Remane filename by switching what is before and after an underscore

Question

I would like to rename a number of files that have the following formatting:

ABCD_20181102.jpg
DEDE_2_20181030.jpg

i.e usually 4 random letters, followed by an underscore and a date (year, month, day). Or 4 random letters followed by an underscore, a random number, an underscore and then a date.

I would like to rename these files to date, (random number if needed), and then random letters, like the following:

20181102_ABCD.jpg
20181030_2_DEDE.jpg

Answer 1

You can use Parameter expansion to extract parts of the strings. # removes from the left, % removes from the right, doubling the symbol removes the longest possible match.

#! /bin/bash
for f in ABCD_20181102.jpg DEDE_2_20181030.jpg ; do
    suffix=${f#*.}
    before_underscore=${f%%_*}
    after_underscore=${f#*_}
    after_underscore=${after_underscore%.$suffix}
    if [[ $after_underscore = *_* ]] ; then
        before2=${after_underscore%_*}
        after2=${after_underscore#*_}
        after_underscore="$after2"_"$before2"
    fi
    echo mv "$f" "$after_underscore"_"$before_underscore.$suffix"
done

Just remove the echo before mv once you have checked the script does what you need.

Answer 2

If you have Larry Wall's rename, then

rename -n -e 's/.*_.*/$2_$1/' «file list»

Remove -n to enable, if output shows that it will do it correctly. (The above only tells you what it would do, but will not do it.)

Answer 3

With mmv:

$ mmv -n '*_*.jpg' '#2_#1.jpg'
ABCD_20181102.jpg -> 20181102_ABCD.jpg
DEDE_2_20181030.jpg -> 2_20181030_DEDE.jpg

(remove the -n once you are sure it's doing the right thing).