How can I run a command after boot?

Depends on distros.

If you are on Debian-derived distros, there are at least two proper places, /etc/rc.local and crontab. You may invoke crontab as root,

   crontab -e

and then insert this line

   @reboot /home/my_name/bin/my_command

where my_command is an executable file (chmod 755 my_command).

Alternatively, you may place a line like this

   /home/my_name/bin/my_command

in /etc/rc.local, and this will be executed last. In any case, pls do remember you are using root environment, not yours. Also for this reason, it is best to use absolute paths.

If you are on a systemd distro (Arch Linux, Fedora 19,...) the first option (crontab) is still valid, while the second one (/etc/rc.local) does not exist any longer. In this case, you should learn how to start a service to be run by systemctl, but this may be more than you bargained for with your simple question.

If your system is running a version of cron that supports it (specifically Vixie cron), you can use @reboot in a cron job.

This is one of 8 special strings that it supports.

Quoting the crontab(5) man page (from my Ubuntu 12.04 system):

Instead of the first five fields, one of eight special strings may appear:

string         meaning
------         -------
@reboot        Run once, at startup.
@yearly        Run once a year, "0 0 1 1 *".
@annually      (same as @yearly)
@monthly       Run once a month, "0 0 1 * *".
@weekly        Run once a week, "0 0 * * 0".
@daily         Run once a day, "0 0 * * *".
@midnight      (same as @daily)
@hourly        Run once an hour, "0 * * * *".

Please note that startup, as far as @reboot is concerned, is the time when the cron(8) daemon startup. In particular, it may be before some system daemons, or other facilities, were startup. This is due to the boot order sequence of the machine.

This is far from being the only way to run something at boot time, but it's an alternative.

Pretty much all variants of Linux (going back a long, long time) have a file /etc/rc.local which runs on startup - you can just add the command to it.

That said, you don't even need to do that. The "correct" way would be to modify / add the line kernel.sysrq = 1 into /etc/sysctl.conf