A Beginner's Guide to the Internet/INTERNET.ISO/text/faqs/unix.faq

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From: [e-mail]
Newsgroups: comp.unix.questions,news.answers
Subject: Welcome to comp.unix.questions [Monthly posting]
Date: 4 May 92 05:00:27 GMT
Followup-To: comp.unix.questions

Archive-name: unix-faq/unix-intro
Version: $Id: unix-intro,v 1.5 1992/04/26 20:17:15 tmatimar Exp $

Comp.unix.questions is one of the most popular and highest volume
newsgroups on Usenet.  This article is a monthly attempt to remind
potential posters about what is appropriate for this newsgroup.
If you would like to make any suggestions about the content of
this article, please contact its maintainer at
[e-mail].

Companion articles include the answers to some Frequently
Asked Questions.  You may save yourself a lot of time by reading
those articles before posting a question to the net.

If you have not already read the overall Usenet introductory material
posted to "news.announce.newusers", please do.  Much of this article
overlaps with the common sense guidelines posted there.

         Should I Post My Unix Question to the Net?

Often the answer is "No, you can get an answer a lot faster without
posting a question." Before you post, you should try -

    o Reading the manual for your system.  Some day you may encounter
      the phrase "RTFM", which stands for "Read the Fine Manual"
      (except 'F' doesn't really stand for "Fine").  If you ask
      someone a question and they tell you to RTFM, it's an
      indication that you haven't done your homework.   For instance,
      if you are having trouble removing a file whose name begins
      with a "-", check the man page for "rm".  It might tell
      you what you need to know.

      When people use terminology like "read(2)", they are referring
      to the "read" man page in section 2 of the manual (which you
      would see by using "man 2 read").

    o Finding a knowledgeable user at your site.  Many sites have
      at least a few Unix experts who will be happy to help you
      figure out how to remove a file whose name begins with "-".
      Many larger sites, particularly universities, may even have
      paid consultants whose job is to help you with Unix problems.
      Check with them first.

    o Find a good introductory book on Unix.  There are plenty of
      such books available, and you will save yourself a lot
      of trouble by having one handy and consulting it frequently.
      (Question 1.5 in the companion articles will let you know
       where you can find a list of good Unix and C books.)

Please remember that the comp.unix.* newsgroups are read by over 80,000
people around the world, and that posting a question to this group will
cost a lot of time and money by the time your article is distributed to
Asia, Australia, Europe (west and east), Africa, the middle east,
and all corners of North, South and Central America.

Also, some people receive these newsgroups as part of a mailing list
rather than a newsgroup.  If you're one of these people, please don't
send a "Remove me from this list" or "UNSUBSCRIBE"  message to the
wrong place.  Take the time to figure out where you're getting this
stuff from, and send your request to the mailing list maintainer, *not*
to the list or newsgroup itself!  Ask your local postmaster for help.
(One of the answers in the companion articles deals with the details of
the mailing list.)

               To Which Newsgroup Should I Post My Question?

The choice of newsgroup is harder than it used to be.  In the old days,
you just had to choose between "comp.unix.questions" and
"comp.unix.wizards".  Now there are a variety of more specific groups.
Choose one of the following groups carefully.  If you aren't sure where
your question belongs or if your question is not specific to some
particular version of Unix, try "comp.unix.questions".  Many
knowledgeable Unix wizards read that group and will be able to help you.

Here are the capsule descriptions of various groups you might consider
(extracted from a monthly posting to "news.announce.newusers")

comp.unix.questions     General questions from UNIX users and sys admins.
            If your question isn't a really good match for one of
            the groups below, post it here.

news.answers        Repository for periodic USENET articles. (Moderated)
            This article is crossposted here.
            Do not try to post here unless you're posting a FAQ.

comp.unix.shell         Using and programming any UNIX shell.

comp.lang.c             Discussion about C.

comp.sources.unix       Postings of complete, UNIX-oriented sources. (Moderated)
comp.std.unix           Discussion for the P1003 committee on UNIX. (Moderated)
comp.unix               Discussion of UNIX* features and bugs. (Moderated)
comp.unix.admin         Administering a Unix-based system.
comp.unix.aix           IBM's version of UNIX.
comp.unix.amiga        Unix on the Commodore Amiga
comp.unix.aux           The version of UNIX for Apple Macintosh II computers.
comp.unix.bsd           Discussions relating to BSD UNIX.
comp.unix.internals     Discussions on hacking UNIX internals.
comp.unix.large         UNIX on mainframes and in large networks.
comp.unix.misc          Various topics that don't fit other groups.
comp.unix.msdos         MS-DOS running under UNIX by whatever means.
comp.unix.programmer    Q&A for people programming under Unix.
comp.unix.sysv286       UNIX System V (not XENIX) on the '286.
comp.unix.sysv386       Versions of Unix (not Xenix) on Intel 80386-based boxes.
comp.unix.ultrix        Discussions about DEC's Ultrix.
comp.unix.xenix.misc    General discussions regarding XENIX (except SCO).
comp.unix.xenix.sco     XENIX versions from the Santa Cruz Operation.

