How do I generate a bootable .iso file from a directory?

Question

Currently, I have selected a distribution of Linux that I like a lot and I have downloaded the .iso image which I can simply dd to an USB or CD media, etc...

I would like to add some additional data to this installer image so that I do not have to bring both a installation disk and removable media with me whenever I need to setup a machine.

Is this something that can be done? If not, how would I go about creating a reasonably sized and bootable iso file after compiling an operating system from source?

I have experimented with mounting the iso file, copying all the files out of it, and trying to repackage it all back into an iso again; however, this resulting image is not bootable.

I am hoping to glean what it is that makes a bootable iso so special when compared to a normal CD with files on it.

Solution

You need mkisofs to generate the new iso and specify boot information. The args of mkisofs and structure of the directories will vary from one distribution to another, so the best is to look for articles about custom iso for your distro. An example: https://access.redhat.com/discussions/1422213