Yes, cgroups can be used to impose limits on resources; and from the outside looking in, can look very similar to setrlimit(). But from the inside of the kernel looking out (generally, the perspective taken by the linux-insides book), they are very different subsystems; I expect that cgroups will come in quite a bit later. If the book were discussing how to limit resources, sure it should mention cgroups; but it is discussing how things work in the kernel, not how to get the kernel to do certain things.

Bash has a few builtin that are clones of commands (mostly from coreutils). To execute the command instead of the builtin, just give the full path to the command (not sure and cannot verify, right now but this might mean either /bin/ulimit or /use/bin/ulimit ). There are other notable ones like echo, and time where the builtins have slightly different behaviour than the commands.

strace -e getrlimit,setrlimit bash -c 'ulimit -s 2 && echo foo'