The Unix `Who` Command • Hacker News Board

Re: The Unix `Who` Command

#41

Post by jf » Fri, Sep 04, 2020, 1:48 AM UTC

forgotpwd16 wrote: post #31

jf wrote: post #23

Earlier quoted context omitted.

Does anybody know why there are line breaks in the sed command?

It runs multiple expressions. That is sed '/pattern1/d pattern2/d' is the same with sed '/pattern1/d; /pattern2/d' and sed -e '/pattern1/d' -e '/pattern2/d' I believe those are portable across all `sed` implementations, and they at least work with GNU `sed --posix` and plan9port's sed.

Thank you! This is exactly the kind of reply I was hoping for.

Re: The Unix `Who` Command

#42

Post by cptnapalm » Fri, Sep 04, 2020, 1:24 PM UTC

lovehashbrowns wrote: post #36

cptnapalm wrote: post #15

Understanding UNIX/LINUX Programming: A Guide to Theory and Practice by Bruce Molay is a work which has the reader learn system programming by re-implementing UNIX commands. who is the focus of Chapter 2.

How useful is that book nowadays? Is it still worth purchasing?

I was messing with it a bit several months ago. You'd have to look up newer data structures, but just looking up the man pages gave me the necessary information. While the implementation may be different now, the fundamentals of UNIX are pretty much the same.

Re: The Unix `Who` Command

#43

Post by eesmith » Sat, Sep 05, 2020, 7:56 PM UTC

eesmith wrote: post #8

BuildTheRobots wrote: post #6

Earlier quoted context omitted.

> Of course, this only mocks the most basic features of the who command and doesn’t handle any option, like the famous `who am i` or `who mom hates`. it's not quite an apples to apples comparison as his rewrite doesn't handle any options or cleverness, but it's still a massive jump in LOC.

Nor is it anywhere near as portability. Though that's not a big issue these days.

Wow, I touched a nerve I don't understand, to get a -1, so I'll explain myself. The GNU code looks like it's portable to a bunch of OSes, some with and some without a given feature. That sort of portability leads to having a lot of #ifdefs. If you're only going to support a handful of OSes - because that's all most people care about - then of course your code is going to be smaller.

As a concrete example, the linked-to "who" uses:

    if (entry.ut_type != USER_PROCESS)
      continue;

The GNU one uses:

      if (IS_USER_PROCESS (utmp_buf))

The IS_USER_PROCESS() macro is in gnulib's readutmp.h (which would need to be included in the overall line cound):

   # define IS_USER_PROCESS(U)                                     \
      (UT_USER (U)[0]                                              \
       && (UT_TYPE_USER_PROCESS (U)                                \
           || (UT_TYPE_NOT_DEFINED && UT_TIME_MEMBER (U) != 0)))

The UT_USER maps to ut_user on some systems, and ut_name on others:

   /* Accessor macro for the member named ut_user or ut_name.  */
   # if HAVE_UTMPX_H
   
   #  if HAVE_STRUCT_UTMPX_UT_USER
   #   define UT_USER(Utmp) ((Utmp)->ut_user)
   #  endif
   #  if HAVE_STRUCT_UTMPX_UT_NAME
   #   undef UT_USER
   #   define UT_USER(Utmp) ((Utmp)->ut_name)
   #  endif

This is because, as https://www.gnu.org/software/libc/manual/html_node/Logging-I... points out, "Note that the ut_user member of struct utmp is called ut_name in BSD. Therefore, ut_name is defined as an alias for ut_user in utmp.h"

Each of those other macros supports cases where the machine supports a given test, and when it doesn't. Including for machines which don't implement ut_type! (That's what the UT_TYPE_NOT_DEFINED case is for, which falls back to using the time field ... which has its own set of variants!)

Portability is neither options nor cleverness, so I added it to the list of considerations. But then again, most people now don't need to support decades of history and scores of Unix variants.