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I'm trying to write an init, but can't figure out the reboot\poweroff thing. apparently reboot is just a link to systemctl? (I'm using arch) So how does this work? init poweroff works and stuff, but reboot/poweroff just seems to be linked to systemctl

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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);
        .
        .
        .
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  • That's a great explanation. Thanks a lot!
    – sef sf
    Commented Dec 25, 2022 at 16:32

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