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Practice Unix/Linux Script - differ.sh
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-IAN! idallen@ncf.ca

All scripts written must conform to the script_checklist.txt checklist.
All user input (arguments or via "read") must be validated before being used.
Do not process bad or missing input data!

Write an executable shell script named differ.sh that will do the
following actions (in the order given below):

  1. Verify that there are more than zero command line
     arguments; otherwise, exit with an error message.	(i.e.
     Exit if there are zero arguments.)
  2. Put the first argument (a file name) into a variable named filnam.
  3. If there is only one command line argument,
     prompt the user to enter the missing directory name and read
     the name from standard input.  Put the directory name
     (either the second command line argument or what the user
     entered from standard input) into the variable dirnam for
     use in the rest of the script.
  4. Display this exact sentence:
	You are processing file 'XXX' and directory 'YYY'.
     where XXX is the contents of the filnam variable and YYY is
     the contents of the dirnam variable containing the directory
     name that the user specified.  Use the exact punctuation.
  5. Verify that the path named by the dirnam variable
     is a directory; otherwise, exit with an error message.
     (i.e. Exit if the path is not a directory.)
  6. Verify that the path named by the dirnam variable
     is writable; otherwise, exit with an error message.  (i.e.
     Exit if the directory is not writable.)
  7. Verify that the path named by the dirnam variable
     is searchable; otherwise, exit with an error message.  (i.e.
     Exit if the directory is not searchable.)
  8. Put the file pathname $dirnam/$filnam into a
     variable named diffout. (Do not create this file yet.)
  9. Verify that the file named by the diffout
     variable is zero size or nonexistent; otherwise, exit with
     an error message.	(i.e. Exit if non-empty.)
  10.
     Make up a pathname for a temporary file and put
     it into a variable named tfile. Use the process ID of the
     current process in the pathname.  The pathname of the
     temporary file should be under the /usr/tmp directory.  (Do
     not create the file yet.)
  11.
     Sort the password file and place the output into
     the temporary file named by the tfile variable.  Exit with a
     non-zero exit status if the command fails.
  12.
     Generate the lines of difference between the
     contents of the password file and the contents of the
     temporary file and put the resulting output into the file
     named by the diffout variable.
  13.
     Save the exit status of the difference command in
     a variable named dstat.
  14.
     If the size of the file named by the diffout
     variable is greater than zero, append this exact message to
     the file named by the diffout variable:
	Difference exit status was XXX.
     where XXX is the contents of the dstat variable from a
     previous step.  Exit with a non-zero exit status if the
     appending command fails.