Modules LM

LM

Print login manager (desktop manager) name and version

Module type	lm
Default order	20 (only used by --gen-config)
Module source	src/modules/lm/lm.c
Detection source	src/detection/lm/

Prints the login manager that owns the current session: a pretty name followed by its version when one is known. The printed key is Login Manager (and it is localised), while the module type is lm.

Login Manager: Logon User Interface 10.0.26100.1

The version is omitted from this line when it is empty. The raw service name — LogonUI in the sample above, gdm, sddm, lightdm, … on Linux — only appears through a custom format or in the JSON.

Platform support

Platform	Implementation	Notes
Linux	lm_linux.c	systemd over DBus, then systemd's private session files, then the process list
FreeBSD / MidnightBSD / DragonFly	lm_linux.c	sysctl(KERN_PROC_UID, 0) process scan
NetBSD	lm_linux.c	sysctl(KERN_PROC2, KERN_PROC_UID, 0) process scan
OpenBSD	lm_linux.c	sysctl(KERN_PROC_UID, 0) process scan
Solaris / illumos	lm_linux.c	Walks /proc/<pid>/psinfo
GNU/Hurd	lm_linux.c	Walks /proc/<pid>/comm
macOS	lm_apple.m	Fixed value, plus a version from an Info.plist
Windows	lm_windows.c	Fixed value, plus the LogonUI.exe file version
Android	lm_nosupport.c	Reports "Not supported on this platform"
Haiku	lm_nosupport.c	Not supported

The same lm_linux.c serves every POSIX platform: only the process-enumeration call differs per platform, and the systemd route is guarded by __linux__ && !__ANDROID__.

Configuration

The module has no options of its own.

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

Format string

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

Variable	Description
{service}	Service or process name as it was found, e.g. gdm3, LogonUI
{pretty-name}	Mapped display name, e.g. GDM, Logon User Interface
{version}	Version string; empty when none was found

None of the three is available in the key format.

JSON output

[
    {
        "type": "LM",
        "result": {
            "service": "LogonUI",
            "prettyName": "Logon User Interface",
            "version": "10.0.26100.1"
        }
    }
]

result is an object, not an array — there is at most one login manager. version is an empty string when it could not be determined. When no login manager was found the module writes { "type": "LM", "error": "No LM service found" }, the same message the text path shows.

Examples

// Keep the service name visible next to the pretty name
{ "type": "lm", "format": "{pretty-name} ({service})" }

// A version-only line, with the key hidden
{ "type": "lm", "key": " ", "format": "{version}" }

// Treat an empty version as a missing variable
{ "type": "lm", "format": "{pretty-name}{?version} {version}{?}" }

