$ 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