M4 as a replacement for sed

Writing a tutorial often involves to replace a placeholder in a file, such as:

Replace FOO with the actual name of your image:

sed 's|IMAGE_NAME|docker.io/foo/bar:latest|g' template.yaml |kubectl apply -f -

But this approach has several drawbacks:

• If you have to replace multiple placeholders, the sed syntax becomes cumbersome.
• If the delimiter appears in your replacement string, you will have to find another delimiter (such as in the previous example where the usual slash has been replaced by a pipe to accomodate the slash in the image name).
• The sed command has some subtleties between the GNU (any Linux distribution) and the BSD (MacOS) flavors.

For this specific use case (replacing placeholders), I would like to introduce another tool: the m4 command.

The m4 command is used in the C/C++ compilation chain to replace pre-processor directives with their actual values. Its syntax is very simple and it is present on most Linux distributions and on MacOS by default.

Let’s have a look at a very simple example:

$ cat > example <<EOF
Hello, my name is NAME and I like THING.
EOF

$ m4 -D NAME=Nicolas -D THING=beers example
Hello, my name is Nicolas and I like beers.

$ m4 -D NAME=John -D THING=wine example
Hello, my name is John and I like wine.

Next time you write a tutorial in which you need to replace a placeholder with its actual value, consider the m4 command!