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# Intsall rename on MacOS with brew
$ brew install rename

# `-n` is use to display the changes will be made. 
# If you want to really rename the files, remove the `-n`

# Remove `n` first charaters
$ rename -n 's/(.{n})/$2/' *.*

# Remove `n` last charaters
$ rename -n 's/(.{n})$/$2/' *.*