Ed

Ah Ed, the standard editor. ed a simple editor, which usefulness can be found when vi doesn't cut it anymore:

vi needs /usr to be mounted, ed doesn't
vi is dynamically linked, ed isn't
ed can be assumed to almost always exist, vi can be often assumed to exist
vi needs $TERM to be set and termcap file exist
bsd.rd ships with ed but not vi:

# vi
ksh: vi: not found
# ed
BEHOLD THE POWER OF MIGHTY ED
?

ed is cooler
?

Most of these are too niche for a day-to-day life, but there are times, ed worth it's manual page in gold. when crisis happens, ed can be your only friend. ed, like vi is a modal editor, which means it has different modes, input mode and command mode. input is for writing text and command is for commands, such as q, w, s and so on.

Input mode

input mode is where you type text into your document, to exit this mode, type:

in a empty line. you will be returned to command mode. to enter command mode, any one of these commands get you into input mode:

a	append after current line
i	insert before current line
c	replace current line

Command mode

basic ed commands

Note that some ed commands can be combined, as they did in vi, for example wq.

e	opens a file to edit (includes `!command` notation too). cleans the buffer
E	same as `e`, but without warning on unsaved file
f	sets default filename
H	verbose errors (instead of "?")
h	explain last error ("?")
n	print current line and it's line number
p	print current line
P	toggles prompt, only if you had `-p` in arguments of `ed`
q	quits `ed`
Q	q, but without warning for unsaved file
u	undo, there is only one level of undo (which includes undo too)
r	read from a file (or a !command)
w	save the buffer
=	shows how many lines current file has (`wc -l`)
any number	sets current line

advanced ed commands

m,nd	deletes mth line until nth line in buffer
m,nG/re/	finds and moves on lines which include regular expression re from mth line, until nth line
m,nj	joins all lines from mth line to nth line
m.ng/re/command	finds all lines containing re regular expression and executes (ed) command on them.
s/re/replacements	replaces all all "re"'s with replacements
m.nV/re/	like G, but reverse (all lines that does not contain re)

ed symbols

, or %	all lines from first line until current line
??	repeats last search done by ?re?
?re?	previous line containing regular expression re
-n or ^n	nth previous line
- or ^	previous line
n	nthn line in buffer
.	current line
$	last line in buffer
+n	n line upper
/re/	next line containing regular expression re
//	repeats last search done by /re/
+	next line
%	file name
'lc	shows (book?)marked line in buffer (see k command)

Examples

Opening a file

$ ed file

Using ed quietly, handy for scripts

$ ed -s file