Today I'll describe the rest of Util Linux's login commands!
After initializing internationalization & parsing commandline flags `last` outputs latest logins by iterating over each of the given files defaulting to /var/log/btmp or wtmp. For each (handling fuzzing slightly specially) it retrieves the boot-time from the kernel, it opens the file with a configured buffersize, carefully reads the binary data `fstat`ing on failure, seeks to end, & repeatedlies...
Repeatedlies reads & validates an entry, possibly & extensively reformats (including a DNS lookup) into human readable output, applies some normalization, tweaks locals and/or output additional info in a similar format, & clears state upon seeing shutdown loglines. After which it attempts to output a timestamp from the file, & it cleans up. Including freeing the filepath that was opened.
Upon processing user-process records it may open /proc/?/loginuid to determine if its a "phantom".
After init'ing internationalization alongside some collections & parsing commandline flags `sulogin` validates its running as superuser, ignores various signals, possibly ensures necessary device-filesystems are mounted (upon exit it'll unmount any it mounted), retrieves the commandline arg or $CONSOLE envvar, retrieves possible console devices via /proc, /sys, /proc/cmdline, or an IOCTL with a fallback validating results are returned, reconnects the pipeline files, retrieves root account,
For each console `sulogin` attempts to open one which isn't overloaded & initializing it to support Plymouth (a.k.a. splashscreens), various IOCTLs, locales, & baudrate.
If successful soft-ignoring SIGCHLD signals `sulogin` iterates over those opened console connections. After validation it forks a child which'll repeatedly ask for a password if needed & run a subshell.
Loops waits for subprocesses to end, closes opened consoles, & wait for subprocesses again. Restores signal handlers.
After initializing internationalization, configuring signal handlers & scheduling, & parsing commandline flags + singular arg `login` configures a process group & the terminal, opens the system logger, configures PAM possibly requesting authentication from it, loads the password entry, loads groups, opens a pam session, clears a watchdog timeout, closes the passwords database, logs some info, switches terminal info, configures environment variables, generates a new process title, ...
... logs some more, optionally outputs login messages, forks a subprocess with watchdog parent, drops privileges, cds to home, and runs the appropriate shell.
This command generates the btmp logs consulted by `last`. Can consult a "hushlogin" file via a mini-sharedlib.
And finally... After initializing internationalizing & parsing/validating commandline flags with the aid of libsmartcols checking if at most one arg remains `lslogins` parses wtmp or btmp files to reformat for libsmartcols.
For people who care about, support, or build Free, Libre, and Open Source Software (FLOSS).