making an init: how exactly does the reboot command work?

Many programs behave differently depending on the name with which they are called. Something like systemctl inspects the value of argv[0] and behaves differently if it is reboot vs if it is systemctl. You can see this taken to the extreme with busybox, which is a single binary that provides almost an entire (minimal) userspace by symlinking all the commands to the single busybox binary.

You can do exactly the same thing with a shell script:

#!/bin/bash if [[ $0 =~ foo ]]; then echo "running foo action" elif [[ $0 =~ bar ]]; then echo "running bar action" else echo "running default action" fi 

Assuming this is multicall.sh, if we set things up like this:

ln -s multicall.sh foo ln -s multicall.sh bar 

And then see:

$ ./foo running foo action $ ./bar running bar action $ ./multicall.sh running default action 

For systemctl in particular, the logic is implemented here:

int systemctl_dispatch_parse_argv(int argc, char *argv[]) { assert(argc >= 0); assert(argv); if (invoked_as(argv, "halt")) { arg_action = ACTION_HALT; return halt_parse_argv(argc, argv); } else if (invoked_as(argv, "poweroff")) { arg_action = ACTION_POWEROFF; return halt_parse_argv(argc, argv); } else if (invoked_as(argv, "reboot")) { if (kexec_loaded()) arg_action = ACTION_KEXEC; else arg_action = ACTION_REBOOT; return halt_parse_argv(argc, argv); . . .