I am trying to swap the first line with last line in a text file in unix
file has:

```
line1
line2
line3
line4
line5
line6

```

I want like this:

```
line6 
line2
line3
line4
line5
line1

```

I am using `sed -n -e '1 s/^.*$/$p' file` . Which is not happening.