Question

Please type your answer, thank you! :)

The subject is UNIX

Using the BASH for and while loops (Textbook question 13.19 page 403)

Write a script that checks each minute and reports on who logs in and who logs out.

You can follow the steps below (not the only way):

1. Using the commands who and cut, extract the list of usernames currently logged in the system
2. To check after a minute, the sleep command can be used: sleep 60
3. Get the new list of users logged in after a minute
4. For each user in the list;
   1. check if he is in the second list using if and grep. You can dump the output of grep to /dev/null when using it in the as a test for the if statement
   2. If the user is not in the second list, print that the user has logged out
5. For each user in the second list,
   1. If he is in the first list, print that the user has logged in
6. Rename the second list as the first list
7. Repeat the process from step 2

Answer 1

Answer:

The command shows the list of users logged into the system

$who -u

The command creates a new file for all the usernames. The user name is the first field of the etc/password file.The cut command extracts the list and stores the sorted list in a file file1

$ cut -d: -f1 /etc/passwd | sort > file1

Sleep for 60 secs

$sleep 60

Get the list of new logged in uses after 60 secs and stores the sorted list of users in another file file2

$ cut -d: -f1 /etc/passwd | sort > file2

Now we shall compare both the files . if user is in file 1 but not in file 2 it means user has logged out . The following command reports the list of users who are present in file 1 but not present ie absent in file 2 meaning logged out hence we should print user has logged out .

diff -u file2 file1 | grep " ^+"

We can also use the bash short cut

diff < file1 <file2

However for the specific purpose sought in the question we shall use the comm command

comm [option] file 1 file 2

for options

-1 suppress lines unique to file 1

-2 suppress lines unique to file 2

-3 suppress lines common to both files

comm -2 -3 < file1<file2 >file3

This will output users who were initially logged in but later logged out

Now we run a loop to display users who have logged out

for i in 'file3'; do

echo $i "has logged out"

done

wait

Similarly we can check for user present in second list but not in first list and for each user print "<user> has logged in "

comm -1 -3 < file1<file2 >file4

for j in 'file4'; do

echo $j "has logged in"

done

wait

Now rename the second file

mv file2 file1

Then we can enter into the loop for step 2