comp.unix.wizards       In-depth discussions of advanced unix topics.
            People should not post to this group unless they
            have used unix as a user, sysadmin and know details
            of the kernel, and how different unix kernels differ.
            In other words, don't post to comp.unix.wizards.

          What Information Should I Include?

It's hard to include too much information.  There are hundreds of
different Unix systems out there, and they all have less in common
than you might think.  If you have a problem and are posting an
article, please be sure to mention:

    o A descriptive subject line.  Many people will decide whether
      to read your article solely on the basis of the subject line,
      so it should be a good statement of your problem.

      NOT GOOD                          GOOD

      "Help"                            "How do I sort a file by line length?"
      "Csh question"            "csh dumps core when I use '$<'"

    o What computer you are using, and what specific version
      of the operating system it uses.  For instance,

        SunOS 4.0.1, Sun 3/50
        4.3BSD-tahoe, Vax 11/780
        SVR3.2, 3b2

    o If possible, the *exact* text of any error message you
      may have encountered.

      WRONG                RIGHT

      "I can't print this file"     "When I type 'lpr Filename', I get
                      lpr: Filename: File too ugly to print
                     What does this mean?  It isn't in
                     the man page.  This is using
                     Mueslix 9.3 on a Fax [phone]"

It's a good idea to post unrelated questions in separate articles,
so that people can keep different discussions separate.   It's also
a *very* good idea to include a line or two like this:

    "Please mail your answers to me and I'll summarize what I get
     and post the results to comp.unix.questions."

This prevents many identical responses from different users to the
same question from clogging up the newsgroup.  And make sure
you really summarize what you get - don't just concatenate
all the mail you've received.

It's also a good idea to read comp.unix.questions for at least a couple
of weeks after you post your article to see what followup articles
are posted.

                Should I Post an Answer to a Question?

It's very tempting to post an answer to a question you read on the net,
especially when you think "Aha, finally - a question I can answer!"
Consider though that when a simple question is asked, such as the
sort about to be answered below, many other people around the
world already know the answer and may be posting their own reply.
In order to avoid dozens of replies to simple questions, please
wait a day or so and see if anyone else has already answered
the question.  If you have something special to contribute, please
do so, but make sure you're not duplicating something someone else has
already done.

You should feel free to reply to any question >by email<.  Even if
the user gets 200 responses to his question, at least the load on the
rest of the net is minimized.

                    What About Posting Source Code?

Posting small amounts of example code is fine (use comp.sources.unix to
distribute complete programs) - but please make sure that your code
runs (or at least compiles) properly.  Don't just type it in while
editing your posting and hope it will work, no matter how sure you are
that it will.  We all make mistakes.

                        What About Those People
       Who Continue to Ask Stupid or Frequently Asked Questions
         In Spite of The Frequently Asked Questions Document?

Just send them a polite mail message, possibly referring them to this document.
There is no need to flame them on the net - it's busy enough as it is.
--
Ted Timar - [e-mail]
Omron Corporation, Shimokaiinji, Nagaokakyo-city, Kyoto 617,  Japan


..
Hope you know what you're doing ...
Data directory is /big/gd
Port is 70
Logging to File /usr/adm/gopherd.l

>From news.cso.uiuc.edu!ux1.cso.uiuc.edu!uwm.edu!wupost!cs.utexas.edu!sun-barr!olivea!uunet!snorkelwacker.mit.edu!bloom-picayune.mit.edu!news Mon Jul  6 13:59:30 CDT 1992
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>From: [e-mail]
Newsgroups: comp.unix.questions,comp.unix.shell,news.answers
Subject: Frequently Asked Questions about Unix (1/4) [Monthly posting]
Message-ID: <unix-faq/[e-mail]>
Date: 3 Jul 92 05:01:17 GMT
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Archive-name: unix-faq/part1
Version: $Id: part1,v 1.5 1992/04/26 20:14:45 tmatimar Exp $

These four 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.

