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From: [e-mail] (Ted M A Timar)
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Subject: Unix - Frequently Asked Questions (4/7) [Frequent posting]
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Date: 24 Dec 1992 06:03:15 GMT
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Archive-name: unix-faq/faq/part4
Version: $Id: part4,v 2.1 92/12/04 07:43:53 tmatimar Exp $

These seven articles contain the answers to some Frequently Asked
Questions often seen in comp.unix.questions and comp.unix.shell.
Please don't ask these questions again, they've been answered plenty
of times already - and please don't flame someone just because they may
not have read this particular posting.  Thank you.

These articles are divided approximately as follows:

      1.*) General questions.
      2.*) Relatively basic questions, likely to be asked by beginners.
      3.*) Intermediate questions.
      4.*) Advanced questions, likely to be asked by people who thought
       they already knew all of the answers.
      5.*) Questions pertaining to the various shells, and the differences.
      6.*) An overview of Unix variants.
      7.*) An comparison of configuration management systems (RCS, SCCS).

This article includes answers to:

      4.1)  How do I read characters from a terminal without requiring the user
              to hit RETURN?
      4.2)  How do I check to see if there are characters to be read without
              actually reading?
      4.3)  How do I find the name of an open file?
      4.4)  How can an executing program determine its own pathname?
      4.5)  How do I use popen() to open a process for reading AND writing?
      4.6)  How do I sleep() in a C program for less than one second?
      4.7)  How can I get setuid shell scripts to work?
      4.8)  How can I find out which user or process has a file open or is using
            a particular file system (so that I can unmount it?)
      4.9)  How do I keep track of people who are fingering me?
      4.10) Is it possible to reconnect a process to a terminal after it has
            been disconnected, e.g. after starting a program in the background
            and logging out?
      4.11) Is it possible to "spy" on a terminal, displaying the output
            that's appearing on it on another terminal?

If you're looking for the answer to, say, question 4.5, and want to skip
everything else, you can search ahead for the regular expression "^4.5)".

While these are all legitimate questions, they seem to crop up in
comp.unix.questions or comp.unix.shell on an annual basis, usually
followed by plenty of replies (only some of which are correct) and then
a period of griping about how the same questions keep coming up.  You
may also like to read the monthly article "Answers to Frequently Asked
Questions" in the newsgroup "news.announce.newusers", which will tell
you what "UNIX" stands for.

With the variety of Unix systems in the world, it's hard to guarantee
that these answers will work everywhere.  Read your local manual pages
before trying anything suggested here.  If you have suggestions or
corrections for any of these answers, please send them to to
[e-mail].

4.1)  How do I read characters from a terminal without requiring the user
      to hit RETURN?

      Check out cbreak mode in BSD, ~ICANON mode in SysV.

      If you don't want to tackle setting the terminal parameters
      yourself (using the "ioctl(2)" system call) you can let the stty
      program do the work - but this is slow and inefficient, and you
      should change the code to do it right some time:

      #include <stdio.h>
      main()
      {
        int c;

        printf("Hit any character to continue\n");
        /*
         * ioctl() would be better here; only lazy
         * programmers do it this way:
         */
        system("/bin/stty cbreak");        /* or "stty raw" */
        c = getchar();
        system("/bin/stty -cbreak");
        printf("Thank you for typing %c.\n", c);

        exit(0);
      }

      You might like to check out the documentation for the "curses"
      library of portable screen functions.  Often if you're interested
      in single-character I/O like this, you're also interested in
      doing some sort of screen display control, and the curses library
      provides various portable routines for both functions.

4.2)  How do I check to see if there are characters to be read without
      actually reading?

      Certain versions of UNIX provide ways to check whether characters
      are currently available to be read from a file descriptor.  In
      BSD, you can use select(2).  You can also use the FIONREAD ioctl
      (see tty(4)), which returns the number of characters waiting to
      be read, but only works on terminals, pipes and sockets.  In
      System V Release 3, you can use poll(2), but that only works on
      streams.  In Xenix - and therefore Unix SysV r3.2 and later - the
      rdchk() system call reports whether a read() call on a given file
      descriptor will block.

