efivarfs

efivarfs is a Linux kernel virtual file system that exposes UEFI firmware variables to user space. It provides a standard, file-based interface for reading and, where permitted, writing the variables stored by the system firmware. The filesystem is typically mounted as efivarfs under the /sys/firmware/efi/efivars path and is used by many user-space utilities that manage EFI variables.

In efivarfs, each UEFI variable is represented by a separate file. The file names encode the variable’s globally unique identifier (GUID) and its name, and the file contents reflect the variable’s data. Access to these files is governed by kernel and firmware permissions; read operations usually require appropriate privileges, and write operations generally require root or capabilities such as CAP_SYS_ADMIN. Some variables may be restricted by the firmware and marked as accessible only during boot or under specific conditions.

The interface is commonly used by system utilities and libraries such as libefivar, fwupd, and boot-management tools to query and modify EFI variables such as BootOrder, Boot####, and Secure Boot settings. This enables tasks like selecting boot options, configuring firmware features, or updating settings from user space without specialized boot-time tools.

efivarfs is available on systems with UEFI firmware and an EFI system partition, and it depends on the kernel’s EFI variable support. While it provides convenient access to firmware state, its use is subject to firmware-imposed restrictions and security considerations. It is not a replacement for firmware documentation but a practical bridge between UEFI variables and Linux user-space tools.