2019-03-15 update

content/blog/m4-as-replacement-for-sed.md

@@ -0,0 +1,45 @@
+---
+title: "M4 as a replacement for sed"
+date: 2019-03-15T00:00:00+02:00
+---
+
+Writing a tutorial often involves to replace a placeholder in a file, such as:
+
+*Replace FOO with the actual name of your image:*
+
+```sh
+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:
+
+```raw
+$ 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!