      There is no way to check whether characters are available to be
      read from a FILE pointer.  (You could poke around inside stdio
      data structures to see if the input buffer is nonempty, but that
      wouldn't work since you'd have no way of knowing what will happen
      the next time you try to fill the buffer.)

      Sometimes people ask this question with the intention of writing
        if (characters available from fd)
            read(fd, buf, sizeof buf);
      in order to get the effect of a nonblocking read.  This is not
      the best way to do this, because it is possible that characters
      will be available when you test for availability, but will no
      longer be available when you call read.  Instead, set the
      O_NDELAY flag (which is also called FNDELAY under BSD) using the
      F_SETFL option of fcntl(2).  Older systems (Version 7, 4.1 BSD)
      don't have O_NDELAY; on these systems the closest you can get to
      a nonblocking read is to use alarm(2) to time out the read.

4.3)  How do I find the name of an open file?

      In general, this is too difficult.  The file descriptor may
      be attached to a pipe or pty, in which case it has no name.
      It may be attached to a file that has been removed.  It may
      have multiple names, due to either hard or symbolic links.

      If you really need to do this, and be sure you think long
      and hard about it and have decided that you have no choice,
      you can use find with the -inum and possibly -xdev option,
      or you can use ncheck, or you can recreate the functionality
      of one of these within your program.  Just realize that
      searching a 600 megabyte filesystem for a file that may not
      even exist is going to take some time.

4.4)  How can an executing program determine its own pathname?

      Your program can look at argv[0]; if it begins with a "/", it is
      probably the absolute pathname to your program, otherwise your
      program can look at every directory named in the environment
      variable PATH and try to find the first one that contains an
      executable file whose name matches your program's argv[0] (which
      by convention is the name of the file being executed).  By
      concatenating that directory and the value of argv[0] you'd
      probably have the right name.

      You can't really be sure though, since it is quite legal for one
      program to exec() another with any value of argv[0] it desires.
      It is merely a convention that new programs are exec'd with the
      executable file name in argv[0].

      For instance, purely a hypothetical example:

    #include <stdio.h>
    main()
    {
        execl("/usr/games/rogue", "vi Thesis", (char *)NULL);
    }

      The executed program thinks its name (its argv[0] value) is
      "vi Thesis".   (Certain other programs might also think that
      the name of the program you're currently running is "vi Thesis",
      but of course this is just a hypothetical example, don't
      try it yourself :-)

4.5)  How do I use popen() to open a process for reading AND writing?

      The problem with trying to pipe both input and output to an
      arbitrary slave process is that deadlock can occur, if both
      processes are waiting for not-yet-generated input at the same
      time.  Deadlock can be avoided only by having BOTH sides follow a
      strict deadlock-free protocol, but since that requires
      cooperation from the processes it is inappropriate for a
      popen()-like library function.

      The 'expect' distribution includes a library of functions that a
      C programmer can call directly.  One of the functions does the
      equivalent of a popen for both reading and writing.  It uses ptys
      rather than pipes, and has no deadlock problem.  It's portable to
      both BSD and SV.  See the next answer for more about 'expect'.

4.6)  How do I sleep() in a C program for less than one second?

      The first thing you need to be aware of is that all you can
      specify is a MINIMUM amount of delay; the actual delay will
      depend on scheduling issues such as system load, and could be
      arbitrarily large if you're unlucky.

      There is no standard library function that you can count on in
      all environments for "napping" (the usual name for short
      sleeps).  Some environments supply a "usleep(n)" function which
      suspends execution for n microseconds.  If your environment
      doesn't support usleep(), here are a couple of implementations
      for BSD and System V environments.

