i tried the question again but this is what i got

The df command shows you how much disk space is available on a Linux system. Use the output from the previous question to write a command pipeline that prints how much disk space is available on each of the disks found in question 1. The only acceptable output consists of two numbers only. NOTE: instead of cut, you may want to use awk. To print the second field in a whitespace-delimited file using awk, use the following command: awk '{print $2}'

and it says $2 but why are you using $4 sir ?

$ df | grep $a | awk '{print $2}'

grep: [sdb]: No such file or directory