$ ls -all
-rw-r--r-- 1 29847 29847 0 Aug 21 14:33 file1.txt
-rw-r--r-- 1 29847 29847 0 Aug 21 14:33 file2.txt
-rw-r--r-- 1 29847 29847 0 Aug 21 14:33 file3.txt
-rw-r--r-- 1 29847 29847 0 Aug 21 14:33 file4.txt
-rw-r--r-- 1 29847 29847 0 Aug 21 14:33 file5.txt
$ for f in *;do mv -v "$f" "${f%.*}hallo.${f##*.}";done
'file1.txt' -> 'file1hallo.txt'
'file2.txt' -> 'file2hallo.txt'
'file3.txt' -> 'file3hallo.txt'
$ ls -all
-rw-r--r-- 1 29847 29847 0 Aug 21 14:33 file1hallo.txt
-rw-r--r-- 1 29847 29847 0 Aug 21 14:33 file2hallo.txt
-rw-r--r-- 1 29847 29847 0 Aug 21 14:33 file3hallo.txt
rename 's/^/1_/' *
where ^ is for starting of the file or end
for f in *.mp3; do mv -- "$f" "${f%.mp3}suffix.mp3"; done