PATH=/bin:/usr/bin
new=/tmp/wwho1.$$
old=/tmp/wwho2.$$
>$old
while :
do
  who >$new
  diff $old $new
  mv $new $old
  sleep 60
done

new=$(who)
if [[ $new != $old ]]
then
    do something
fi
old=$new

% a="a" b="b";diff <(echo "$a") <(echo "$b") 
1c1
< a
---
> b

% echo "$var1"
line1
line2
line3
% echo "$var2"
line1
line2
% diff <(echo "$var1") <(echo "$var2") 
3d2
< line3

man diff

Display the differences between two files, or each corresponding file in two directories.
Each set of differences is called a "diff" or "patch". For files that are identical, `diff' normally produces no output; for binary (non-text) files, `diff' normally reports only that they are different.
...
In the simplest case, diff compares the contents of the two files from-file and to-file. A file name of - stands for text read from the standard input.

If from-file is a directory and to-file is not, diff compares the file in from-file whose file name is that of to-file, and vice versa. The non-directory file must not be -.

If both from-file and to-file are directories, diff compares corresponding files in both directories, in alphabetical order; this comparison is not recursive unless the -r or --recursive option is given.

GNU `diff' can show whether files are different without detailing the differences.
It also provides ways to suppress certain kinds of differences that are not important to you.

Most commonly, such differences are changes in the amount of white space between words or lines. `diff' also provides ways to suppress differences in alphabetic case or in lines that match a regular expression that you provide.