I guess the 'diff' files in some SlackBuilds (for example, blas) (and located on different websites) are meant to somehow 'patch' the project. I've read thru the manpage for 'patch' several times without really understanding it.
Surely there is some standard, simple way to apply a 'diff' file to a particular SlackBuild for which it was designed. After looking around for a while, I've not found such information, and would be most grateful for any help with doing this.
Thanks in advance, jrc
Using Slack 14.2 64-bit with Alien multilib on Dell XPS-410

You don't have to do anything. Whenever there's a patch included in a SlackBuild, the SlackBuild script does all that for you. Just run the SlackBuild. In the case of blas, it happens here:

But to answer the question you asked, imagine you have two files. To see the differences between an original file and a new modified version, you use the diff command. Ok so far? You can save the output of the diff command in a file. The patch command performs the reverse process: by reading the file containing the output of the diff command, and a copy of the original file, it outputs a copy of the new modified version.

2 members found this post helpful.

If the diff is very simple (just added, deleted or edited lines) I do this way
patch old-file <(diff old-file new-file)

For more complex diff, like files in /etc that slackpkg update with .new files,
I run kompare to apply one by one.

2 members found this post helpful.

Thanks people. Very helpful, both with specific problem and
for general use. - jrc