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E.2.27.  /proc/sysrq-trigger

Using the echo command to write to this file, a remote root user can execute most System Request Key commands remotely as if at the local terminal. To echo values to this file, the /proc/sys/kernel/sysrq must be set to a value other than 0. For more information about the System Request Key, refer to Section E.3.9.3, “ /proc/sys/kernel/ ”.
Although it is possible to write to this file, it cannot be read, even by the root user.