dd, echo * as a poor man's ls if ls is accidentally deleted, busybox, cpio, fsck and fsdb.

Used all of these and more, in Unix, not just Linux crisis situations.

Those are already there. We are talking about diagnostic and recovery tools that should be installed by policy, in advance, so that they are already in place to aid in emergencies.

Okay, my mistake. But busybox is not always already there, right? Installed, I mean? Not at a box right now.

Busybox has one big downside: the tools it provides tend to have a rather ... limited set of options available. The easy stuff you can do in a standard shell might not be supported.