Pitfalls

• The process-list route only knows 16 names. A process is recognised only if its name is exactly one of atrium, cdm, entrance, gdm, gdm3, greetd, lemurs, lightdm, lxdm, ly, plasmalogin, sddm, slim, tbsm, xdm or xenodm (case-insensitively). Anything else — a custom or a newly renamed display manager — is reported as No LM service found even while it is running.
• The process scan only looks at root-owned processes. On Linux a pid is skipped unless /proc/<pid>/loginuid reads "no login uid" (4294967295), which in practice leaves root-owned system daemons; the BSD variants ask for uid 0 explicitly, and Solaris skips every psinfo whose pr_uid is not 0. A per-user login manager is therefore invisible.
• {pretty-name} falls back to the raw service name. Only the 16 names above (and gdm-prefixed, sddm-prefixed, xfwm-prefixed and lightdm services coming from systemd) get a mapped display name; everything else is printed verbatim, so a session running sshd prints sshd twice.
• Version lookups both run programs and cover only five prefixes. gdm / gdm3 (runs gdm3 --version, falling back to gdm --version), lightdm (runs lightdm --version and reads stderr), xfwm (runs xfwm4 --version), sshd (runs sshd -V, reads stderr, keeps what is between the first _ and the first ,), and sddm (decompresses /usr/share/man/man1/sddm.1.gz and parses its .TH line). Everything else gets an empty version, and each of these parses a fixed output shape with string surgery, so a changed output format degrades to an empty or wrong version rather than an error.
• xfwm and sshd are unreachable through the process-list route. Both have version lookups but neither is in the 16-name table, so those versions only ever appear when the service name comes from systemd (a sshd session, for instance).
• SDDM's version depends on a hardcoded man page path and on libz. It is read from /usr/share/man/man1/sddm.1.gz; a build without libz reports Fastfetch is built without libz support internally and simply ends up with an empty version, and an install whose man pages live elsewhere (a non-/usr prefix) is not found.
• --no-detect-version is honoured everywhere except macOS. lm_apple.m reads CFBundleShortVersionString from /System/Library/CoreServices/loginwindow.app/Contents/Info.plist unconditionally, while the Linux and Windows backends check detectVersion before spawning anything or reading a file version.
• On Linux without DBus, the private systemd files are the next resort. $XDG_SESSION_ID is used, and when it is unset the session id is guessed from /run/systemd/users/<uid>'s DISPLAY= property. The source comments call this "private data. Do not parse" and describe the guess as "actually buggy, and assumes current user is using DE" — a multi-seat machine can therefore report the wrong session's service.
• Windows and macOS always report something. Both short-circuit the whole detection with a constant (LogonUI / Logon User Interface, loginwindow / Login Window) and only the version is read from the system, so No LM service found cannot happen there.

Implementation

ffDetectLM() fills an FFLMResult — three FFstrbufs, service, prettyName and version — and returns an error string. ffPrintLM() and ffGenerateLMJsonResult() each call it themselves; nothing is cached, so a --dynamic-interval run repeats the whole lookup every round.

The detection is a chain of three (Linux) or one (all other POSIX platforms) sources, followed by two lookup tables:

1. Get a service name from the first source that works.
2. Map it to a pretty name, falling back to the service name itself.
3. If detectVersion is on, look the version up by service-name prefix.

Linux

In order:

• systemd over DBus (only when built with DBus support): reads the Service property of /org/freedesktop/login1/session/auto from org.freedesktop.login1 on the system bus.
• systemd's private files: SERVICE= from /run/systemd/sessions/<session id>, where the session id comes from $XDG_SESSION_ID or from the DISPLAY= line of /run/systemd/users/<uid>.
• The process list: /proc is walked, keeping only directories whose name starts with a digit and whose loginuid reads "no login uid"; <pid>/comm is then matched against the name table.

The pretty-name mapping is a switch on the first character of the service name, and the version lookup a chain of prefix comparisons (both described in the Pitfalls above).

FreeBSD / MidnightBSD / DragonFly, NetBSD, OpenBSD

The process list comes from sysctl(): KERN_PROC_UID with uid 0 (NetBSD uses KERN_PROC2), yielding a kinfo_proc array whose ki_comm (NetBSD: p_comm) is matched against the name table. On DragonFly ki_comm is a macro for kp_comm. The first match wins.

Solaris / illumos and GNU/Hurd

Both walk /proc. Solaris reads sizeof(psinfo_t) bytes from each <pid>/psinfo and matches pr_fname after skipping every entry whose pr_uid is not 0. GNU/Hurd reads <pid>/comm, which makes its scan effectively the same as the Linux fallback.

macOS

The constants are hardcoded: loginwindow / Login Window. The version is CFBundleShortVersionString from /System/Library/CoreServices/loginwindow.app/Contents/Info.plist, read through NSDictionary; when the file cannot be read the version stays empty.

Windows

The constants are hardcoded: LogonUI / Logon User Interface. When detectVersion is on, the file version of %SystemRoot%\system32\LogonUI.exe is read with ffGetFileVersion(), which is the same mechanism the Kernel module uses for its version.