env | grep $USER vs env | grep USER
Solution 1:
In the first form,
env | grep $USER
The variable $USER is expanded to its value for grep. So, grep will show all lines which contains the value of $USER variable. For me, the value of $USER is anwar. So the command is equivalent to env | grep anwar
If you want to grep the literal $USER, use single quote as suggested in this answer
In the second form,
env | grep USER
only the literal USER is used for grepping and thus grep will only show those lines containing USER.