      The following code is adapted from Doug Gwyn's System V emulation
      support for 4BSD and exploits the 4BSD select() system call.
      Doug originally called it 'nap()'; you probably want to call it
      "usleep()";

      /*
        usleep -- support routine for 4.2BSD system call emulations
        last edit:    29-Oct-1984    D A Gwyn
      */

      extern int    select();

      int
      usleep( usec )                /* returns 0 if ok, else -1 */
        long        usec;        /* delay in microseconds */
        {
        static struct            /* `timeval' */
            {
            long    tv_sec;        /* seconds */
            long    tv_usec;    /* microsecs */
            }    delay;        /* _select() timeout */

        delay.tv_sec = usec / 1000000L;
        delay.tv_usec = usec % 1000000L;

        return select( 0, (long *)0, (long *)0, (long *)0, &delay );
        }

      On System V you might do it this way:

      /*
      subseconds sleeps for System V - or anything that has poll()
      Don Libes, 4/1/1991

      The BSD analog to this function is defined in terms of
      microseconds while poll() is defined in terms of milliseconds.
      For compatibility, this function provides accuracy "over the long
      run" by truncating actual requests to milliseconds and
      accumulating microseconds across calls with the idea that you are
      probably calling it in a tight loop, and that over the long run,
      the error will even out.

      If you aren't calling it in a tight loop, then you almost
      certainly aren't making microsecond-resolution requests anyway,
      in which case you don't care about microseconds.  And if you did,
      you wouldn't be using UNIX anyway because random system
      indigestion (i.e., scheduling) can make mincemeat out of any
      timing code.

      Returns 0 if successful timeout, -1 if unsuccessful.

      */

      #include <poll.h>

      int
      usleep(usec)
      unsigned int usec;        /* microseconds */
      {
        static subtotal = 0;    /* microseconds */
        int msec;            /* milliseconds */

        /* 'foo' is only here because some versions of 5.3 have
         * a bug where the first argument to poll() is checked
         * for a valid memory address even if the second argument is 0.
         */
        struct pollfd foo;

        subtotal += usec;
        /* if less then 1 msec request, do nothing but remember it */
        if (subtotal < 1000) return(0);
        msec = subtotal/1000;
        subtotal = subtotal%1000;
        return poll(&foo,(unsigned long)0,msec);
      }

      Another possibility for nap()ing on System V, and probably other
      non-BSD Unices is Jon Zeeff's s5nap package, posted to
      comp.sources.misc, volume 4.  It does require a installing a
      device driver, but works flawlessly once installed.  (Its
      resolution is limited to the kernel HZ value, since it uses the
      kernel delay() routine.)

4.7)  How can I get setuid shell scripts to work?

      [ This is a long answer, but it's a complicated and frequently-asked
        question.  Thanks to Maarten Litmaath for this answer, and
        for the "indir" program mentioned below. ]

      Let us first assume you are on a UNIX variant (e.g. 4.3BSD or
      SunOS) that knows about so-called `executable shell scripts'.
      Such a script must start with a line like:

    #!/bin/sh

      The script is called `executable' because just like a real (binary)
      executable it starts with a so-called `magic number' indicating
      the type of the executable.  In our case this number is `#!' and
      the OS takes the rest of the first line as the interpreter for
      the script, possibly followed by 1 initial option like:

    #!/bin/sed -f

      Suppose this script is called `foo' and is found in /bin,
      then if you type:

    foo arg1 arg2 arg3

      the OS will rearrange things as though you had typed:

    /bin/sed -f /bin/foo arg1 arg2 arg3

      There is one difference though: if the setuid permission bit for
      `foo' is set, it will be honored in the first form of the
      command; if you really type the second form, the OS will honor
      the permission bits of /bin/sed, which is not setuid, of course.

      ----------

      OK, but what if my shell script does NOT start with such a `#!'
      line or my OS does not know about it?

      Well, if the shell (or anybody else) tries to execute it, the OS
      will return an error indication, as the file does not start with
      a valid magic number.  Upon receiving this indication the shell
      ASSUMES the file to be a shell script and gives it another try:

    /bin/sh shell_script arguments

      But we have already seen that a setuid bit on `shell_script' will
      NOT be honored in this case!

