Write a shell script, change-lines, which will substitute a string for
a replacement string for each occurance of the string in files
specified.  The original file will be saved, with the same filename
with the .keep extension unless the -n option is present.  You may use
a temporary file for this script. Remember the search string and or the
replacement string can have special characters in them. Use proper
quoting techniques.

The script will use

   * the option -s "string" for the search string
   * the option -r "string" for the replacement string
   * the -n option to prevent a backup file from being created
   * the -h option to print a usage statement and exit

The script will exit with an error status and print  an error message:

   * if the -s option is not present.
   * if the -r option is not present.
   * if the string is not specified with the -s option
   * if the string is not specified with the -r option
   * if there are any other options specified
   * if there are no input files specified
   * if the input file cannot be read by the current process
   * if the input file cannot be written by the current process
   * if you cannot create the backup file.

Here is the usage statement for the script:


     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

Examples:

     prompt> change-lines -s foobar -r FooBar *.c
     This script will edit all the files *.c, and each occurrence
     of foobar it will be replaced by FooBar it will be replaced by
     FooBar. Each file will be backed up.

     prompt> change-lines -s "SHELL IS NOT FUN" -r "SHELL PROGRAMING IS A LOT OF FUN"  /tmp/file1.txt 
     This script will edit all the file /tmp/file1.txt , and each
     occurrence of SHELL IS NOT FUN will replaced by the text, SHELL
     PROGRAMMING IS A LOT OF FUN. A backup file /tmp/file1.txt.keep
     will be created.

You may use temporary files in this project.