rename the suffix

common.lib

# common.lib
# Note no #!/bin/sh as this should not spawn 
# an extra shell. 

STD_MSG="About to rename some files..."

rename()
{
  # expects to be called as: rename .txt .bak 
  FROM=$1
  TO=$2

  for i in *$FROM # = *.txt
  do
    j=`basename $i $FROM` # get the basename of the file
    mv $i ${j}$TO # ${j}$TO = someBaseName.bak
  done
}

function.sh

#!/bin/sh
# function.sh

. ./common.lib
echo $STD_MSG
rename .txt .bak