Sysctl is a tool for examining and changing kernel parameters at runtime on Linux systems. It makes use of the procfs virtual filesystem mounted at /proc/ to allow modifying kernel parameters without rebooting. The /proc/sys/ directory contains files that expose kernel settings, and sysctl or echo commands can be used to view or change these settings temporarily. For changes to persist after a reboot, settings must be added to /etc/sysctl.conf.