Understanding the apt-get build-dep process

Addendum added to the end of this posted question to respond to the answer of Stephen Kitt. ----- Cross-posted : no joy posting on the asymptote (sourceforge) forum. In this posting I describe experiments that I took trying to execute the apt-get build-dep command and also executing the apt-update command. I would like to **understand** what is causing these errors, and whether (using the extracted source code, as is) there is a remedy. In Linux Mint 22, I installed VirtualBox 7 and created a Linux Mint 22 guest, for experimentation. In the guest, I first installed TexLive, and then **successfully** installed Asymptote 3.00, a latex plug-in, from its source code. To do this, I had to first install the following packages: freeglut, cmake, libx11-dev, bison, flex, libgl1-mesa-dev, libglu1-mesa-dev zlib1g-dev Following this successful experiment, I restored the VirtualBox snapshot taken just after installing TexLive and extracting the Asymptote 3.00 source code. As a second experiment, I then attempted to shortcut the process. I had been advised that I can let a single install handle the dependencies, via the following commands: apt-get build-dep asymptote ./configure make all make install Based on preliminary research, I sudo xed updated \etc\apt\sources.list with the following line: deb-src file:/home/steve/Downloads/asymptote-3.00 unstable main contrib I then navigated to /home/steve/Downloads/asymptote-3.00 and then executed sudo apt-get build-dep asymptote. This failed with the message: $ sudo apt-get build-dep asymptote Reading package lists... Done E: You must put some 'deb-src' URIs in your sources.list To investigate, I then executed sudo apt update, which yielded Get:1 file:/home/steve/Downloads/asymptote-3.00 unstable InRelease Ign:1 file:/home/steve/Downloads/asymptote-3.00 unstable InRelease Get:2 file:/home/steve/Downloads/asymptote-3.00 unstable Release Err:2 file:/home/steve/Downloads/asymptote-3.00 unstable Release File not found - /home/steve/Downloads/asymptote-3.00/dists/unstable/Release (2: No such file or directory) ... Based on the above apt update report, I retried sudo apt-get build-dep asymptote, after replacing the appended line with: deb-src file:/home/steve/Downloads/asymptote-3.00 This yielded $ sudo apt-get build-dep asymptote E: Malformed entry 8 in list file /etc/apt/sources.list (Suite) E: The list of sources could not be read. E: Malformed entry 8 in list file /etc/apt/sources.list (Suite) E: The list of sources could not be read. Again investigating, I re-executed sudo apt update, which yielded $ sudo apt update E: Malformed entry 8 in list file /etc/apt/sources.list (Suite) E: The list of sources could not be read. Investigating further, the complete list of subdirectories to the asymptote 3.0 directory is: backports examples patches base gc prc build-scripts gl-matrix-2.4.0-pruned tests cmake-preset-files GUI thirdparty_impl cmake-scripts libatomic_ops tinyexr cudareflect LspCpp webgl cxxtests misc windows doc ----- **Addendum**
I test-verified the answer of Stephen Kitt. After reading his answer, and reloading the (previous) VirtualBox snapshot, I found that the deb rather than deb-src entries were already included in /etc/apt/sources.list.d/official-source-repositories.list. I had already determined that in this situation, the apt-get build-dep asymptote command would not work. It was necessary to add the deb-src entries into \etc\apt\sources.list. Per his answer, I added the five deb-src entries to \etc\apt\sources.list. Then, I executed menu : software-sources : enable-source-code-repositories : update-cache. Then I executed apt update. Then, I navigated to the directory of the extracted asymptote code and executed the apt-get build-dep asymptote command. Presumably, this worked nicely (i.e. I aborted after 10 minutes because VirtualBox processing is slow, and the process was adding an additional 1+ gb of packaging). Based on a comment in Stephen Kitt's answer, I re-executed the entire experiment, with one exception: instead of adding all 5 deb-src lines to \etc\apt\sources.list, I only added the deb-src http://archive.ubuntu.com/ubuntu noble main restricted universe multiverse line. This also seemed to work perfectly. Again, I aborted the VirtualBox running after 10 minutes, for the same reason. Overall, this has been a great learning experience for me.