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 Previous work on "9.6 million links in source code comments" showed that links are prone to decay, become outdated, and lack bidirectional traceability.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.

翻译：提交信息包含软件维护与演化各个方面的多样且宝贵的知识类型。链接是这类知识的一个实例。先前关于"源代码注释中的960万个链接"的研究表明，链接容易发生消亡、变得过时，并且缺乏双向可追溯性。我们对来自23,110个GitHub仓库提交中的18,201,165个链接进行了大规模研究，以调查它们是否面临同样的命运。结果表明，引用外部资源十分普遍，除github.com外，出现频率最高的域名为Stack Overflow和Google Code的外部域名。同样，链接作为提交信息的源代码上下文，其中不可访问的链接频繁出现。尽管重复引用链接的情况很少见（4%），但14%易于演化的链接会随时间推移变得不可用；例如，教程或文章以及软件主页随时间推移变得不可用。此外，我们发现70%的不同链接遭受消亡；出现频率最高的域名与Subversion仓库相关。我们总结出，提交中的链接与代码中的链接面临相同的命运，这为未来工作开辟了研究方向。