Passing Parameters to Tasks in Makefile

In this article, part of our ongoing Makefile series, I will demonstrate how to pass parameters to tasks in a Makefile.

Example

Consider the useful tool youtube-dl, which is used to download videos from YouTube. It also has a flag to download the JSON list of a playlist. The command looks like this:

youtube-dl -J https://www.youtube.com/playlist?list=<playlist_id>

I want to create a Makefile task to download the JSON file simply, as shown below:

download-youtube-playlist-json:
	@youtube-dl -J "https://www.youtube.com/playlist?list=<playlist_id>" > <playlist_id>.json

In this example, the playlist_id is hardcoded in the Makefile. If I want to download another JSON list, I need to update the Makefile.

Solution

To simplify the task, we can update the Makefile to accept a parameter, allowing us to download different playlist JSON files with the same task but different parameters. The modified Makefile is shown below:

PLAYLIST_ID=$(list)

download-youtube-playlist-json:
ifeq ($(strip $(PLAYLIST_ID)),)
	$(error list is empty. Please run the task like: make download-youtube-playlist-json list=<playlist_id>)
endif
	@youtube-dl -J "https://www.youtube.com/playlist?list=$(PLAYLIST_ID)" > $(PLAYLIST_ID).json

Explanation

Variable Definition

PLAYLIST_ID=$(list)

• This line defines a variable PLAYLIST_ID which is assigned the value of another variable list. When you run the make command, you can pass a value to list with list=<playlist_id>.

Conditional Check

ifeq ($(strip $(PLAYLIST_ID)),)
	$(error list is empty. Please run the task like: make download-youtube-playlist-json list=<playlist_id>)
endif

• The ifeq ($(strip $(PLAYLIST_ID)),) directive checks if PLAYLIST_ID is empty after stripping any leading or trailing whitespace.
• If PLAYLIST_ID is empty, the $(error ...) function is invoked, which stops the make process and prints the error message: "list is empty. Please run the task like: make download-youtube-playlist-json list=<playlist_id>".
• Note that the directive should not be indented.

Usage

To use the task to download the JSON of a YouTube playlist, you can use the following command:

make download-youtube-playlist-json list=<playlist_id>

By using this approach, you can easily pass different playlist IDs to the Makefile task without modifying the Makefile itself each time. This makes the process more efficient and flexible.