There are a few good tutorials on the 'ed' editor floating around
gopherspace, the one I've most recently seen is from KatolaZ [0][1].
It's nice and simple, but I find the actual line editing a bit
sparse - neither the GNU nor BSD versions of ed are linked with GNU
readline, so you don't have access to the usual arrow, ctrl-a,
ctrl-e, alt-d, etc. keys. But it's really easy to add this
functionality, using 'rlwrap', like so:
'rlwrap ed'
Obviously you might need to install rlwrap, but it is packaged
for many linux distros as well as BSDs. It's installed here
on the Republic, and I have a shell alias setup for it:
ed='/usr/local/bin/rlwrap /bin/ed'
[0]
gopher://r.circumlunar.space/0/%7ekatolaz/phlog/20190505_ed_lists.txt
[1]
gopher://r.circumlunar.space/0/%7ekatolaz/phlog/20190830_ed_lists_2.txt