Fun tip #1: Apply a diff with ed

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) -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) 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.