Kernel

Kernel

Print system kernel version

Module type	kernel
Default order	9 (only used by --gen-config)
Module source	src/modules/kernel/kernel.c
Detection source	— (no detection layer)

Prints the kernel name and release, which is exactly what uname reports as sysname and release.

Kernel: Linux 6.12.4-arch1-1
Kernel: Darwin 24.3.0

Platform support

This module has no detection layer. It reads the FFPlatformSysinfo structure that FFPlatform fills once at startup, so it is available on every platform fastfetch supports and has no per-platform implementation to describe. That is why its row in the platform matrix is all dashes.

Configuration

Key	Type	Default	Description
key	string	module name	Module key. A single space hides the key and the separator.
keyColor	color	–	Overrides display.color.keys.
keyWidth	integer	–	Overrides display.key.width.
keyIcon	string	built-in glyph	Printed when display.key.type includes the icon bit. Any glyph works; "" prints none.
outputColor	color	–	Overrides display.color.output.
format	string	–	Custom output format (see below).
condition	object	–	Show the module only if the conditions match.

There are no module-specific keys.

Format string

Run fastfetch -h kernel-format for the authoritative list.

Variable	Description
{sysname}	Kernel name (Linux, Darwin, …)
{release}	Kernel release
{version}	Full build string — very long
{arch}	Machine architecture
{page-size}	Memory page size, as a formatted size

JSON output

{
    "type": "kernel",
    "result": {
        "architecture": "x86_64",
        "name": "Linux",
        "release": "6.12.4-arch1-1",
        "version": "#1 SMP PREEMPT_DYNAMIC Fri, 20 Dec 2024 14:30:00 +0000",
        "pageSize": 4096
    }
}

Examples

{ "type": "kernel", "format": "{release}" }

{ "type": "kernel", "key": "Kernel", "format": "{sysname} {release} ({arch})" }

Pitfalls

• {page-size} is not a number. It is rendered through the shared size formatter, so it follows display.size: with the default iec it prints 16.00 KiB, with si it prints 16 kB, and display.size.ndigits changes the decimals. The JSON output carries the raw byte count instead ("pageSize": 16384), so console and JSON are not interchangeable here.
• {version} is the entire uname -v string, which includes the build host, the toolchain and timestamps. It is several times longer than the rest of the line and is normally left out of a default config.
• {sysname} here and {sysname} in the OS module are the same value. Both read instance.state.platform.sysinfo.name, so a format that prints both always duplicates it. The kernel module calls it sysname; the same field is what the OS module defaults to when the distribution did not report a name.
• The default line has no label distinguishing kernel from OS. Kernel: Darwin 24.3.0 alongside OS: macOS Sequoia 15.3 is easy to misread, since on macOS the kernel name is Darwin while the OS name is macOS.
• Nothing is configurable beyond the shared keys — there is no way to ask for a specific uname field beyond what the format variables expose.

Implementation

Everything at once

ffPrintKernel() is short. It takes const FFPlatformSysinfo* info = &instance.state.platform.sysinfo — a structure filled once during startup by the FFPlatform detection code, not by this module — and then either prints name release, or formats the template with five arguments.

ffGenerateKernelJsonResult() reads the same structure and writes architecture, name, release, version and pageSize (as a uint64_t).

The one piece of arithmetic is the page size: FFPlatformSysinfo stores a shift, not a byte count, so both the console and the JSON writer compute (uint64_t) 1 << pageSizeShift. The console formatter then feeds that number to ffSizeAppendNum(), which reads instance.config.display.sizeBinaryPrefix and picks the unit table accordingly — IEC (KiB/MiB, base 1024), SI (kB/MB, base 1000), JEDEC (KB/MB, base 1024) or a bytes-only fallback for none.

Why the module has no detection layer

Most modules split into a UI layer under src/modules/<name>/ and a platform layer under src/detection/<name>/. Kernel does not, because uname(2) is POSIX and FFPlatform already calls it for the OS, host, shell and terminal modules. Adding a detection layer would only duplicate work the process has already done. The same applies to Title, Separator, Custom, Break, Colors and DateTime.

Keeping the two argument lists in sync

The formatArgs list and the argument array in ffPrintKernel() are maintained separately, and nothing enforces that they agree. They currently do — the same five names on each side:

Declared in formatArgs	Passed to the formatter
sysname, release, version, arch, page-size	sysname, release, version, arch, page-size

formatArgs is what -h kernel-format reads, while the argument array is what actually resolves a placeholder. A name that is declared but not passed is emitted verbatim rather than reported, so a drift between the two lists is silent unless you print the variable. The module used to advertise a {display-version} that printed as literal text for exactly that reason.