1)
Using awk:
awk -F':' '{print $5}' /etc/passwd

Using cut:
cut -d ':' -f 5 /etc/passwd


2)
sed -i -e 's/\[/\(/g' -e  's/\]/\)/g' bad.py



3)
sed -n '100,200p' alongfile.txt


4) 
tail -n 100 alongfile.txt

5)

We are asked to explain the expression 's/(.{4})/& /g;s/ $//'

This expression has two replace commands: 's/(.{4})/& /g' and 's/ $//'

For the first command, (.{4}) will match and capture as back references groups of 4 characters. The '& ' outputs the matched pattern and a space after it, and since everery line has 16 digits, then every line will have produce a line with groups of four characters followed by a space.

The command 's/ $//' simple removes the last space at the end of every line.


6)
awk '{print NR " " $0}' longfile.txt