[sf-lug] What purpose is served by the first two parts? (cd / && umask 022 ...

[Rick Moen]

Quoting Michael Paoli (Michael.Paoli at cal.berkeley.edu):

> cd /
> at least many versions of shutdown will typically umount non-root
> filesystems (at least before eventually dealing with root filesystem)
> in order to umount those filesystems, they can't have any processes on
> them with open files or current directories there - that's generally
> dealt with by shutdown using fuser or the like to terminate/kill such
> offending processes ... and if your shutdown itself didn't chdir(2) to
> avoid that, then it may terminate itself.

You _could_ just reboot this way:

# echo b > /proc/sysrq-trigger

That doesn't even try to umount or sync any filesystems, let alone warn
those pesky users.  The strong will survive.  ;->

(More seriously, above works even if /sbin/reboot & /sbin/shutdown
cannot function for reasons that might include disk media problems.  
And it supports not just an unconditional reboot command but also a 
sync command 's' and a umount command 'u' -- among others.  So, sending 
s, then u, then b is a safe forced reboot.)

For above to work, the kernel's magic SysRq option must be enabled,
which I think ought to be routinely for reasons entirely aside from
being able to terrorise users.  This sets that:

# echo "kernel.sysrq = 1" >> /etc/sysctl.conf 

To enable it only for the current runtime:

# echo 1 > /proc/sys/kernel/sysrq

For more about System Request Key coolness:
https://kernel.org/doc/html/latest/admin-guide/sysrq.html