Title: Fun tip #1: Apply a diff with ed

	Title: Fun tip #1: Apply a diff with ed
	Author: Solène
	Date: 13 November 2018
	Tags: fun-tip unix openbsd
	Description:

	I am starting a new kind of articles that I chose to name it ”fun
	facts“.
	Theses articles will be about one-liners which can have some kind of
	use, or
	that I find interesting from a technical point of view. While not
	useless,
	theses commands may be used in very specific cases.

	The first of its kind will explain how to programmaticaly use diff to
	modify
	file1 to file2, using a command line, and without a patch.

	First, create a file, with a small content for the example:

	$ printf "first line\nsecond line\nthird line\nfourth line with
	text\n" > file1
	$ cp file1{,.orig}
	$ printf "very first line\nsecond line\n third line\nfourth line\n"
	> file1

	We will use [diff(1)](https://man.openbsd.org/diff) `-e` flag with the
	two
	files.

	$ diff -e file1 file1.orig
	4c
	fourth line
	.
	1c
	very first line
	.

	The diff(1) output is batch of [ed(1)](https://man.openbsd.org/ed)
	commands,
	which will transform file1 into file2. This can be embedded into a
	script as
	in the following example. We also add `w` as the last command in order
	to save the file after changes.

	#!/bin/sh
	ed file1 <<EOF
	4c
	fourth line
	.
	1c
	very first line
	.
	w
	EOF

	This is a convenient way to transform a file into another file, without
	pushing the entire file. This can be used in a deployment script. This
	is less error prone than a sed command.

	In the same way, we can use ed to alter configuration file by writing
	instructions without using diff(1). The following script will change
	the
	first line containing "Port 22" into Port 2222 in /etc/ssh/sshd_config.

	#!/bin/sh
	ed /etc/ssh/sshd_config <<EOF
	/Port 22
	c
	Port 2222
	.
	w
	EOF

	The sed(1) equivalent would be:

	sed -i'' 's/.Port 22./Port 2222/' /etc/ssh/sshd_config

	Both programs have their use, pros and cons. The most important is to
	use the
	right tool for the right job.