This article includes answers to:

      1.1)  Who helped you put this list together?
      1.2)  When someone refers to 'rn(1)' or 'ctime(3)', what does
              the number in parentheses mean?
      1.3)  What does {some strange unix command name} stand for?
      1.4)  How does the gateway between "comp.unix.questions" and the
              "info-unix" mailing list work?
      1.5)  What are some useful Unix or C books?
      1.6)  What happened to the pronunciation list that used to be
              part of this document?

    If you're looking for the answer to, say, question 1.5, and want to skip
    everything else, you can search ahead for the regular expression "^5)".

While these are all legitimate questions, they seem to crop up in
comp.unix.questions 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].

1)  Who helped you put this list together?

    I have just taken over the maintenance of this list.  Almost all of
    the work (and the credit) for generating this compilation was done
    by Steve Hayman.

    We also owe a great deal of thanks to dozens of Usenet readers who
    submitted questions, answers, corrections and suggestions for this
    list.  Special thanks go to Maarten Litmaath, Guy Harris and
    Jonathan Kamens, who have all made many especially valuable
    contributions.

2)  When someone refers to 'rn(1)' or 'ctime(3)', what does
    the number in parentheses mean?

    It looks like some sort of function call, but it isn't.
    These numbers refer to the section of the "Unix manual" where
    the appropriate documentation can be found.  You could type
    "man 3 ctime" to look up the manual page for "ctime" in section 3
    of the manual.

    The traditional manual sections are:

    1    User-level  commands
    2    System calls
    3    Library functions
    4    Devices and device drivers
    5    File formats
    6    Games
    7    Various miscellaneous stuff - macro packages etc.
    8    System maintenance and operation commands

    Some Unix versions use non-numeric section names.  For instance,
    Xenix uses "C" for commands and "S" for functions.

    Each section has an introduction, which you can read with "man # intro"
    where # is the section number.

    Sometimes the number is necessary to differentiate between a
    command and a library routine or system call of the same name.  For
    instance, your system may have "time(1)", a manual page about the
    'time' command for timing programs, and also "time(3)", a manual
    page about the 'time' subroutine for determining the current time.
    You can use "man 1 time" or "man 3 time" to specify which "time"
    man page you're interested in.

    You'll often find other sections for local programs or
    even subsections of the sections above - Ultrix has
    sections 3m, 3n, 3x and 3yp among others.

3)  What does {some strange unix command name} stand for?

    awk = "Aho Weinberger and Kernighan"

    This language was named by its authors, Al Aho, Peter Weinberger and
    Brian Kernighan.

    grep = "Global Regular Expression Print"

    grep comes from the ed command to print all lines matching a
    certain pattern

            g/re/p

    where "re" is a "regular expression".

    fgrep = "Fixed GREP".

    fgrep searches for fixed strings only.  The "f" does not
    stand for "fast" - in fact, "fgrep foobar *.c" is usually slower
    than "egrep foobar *.c"  (Yes, this is kind of surprising. Try it.)

    Fgrep still has its uses though, and may be useful when searching
    a file for a larger number of strings than egrep can handle.

    egrep = "Extended GREP"

    egrep uses fancier regular expressions than grep.
    Many people use egrep all the time, since it has some more
    sophisticated internal algorithms than grep or fgrep,
    and is usually the fastest of the three programs.

    cat = "CATenate"

    catenate is an obscure word meaning "to connect in a series",
    which is what the "cat" command does to one or more files.
    Not to be confused with C/A/T, the Computer Aided Typesetter.

    gecos = "General Electric Comprehensive Operating System"

    When GE's large systems division was sold to Honeywell,
    Honeywell dropped the "E" from "GECOS".

    Unix's password file has a "pw_gecos" field.  The name is
    a real holdover from the early days.  Dennis Ritchie
    has reported:

        "Sometimes we sent printer output or batch jobs
         to the GCOS machine.  The gcos field in the
         password file was a place to stash the information
         for the $IDENT card.  Not elegant."

    nroff = "New ROFF"
    troff = "Typesetter new ROFF"

    These are descendants of "roff", which was a re-implementation
    of the Multics "runoff" program (a program that you'd use to
    "run off" a good copy of a document).

    tee    = T

    From plumbing terminology for a T-shaped pipe splitter.

    bss = "Block Started by Symbol"

    Dennis Ritchie says:

        Actually the acronym (in the sense we took it up; it may
        have other credible etymologies) is "Block Started by Symbol."
        It was a pseudo-op in FAP (Fortran Assembly [-er?] Program), an
        assembler for the IBM 704-709-7090-7094 machines.  It defined
        its label and set aside space for a given number of words.
        There was another pseudo-op, BES, "Block Ended by Symbol"
        that did the same except that the label was defined by
        the last assigned word + 1.  (On these machines Fortran
        arrays were stored backwards in storage and were 1-origin.)

