How to check which SysRq functions are enabled?

These are the available SysRq functions:

0 - disable every SysRq function.
1 - enable every SysRq function.
2 - enable control of console logging level
4 - enable control of keyboard (SAK, unraw)
8 - enable debugging dumps of processes etc.
16 - enable sync command
32 - enable remount read-only
64 - enable signalling of processes (term, kill, oom-kill)
128 - allow reboot/poweroff
256 - allow nicing of all RT tasks

438 = 2 + 4 + 16 + 32 + 128 + 256, so only the functions associated with those numbers are allowed. Read all about it in the documentation.

If you convert 438 to base 2 (110110110) it is even easier to see.

1     1     0    1    1    0   1   1   0
^256  ^128  ^64  ^32  ^16  ^8  ^4  ^2  ^1

Depending on your distribution, you may be able to tell if the kernel was compiled with CONFIG_MAGIC_SYSRQ using this command:

$ grep SYSRQ /boot/config-$(uname -r)

This works for me on Ubuntu.

Here is a Bash one-liner which will print you the enabled options:

for i in $(seq 1 8); do (( ($(</proc/sys/kernel/sysrq) & $((1<<$i))) > 0 )) && echo $((1<<$i)); done

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Which SysRq functions are allowed/disallowed when the bitmask is set to 438?

$ for i in $(seq 1 8); do (( (438 & $((1<<$i))) > 0 )) && echo $((1<<$i)); done
2
4
16
32
128
256

For the meaning, refer to William's answer.

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To enable all options, run:

echo 1 | sudo tee /proc/sys/kernel/sysrq

To make it persistent, run:

echo kernel.sysrq=1 | sudo tee /etc/sysctl.d/20-sysrq.conf