mjhjmtu

in base directory i have folders like this:

1
2
3
4
5
10
110

so in each of them i have files like

 0010011.mp3 0010031.mp3 0010051.mp3 0010071.mp3 0010021.mp3 0010041.mp3 0010061.mp3

so i want to remove first 3 characters and last 1 character so files will look like this

 001.mp3 003.mp3 005.mp3 007.mp3 002.mp3 004.mp3 006.mp3

i tried this

for file in ??????*; do echo mv $file `echo $file | cut -c4-`; done

also this is not working in subdirectories it is just if files are in base directory

I would not advise just collecting all files that match *.mp3
and then trimming the file names. You MAY want to run this same
script AGAIN later (like next week) and it should NOT try to
rename the same files a second time, making them even shorter
and have naming conflicts.

Your example file names seem to be 001xyz1.mp3 and you want xyz.mp3

 KEEP_DIR=$PWD 
 cd /your/music/base_dir # sub-dirs holding .mp3

 RCOUNT=0 ; SKIP=0 ; FCC=0


 for FFF in */001???1.mp3 
 do
 LOC="$(dirname $FFF)" 
 BAS="$(basename $FFF)"
 TRIM1="$BAS#1.mp3" #could use $BAS:4:3
 NEWF="$LOC/$TRIM1%001.mp3"
 if [ -e "$NEWF" ] ; then
 echo "#-- File $NEWF already exists, not renaming $FFF"
 SKIP=$((SKIP+1))
 else
 mv $FFF $NEWF
 MCOUNT=$((MCOUNT+1))
 fi
 FCC=$((FCC+1))
 done

 echo "Renamed $MCOUNT mp3 files, of $FCC found. (Skipped $SKIP)"
 cd $KEEP_DIR

 #--[eof]

If you're got rename (sometimes known as prename) you can do this in one command:

rename -n 's!(.*/)...(.*).(.mp3)!$1$2$3!' */?????*.mp3

Alternatively you can loop through the files:

for f in */?????*.mp3
do
 echo mv -v "$f" "$(echo "$f" | sed -r 's!(.*/)...(.*).(.mp3)!123!')"
done

In the first instance remove -n (or replace it with -v) to make it perform the work. In the second instance remove the first echo to allow it to apply the changes.

You may use sed to create one mv command line for each mp3 with a 7 characters long basename:

$ find . -name "???????.mp3" | sed 's/(.*)(...)(...)(.)(.mp3)/mv 12345 135/'

After reviewing the output you can redirect it into a file and execute.