Change multiple filenames unix - unix

Question

I had to download 15GB of data and for some reason during the downloading process the filenames were messed up in a way so that instead of
test_file.txt
the filenames are doubled, so it's
test_file.txttest_file.txt
instead. My only idea was whether there is any way to count the letters and then rename each file with deleting the first/ or second half of the filename? The filenames are not consistent, so for example in the same folder there might also be files named
files_are_great.txtfiles_are_great.txt
so I'm struggling to find a way to loop over them.
Thanks a lot!

Answer 1

The command sed 's/\(.*\)\1/\1/' will replace all duplicated strings with the single string without requiring a certain part of the file name like .txt. It allows spaces in the string.
Example:
echo 'abc defabc def' | sed 's/^\(.*\)\1$/\1/'
prints
abc def
Explanation of the sed command:
^ anchors the pattern to the beginning of the line
.* is 0 or more occurrences of any character
\(...\) captures what matches the pattern in between
\1 is a reference to the first capture group, i.e. the text that was found before
$ anchors the search pattern to the end of the line
This results in a search pattern that matches a whole line that consists of any text followed by the same text.
\1 in the replacement is the same reference to the matched text, i.e. a single occurrence of the duplicated text.
Any input that does not match the pattern will remain unchanged.
Assuming you want to rename all files in the current directory you can use it like this
for file in *
do
new=$(echo $file|sed 's/\(.*\)\1/\1/')
[ "$file" = "$new" ] || mv "$file" "$new"
done
As the sed command does not change non-matching input, $new will be the same as $file for file names that don't consist of a duplicated string. This would result in an error message from mv. That's why the renaming will be skipped in this case.

Answer 2

Using sed
sed 's#\(\.txt\)#& #g'
Explanation: using \( \) we group the expression which can be accessed using &
Demo:
echo "files_are_great.txtfiles_are_great.txt" | sed 's#\(\.txt\)#& #g'
files_are_great.txt files_are_great.txt
For renaming:
for file_name in $(ls -1 *txt*txt)
do
new_file_name=$(echo $i |sed 's#\(\.txt\)#& #g' | cut -d' ' -f1)
mv $file_name $new_file_name
done