Bootflags

Bootflags are parameters that are passed to a computer's operating system kernel at startup to alter its behavior. In Unix-like systems, especially Linux, they are most often supplied via the bootloader (GRUB, LILO, or UEFI boot manager) on the kernel command line. The kernel parses these flags and uses them to enable debugging, adjust hardware settings, mount filesystems, set console output, and control early user‑space init processes.

Typical bootflags include options such as console=, root=, init=/sbin/init, quiet, splash, acpi=off, drm.debug=, ip=dhcp, plus many kernel‑specific or module parameters that begin with a dash, e.g. nmi_watchdog=0. The syntax is usually a key=value pair separated by spaces, or a boolean flag that is added without a value. Flags may be combined on a single line separated by spaces. When the system boots, the kernel’s command line parser stores a string of all supplied flags in /proc/cmdline for later reference by modules or scripts.

Bootflags are crucial for troubleshooting. Temporarily adding debug=3 or loglevel=7 can generate verbose kernel messages that aid diagnosis. On embedded devices, bootflags can select between different device trees or set secure boot modes. In corporate environments, bootflags may enforce minimal services or network configuration during initial boot for security or performance reasons.

Bootflags can also be overridden or supplemented by the initramfs. In many distributions, /etc/initramfs-tools/conf.d/ contains files that add custom flags at initramfs build time. The initramfs treats bootflags as runtime parameters, passing them to the real kernel once it hands control from the initrd.

Because bootflags influence early system behavior, mishandling them can prevent the machine from starting. Therefore, changes are usually tested in a staging environment, or a recovery kernel is kept on a separate boot menu entry.