Bash script to uncompress any (compressed) file under a directory tree

uncompress.sh

#!/bin/bash
PROG_NAME=$(basename $0)

# Bash script developed to uncompress student exercises, regardless
# of the compressed file type. The bash script takes an argument, that
# corresponds to the directory that includes all student-subdirectories
# that contain the compressed exercises.
# E.g 
#                          students/
#        sdi000001/        sdi000002/               sdi00003/      .....



if [ "$#" -eq 0 ]
then
    echo "usage: ${PROG_NAME} [students_dir]"
    echo "example: ${PROG_NAME} students"
    exit
fi

cd $1
currentdir=`pwd`
student_dirs=`ls`

for student in $student_dirs; do
    cd $currentdir/$student
	 compressed_files=`ls`
	 for compressed_file in $compressed_files; do
    	 if  file $compressed_file | grep 'gzip';
    	      then 
    	        tar -xzvf $compressed_file
    	      elif  file $compressed_file | grep 'tar';
    	      then 
    	        tar -xvf $compressed_file
    	      elif  file $compressed_file | grep 'bzip2';
    	      then 
    	        tar -xvjf $compressed_file
    	      elif  file $compressed_file | grep '7-zip';
    	      then 
    	         7z x $compressed_file
    	      elif  file $compressed_file | grep 'zip';  
    	      then 
    	          unzip $compressed_file
    	      else
    	        case $compressed_file in
    	           *.tar.bz2) tar -xvjf $compressed_file ;;
    	           *.bz2)     tar -xvjf $compressed_file ;;
    	           *.tar.gz)  tar -xvzf $compressed_file ;;
    	           *.tgz)     tar -xvzf $compressed_file ;;
    	           *.gz)      gunzip $compressed_file ;;
    	           *.zip)     unzip $compressed_file ;;
    	           *.7z)      7z x $compressed_file ;;
    	           *)         echo "I cannot unpack it"dd
    	        esac;
    	 fi  
	done
done