captured value so it is denoted \3. Likewise, the month was the first captured value so it is denoted \1 and the day the second so it is \2.

To eliminate some blackslashes, I used the -r option to invoke extended regular expressions. If you are on a Mac or other non-GNU system, use sed -E in place of sed -r. Otherwise, use:

sed  's/[^_]*_\([0-9]\{2\}\)-\([0-9]\{2\}\)-\([0-9]\{2\}\)/newyears_20\3-\1-\2/'

Answer 2

This is simple to do with awk

echo "img_MM-DD-YY_XX.jpg"  | awk -F"[_-]" '{print "newyears_20"$4"-"$2"-"$3"_0"$5}'
newyears_20YY-MM-DD_0XX.jpg