  Commit messages contain diverse and valuable types of knowledge in all
aspects of software maintenance and evolution. Links are an example of such
knowledge. Previous work on "9.6 million links in source code comments" showed
that links are prone to decay, become outdated, and lack bidirectional
traceability. We conducted a large-scale study of 18,201,165 links from commits
in 23,110 GitHub repositories to investigate whether they suffer the same fate.
Results show that referencing external resources is prevalent and that the most
frequent domains other than github.com are the external domains of Stack
Overflow and Google Code. Similarly, links serve as source code context to
commit messages, with inaccessible links being frequent. Although repeatedly
referencing links is rare (4%), 14% of links that are prone to evolve become
unavailable over time; e.g., tutorials or articles and software homepages
become unavailable over time. Furthermore, we find that 70% of the distinct
links suffer from decay; the domains that occur the most frequently are related
to Subversion repositories. We summarize that links in commits share the same
fate as links in code, opening up avenues for future work.