      ----------

      Right, but what about the security risks of setuid shell scripts?

      Well, suppose the script is called `/etc/setuid_script', starting
      with:

    #!/bin/sh

      Now let us see what happens if we issue the following commands:

    $ cd /tmp
    $ ln /etc/setuid_script -i
    $ PATH=.
    $ -i

      We know the last command will be rearranged to:

    /bin/sh -i

      But this command will give us an interactive shell, setuid to the
      owner of the script!
      Fortunately this security hole can easily be closed by making the
      first line:

    #!/bin/sh -

      The `-' signals the end of the option list: the next argument `-i'
      will be taken as the name of the file to read commands from, just
      like it should!

      ---------

      There are more serious problems though:

    $ cd /tmp
    $ ln /etc/setuid_script temp
    $ nice -20 temp &
    $ mv my_script temp

      The third command will be rearranged to:

    nice -20 /bin/sh - temp

      As this command runs so slowly, the fourth command might be able
      to replace the original `temp' with `my_script' BEFORE `temp' is
      opened by the shell!  There are 4 ways to fix this security hole:

    1)  let the OS start setuid scripts in a different, secure way
        - System V R4 and 4.4BSD use the /dev/fd driver to pass the
        interpreter a file descriptor for the script

    2)  let the script be interpreted indirectly, through a frontend
        that makes sure everything is all right before starting the
        real interpreter - if you use the `indir' program from
        comp.sources.unix the setuid script will look like this:

        #!/bin/indir -u
        #?/bin/sh /etc/setuid_script

    3)  make a `binary wrapper': a real executable that is setuid and
        whose only task is to execute the interpreter with the name of
        the script as an argument

    4)  make a general `setuid script server' that tries to locate the
        requested `service' in a database of valid scripts and upon
        success will start the right interpreter with the right
        arguments.

      ---------

      Now that we have made sure the right file gets interpreted, are
      there any risks left?

      Certainly!  For shell scripts you must not forget to set the PATH
      variable to a safe path explicitly.  Can you figure out why?
      Also there is the IFS variable that might cause trouble if not
      set properly.  Other environment variables might turn out to
      compromise security as well, e.g. SHELL...  Furthermore you must
      make sure the commands in the script do not allow interactive
      shell escapes!  Then there is the umask which may have been set
      to something strange...

      Etcetera.  You should realise that a setuid script `inherits' all
      the bugs and security risks of the commands that it calls!

      All in all we get the impression setuid shell scripts are quite a
      risky business!  You may be better off writing a C program instead!

4.8)  How can I find out which user or process has a file open or is using
      a particular file system (so that I can unmount it?)

      Use fuser (system V), fstat (BSD), ofiles (public domain) or
      pff (public domain).  These programs will tell you various things
      about processes using particular files.

      A port of the 4.3 BSD fstat to Dynix, SunOS and Ultrix
      can be found in archives of comp.sources.unix, volume 18.

      pff is part of the kstuff package, and works on quite a few systems.
      Instructions for obtaining kstuff are provided in question 3.10.

4.9)  How do I keep track of people who are fingering me?

      From: [e-mail] (Jonathan I. Kamens)
      From: [e-mail] (Nikola Malenovic)
      Date: Mon, 23 Nov 1992 16:01:45 -0600

      Generally, you can't find out the userid of someone who is
      fingering you from a remote machine.  You may be able to
      find out which machine the remote request is coming from.
      One possibility, if your system supports it and assuming
      the finger daemon doesn't object, is to make your .plan file a
      "named pipe" instead of a plain file.  (Use 'mknod' to do this.)

      You can then start up a program that will open your .plan file
      for writing; the open will block until some other process (namely
      fingerd) opens the .plan for reading.  Now you can whatever you
      want through this pipe, which lets you show different .plan
      information every time someone fingers you.

