`systemctl hibernate` not executed on crontab script

Question

I have a script that verifies the battery level using acpi and if it's below certain threshold it should lock the machine and hibernate. The script is executed every minute using crontab

The problem is that the machine gets locked but never hibernates.

The script:

#!/bin/sh
acpi -b | awk -F'[,:%]' '{print $2, $3}' | {
    read -r status capacity

    if [ "$status" = Discharging -a "$capacity" -lt 10 ]; then
        echo 'Success' >> /tmp/low;
        logger "Critical battery threshold";
        DISPLAY=:0 i3lock -t -i $(ls -d ~/.wallpapers/* | shuf | head -n 1);
        echo 'Locked' >> /tmp/low;
        systemctl hibernate;
    fi
}

The /tmp/low log file shows the following:

$ cat /tmp/low 
Success
Locked
Success
Locked
Success
Locked

I tried to directly run a similar script (Without the ACPI check) and it worked perfectly

The testing script:

#!/bin/sh
acpi -b | awk -F'[,:%]' '{print $2, $3}' | {
    read -r status capacity

        echo 'Success' >> /tmp/low;
        logger "Critical battery threshold";
        DISPLAY=:0 i3lock -t -i $(ls -d ~/.wallpapers/* | shuf | head -n 1);
        echo 'Locked' >> /tmp/low;
        systemctl hibernate;
}

Same testing script was run using at but it didn't hibernate the machine. Any ideas why crontab can't execute systemctl hibernate?

The systemctl command takes two parameters: the COMMAND (i.e., 'start', 'stop', 'status', etc.) and the NAME of the service to execute the COMMAND on. In your test instance , it's missing the COMMAND part of that setup. — Thomas N
– Thomas N, Commented Jan 22, 2020 at 22:39
@ThomasN This is not the case, running systemctl hibernate from terminal works perfectly fine. The script also works fine, except when tools such as cron or at runs it — Mr. E
– Mr. E, Commented Jan 23, 2020 at 12:53
I stand corrected... should have read the manual pages a little further. — Thomas N
– Thomas N, Commented Jan 24, 2020 at 20:18

Mr. E · Accepted Answer · 2020-01-24 13:10:36Z

I have found a solution. Apparently the problem was in the polkit package that defines the policies for users to shutdown, reboot, suspend, hibernate, etc

As I had no rule file in /etc/polkit-1/rules.d the default is not to allow users to hibernate or suspend the machine while a user is logged in (I believe the problem here is that I'm locking the machine before hibernating, and therefore there is an open session)

To solve it I had to create the file /etc/polkit-1/rules.d/99-allow-hibernate-on-low-battery.rules with the following content:

polkit.addRule(function(action, subject) {
    if (action.id == "org.freedesktop.login1.suspend" ||
        action.id == "org.freedesktop.login1.suspend-multiple-sessions" ||
        action.id == "org.freedesktop.login1.hibernate" ||
        action.id == "org.freedesktop.login1.hibernate-multiple-sessions") {
        return polkit.Result.YES;
    }
});

After that, cron and at can hibernate the machine correctly

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