How to replace using sed?

Question

linux

How to replace the first character of each line with the first character of the previous line, leaving the first line unchanged? Awk cannot be used, only sed :(

Answer 1

#! /bin/sed -nf

1{p;h;}
2~1{H;x;s/\n.//;p;}
x
s/^\(.\).*$/\1/
h

Now try to understand how it works.

Answer 2

sed '
1 { h; s/^\(.\).*$/\1/; x; }
H
s/^\(.\).*$/\1/
x
s/^\(.\)../\1/
'