If Statements with Exit Code Statements

Question

I'm a beginner at Linux/Unix. Here is my assignment, it is too long to post everything here: https://i.sstatic.net/v2AJR.jpg

Here is the logic that I have:

This code will check if the user entered an argument or not. I do not know what to put under else. We never talked about the which statement which I learned from another user here so it is not expected to be used. Just basic commands this isn't a super advanced assignment.

if [ $# -ne 1 ]; then
        echo Please enter a single, valid user id: 
        read userid
else
        userid=$1
        **what goes here?**
fi

To display a user's full name (i.e. Ben Franklin) this is the code that I have written and tested:

grep "$userid" /etc/passwd | cut -d: -f5 | sort | sed 's/^\(.*\), \(.*\)$/\2 \1/'

Determine whether the user is logged in or not:

If user is not logged in (will return exit code 1): who | grep $userid

1. Then display grep "$userid" /etc/passwd | cut -d: -f5 | sort | sed 's/^\(.*\), \(.*\)$/\2 \1/' AND echo is NOT currently logged on then end script with a code of 1.

If user is logged in (will return exit code 0): who | grep $userid

2. Then display grep "$userid" /etc/passwd | cut -d: -f5 | sort | sed 's/^\(.*\), \(.*\)$/\2 \1/' AND echo is currently logged on then end script with a code of 0.

Determining whether a user is valid:

If user is invalid (will return exit code 1): grep $userid /etc/passwd

3. Then it should display echo "The user you entered, $userid is not a valid user on this system."

If user is valid (will return exit code 0): grep $userid /etc/passwd

How do I write this script? What's the full structure? I am stuck on the structure/the setup.

Answer 1

As this looks like an assignment, I will only give you the tools and some hints:

1. If no argument was given to the script, then $1 (the first argument to the script) will be empty. You can test this with [[ -z $1 ]]. If you want to display an error for too many arguments, test the number of arguments with [[ $# -eq 1 ]] (will be true for a single argument).

2. The gentent passwd "$username" command can be used to get the passwd entry for the user $username. The command will fail with a non-zero exit status if the user does not exist. This means that you can use if ! getent passwd "$username" >/dev/null; then ...; fi to react to a non-existing user $username.

3. The 5th field of the passwd entry for a user usually contains the user's full name (there is really no guarantee that it will be in "firstname lastname" form). The entry may have extra commas at the end that you have to delete. You can parse the output of getent passwd "$username" using cut -d: -f 5 to get at the fifth field, and then delete from the first comma with name=${name%%,*}.

   On some systems, the full name of the user may contain a & character. On those systems, this & character should be replaced by the username (so that the name System & for the user operator becomes System operator). I'm not sure Linux generally does this, but that could be done with name=${name/&/$username}.

4. The who utility outputs a list of users that are currently logged on. To match $username against that, use grep -e "^$username\>". The \> matches the end of a word so that e.g. antony is not matched when looking or anton. grep -q is useful in an if statement.

This follows the code structure:

1. Check and handle invalid arguments (missing: read; too many: exit or just use first). One if statement.
2. Check and handle invalid user name. One if statement.
3. Get full name of user. Variable assignment(s) with command substitution.
4. Check if user is logged on (and output). One if statement, possibly with an else branch.

I don't see a loop in this script, so it will be a number of if-statements and variable assignments.