Shell Programming and Scripting

#!/bin/sh

# a shell function to print and error message and exit the script
error_and_die ()
{
echo "$@" >&2
exit 1

}
# a shell function to print and error message and a usage message and
# to exit the script
error_and_die_with_usage ()
{
echo "$@" >&2
usage
exit 1
}

# a shell function to print a usage message
usage ()
{
echo "
change-lines [-n] -s search string -r replace string files ...

-n do not backup the original file
-s search string the search for this string
-r replace string replace the search string with this string
-h print this message
" >&2
}


backup_file=TRUE
search_string=
replace_string=

while :
  do
   case $1 in
   -n)
   backup_file=FALSE
   ;;
   -s)
     if [ ! "$2" ]
     then
     error_and_die_with_usage "Search String not specified with -s"
     fi
   search_string="$2"
   shift 
   ;;
   -r)
     if [ ! "$2" ]
     then
     error_and_die_with_usage "Search String not specified with -r"
     fi
   replace_string="$2"
   shift 
   ;;
   -h)
   usage
   exit 0
   ;;
   -?)
   error_and_die_with_usage "This should help!"
   ;;
   *)
   break
   ;;
   esac
   shift
  done

if [ ! "$search_string" -a ! "$replace_string" -a $# -eq 0 ]
then
error_and_die_with_usage "No Search String, Replace String, or file to edit entered."
fi

if [ ! "$search_string" -a ! "$replace_string" ]
then
error_and_die_with_usage "No Search or Replace String entered.
fi

if [ ! "$search_string" -a $# -eq 0 ]
then
error_and_die_with_usage "No Search String or file to edit entered."
fi

if [ ! "$search_string" ]
then
error_and_die_with_usage "No Search String entered."
fi

if [ ! "$replace_string" -a $# -eq 0 ]
then
error_and_die_with_usage "No Replacement String or file to edit entered."
fi

if [ ! "$replace_string" ]
then
error_and_die_with_usage "No Replacement String entered."
fi

if [ $# -eq 0 ]
then
error_and_die_with_usage "No file to edit entered."
fi

# here is where I am pretty sure I am getting the error.
for file in $*
     do
      backup=$1
        if ( backup_file == TRUE )
        then
        cp $backup $backup.keep
        fi
     sed 's/$search_string/$replace_string/g' $backup
     shift
     done

exit 0;

...
if [ ! "$search_string" -a ! "$replace_string" ]
then
   error_and_die_with_usage "No Search or Replace String entered."
fi
...

# here is where I am pretty sure I am getting the error.
for file in $*
do
        backup=$file
        if [ $backup_file == "TRUE" ]
        then
                cp $backup $backup.keep
        fi
        sed "s/$search_string/$replace_string/g" $backup
        #shift <<<< You don't need this here now
done

error_and_die_with_usage "No Search or Replace String entered.

$ A=a; export A;
$ echo "a" | sed "s/$A/b/g"
b
$ echo "a" | sed 's/$A/b/g'
a
$ echo "a" | sed 's/'$A'/b/g'
b
$