      Of course, this may not work at all if your system doesn't
      support named pipes or if your local fingerd insists
      on having plain .plan files.

      Your program can also take the opportunity to look at the output
      of "netstat" and spot where an incoming finger connection is
      coming from, but this won't get you the remote user.

      Getting the remote userid would require that the remote site be
      running an identity service such as RFC 931.  There are now three
      RFC 931 implementations for popular BSD machines, and several
      applications (such as the wuarchive ftpd) supporting the server.
      For more information join the rfc931-users mailing list,
      [e-mail].

      There are three caveats relating to this answer.  The first is
      that many NFS systems won't recognize the named pipe correctly.
      This means that trying to read the pipe on another machine will
      either block until it times out, or see it as a zero-length file,
      and never print it.

      The second problem is that on many systems, fingerd checks that
      the .plan file contains data (and is readable) before trying to
      read it.  This will cause remote fingers to miss your .plan file
      entirely.

      The third problem is that a system that supports named pipes
      usually has a fixed number of named pipes available on the
      system at any given time - check the kernel config file and
      FIFOCNT option.  If the number of pipes on the system exceeds the
      FIFOCNT value, the system blocks new pipes until somebody frees
      the resources.  The reason for this is that buffers are allocated
      in a non-paged memory.

4.10) Is it possible to reconnect a process to a terminal after it has
      been disconnected, e.g. after starting a program in the background
      and logging out?

      Most variants of Unix do not support "detaching" and "attaching"
      processes, as operating systems such as VMS and Multics support.
      However, there are two freely redistributable packages which can
      be used to start processes in such a way that they can be later
      reattached to a terminal.

      The first is "screen," which is described in the
      comp.sources.unix archives as "Screen, multiple windows on a CRT"
      (see the "screen-3.2" package in comp.sources.misc, volume 28.)
      This package will run on at least BSD, System V r3.2 and SCO UNIX.

      The second is "pty," which is described in the comp.sources.unix
      archives as a package to "Run a program under a pty session" (see
      "pty" in volume 23).  pty is designed for use under BSD-like
      system only.

      Neither of these packages is retroactive, i.e. you must have
      started a process under screen or pty in order to be able to
      detach and reattach it.

4.11) Is it possible to "spy" on a terminal, displaying the output
      that's appearing on it on another terminal?

      There are a few different ways you can do this, although none
      of them is perfect:

      * kibitz allows two (or more) people to interact with a shell
        (or any arbitary program).  Uses include:

    - watching or aiding another person's terminal session;
    - recording a conversation while retaining the ability to
      scroll backwards, save the conversation, or even edit it
      while in progress;
    - teaming up on games, document editing, or other cooperative
      tasks where each person has strengths and weakness that
      complement one another.

        kibitz comes as part of the expect distribution.  See question 3.9.

        kibitz requires permission from the person to be spyed upon.  To
        spy without permission requires less pleasant approaches:

      * You can write a program that grovels through Kernel structures
        and watches the output buffer for the terminal in question,
        displaying characters as they are output.  This, obviously, is
        not something that should be attempted by anyone who does not
        have experience working with the Unix kernel.  Furthermore,
        whatever method you come up with will probably be quite
        non-portable.

      * If you want to do this to a particular hard-wired terminal all
        the time (e.g. if you want operators to be able to check the
        console terminal of a machine from other machines), you can
        actually splice a monitor into the cable for the terminal.  For
        example, plug the monitor output into another machine's serial
        port, and run a program on that port that stores its input
        somewhere and then transmits it out *another* port, this one
        really going to the physical terminal.  If you do this, you have
        to make sure that any output from the terminal is transmitted
        back over the wire, although if you splice only into the
        computer->terminal wires, this isn't much of a problem.  This is
        not something that should be attempted by anyone who is not very
        familiar with terminal wiring and such.

--
Ted Timar - [e-mail]
Empress Software, 3100 Steeles Ave E, Markham, Ont., Canada L3R 8T3