Shell Input and Conditional Constructs

Part 1

Write a script named lastname_firstname_checking.sh that does the following:

1. Take a command-line argument, a user’s login name.
2. If no argument was given, prompt the user for a login name and read it in.
3. Check to see if the user is logged in (use the who command and pipe it to grep).
4. If the user is not logged in, print a message like:

   username is not logged on.

5. If the user is logged in, print all the processes that the user is running. (Use ps -aux and pipe it to grep.)

Part 2

Write a “calculator” program in a script named lastname_firstname_calculator.sh. This script will ask users to enter a simple expression of the form number operator number, where the numbers are integers and the operator is either +, -, *, or /. If the operator is not one of those four, you must produce an error message. Your script must use the case construction.

In order to assign an asterisk * to a variable without having bash expand it as a pathname, you must put the following line in your script before you start processing user input:

GLOBIGNORE='*'

Hints

• You will have to store the result of executing shell commands into variables. You do this the same way you did it in the previous assignment. For example, if I needed to store the month name in a variable named month, I would use the result of the date command like this:

  month=$(date '+%B')

• Your script must prompt the user for input. Use echo -n to keep the cursor on the same line as your prompt.
• Use read to read the user’s input into a variable.
• To get the first number, you will echo the input to an awk command. That command will set the awk delimiter string to '[+*/-]'. (The dash must come either first or last in the square brackets to avoid being interpreted as a character range.) The rest of the awk command will print field 1; that is the first number in the input.
• To get the second number, do another echo piped to awk exactly as described previously, but print field 2; that is the second number in the input. described above and have awk print field 2.
• To get the operator, echo the input string and pipe it to grep. Use the --only-matching option to grep to extract the operator. Normally, grep echoes the entire line when it finds a pattern; the --only-matching option shows only the part of the line that matched. Example:

  string1=$(echo "concatenate" | grep 'c[aeiou]t')
  string2=$(echo "concatenate" | grep --only-matching 'c[aeiou]t')
  echo "String 1 is $string1"
  echo "String 2 is $string2"
  cat

• Now that you have the first and second numbers and the operator, you can use the case statement to correctly set a variable named result to the sum, difference, product, or quotient of the two numbers. In the case statement, use the “default” *) case to set result to an error message.
• Finally, echo $result to display the answer (or error).

Here is sample output from several runs of the program. Your program does not have to give exactly these prompts and error messages, but it must exhibit similar behavior.

Enter expression: 3 + 5
8

Enter expression: 15 - 2
13

Enter expression: 12*144
1728

Enter expression: 1892/4
473

Enter expression: 3^2
Unknown or missing operator in 3^2

When You Finish

Make sure the script files are named in the form lastname_firstname_checking.sh and lastname_firstname_calculator.sh, and email them to the instructor. Put the word CIT052 in the subject line of your email, please.