Title : Fun tip #1: Apply a diff with ed
Author: Solène
Date  : 13 November 2018
Tags  : fun-tip unix openbsd68

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` last commands to save the file after
edition.

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

This is a quite convenient way to transform a file into another file, without
pushing the entire file. This can be used in a deployment script. This is more
precise and 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 whole
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.