        The usage is reasonably appropriate, because just as with
        standard Unix loaders, the space assigned didn't have to
        be punched literally into the object deck but was represented
        by a count somewhere.

    biff = "BIFF"

        This command, which turns on asynchronous mail notification,
    was actually named after a dog at Berkeley.

        I can confirm the origin of biff, if you're interested.  Biff
        was Heidi Stettner's dog, back when Heidi (and I, and Bill Joy)
        were all grad students at U.C. Berkeley and the early versions
        of BSD were being developed.   Biff was popular among the
        residents of Evans Hall, and was known for barking at the
        mailman, hence the name of the command.

    Confirmation courtesy of Eric Cooper, Carnegie Mellon
    University

    rc (as in ".cshrc" or "/etc/rc") = "RunCom"

    "rc" derives from "runcom", from the MIT CTSS system, ca. 1965.

        'There was a facility that would execute a bunch of commands
        stored in a file; it was called "runcom" for "run commands",
        and the file began to be called "a runcom."

        "rc" in Unix is a fossil from that usage.'

    Brian Kernighan & Dennis Ritchie, as told to Vicki Brown

    "rc" is also the name of the shell from the new Plan 9
    operating system.

    Perl = "Practical Extraction and Report Language"

    The Perl language is Larry Wall's highly popular
    freely-available completely portable text, process, and file
    manipulation tool that bridges the gap between shell and C
    programming (or between doing it on the command line and
    pulling your hair out).  For further information, see the
    Usenet newsgroup comp.lang.perl.

    Don Libes' book "Life with Unix" contains lots more of these
    tidbits.

4)  How does the gateway between "comp.unix.questions" and the
    "info-unix" mailing list work?

    "Info-Unix" and "Unix-Wizards" are mailing list versions of
    comp.unix.questions and comp.unix.wizards respectively.
    There should be no difference in content between the
    mailing list and the newsgroup.

    To get on or off either of these lists, send mail to
    [e-mail] or [e-mail] .
    Be sure to use the '-Request'.  Don't expect an immediate response.

    Here are the gory details, courtesy of the list's maintainer, Bob Reschly.

    ==== postings to info-UNIX and UNIX-wizards lists ====

       Anything submitted to the list is posted; I do not moderate incoming
    traffic -- BRL functions as a reflector.  Postings submitted by Internet
    subscribers should be addressed to the list address (info-UNIX or UNIX-
    wizards);  the '-request' addresses are for correspondence with the list
    maintainer [me].  Postings submitted by USENET readers should be
    addressed to the appropriate news group (comp.unix.questions or
    comp.unix.wizards).

       For Internet subscribers, received traffic will be of two types;
    individual messages, and digests.  Traffic which comes to BRL from the
    Internet and BITNET (via the BITNET-Internet gateway) is immediately
    resent to all addressees on the mailing list.  Traffic originating on
    USENET is gathered up into digests which are sent to all list members
    daily.

       BITNET traffic is much like Internet traffic.  The main difference is
    that I maintain only one address for traffic destined to all BITNET
    subscribers. That address points to a list exploder which then sends
    copies to individual BITNET subscribers.  This way only one copy of a
    given message has to cross the BITNET-Internet gateway in either
    direction.

       USENET subscribers see only individual messages.  All messages
    originating on the Internet side are forwarded to our USENET machine.
    They are then posted to the appropriate newsgroup.  Unfortunately,
    for gatewayed messages, the sender becomes "news@brl-adm".  This is
    currently an unavoidable side-effect of the software which performs the
    gateway function.

       As for readership, USENET has an extremely large readership - I would
    guess several thousand hosts and tens of thousands of readers.  The
    master list maintained here at BRL runs about two hundred fifty entries
    with roughly ten percent of those being local redistribution lists.
    I don't have a good feel for the size of the BITNET redistribution, but
    I would guess it is roughly the same size and composition as the master
    list.  Traffic runs 150K to 400K bytes per list per week on average.

5)  What are some useful Unix or C books?

    Mitch Wright ([e-mail]) maintains a useful list of Unix and
    C books, with descriptions and some mini-reviews.  There are currently
    77 titles on his list.

    You can obtain a copy of this list by anonymous ftp from
    ftp.wg.omron.co.jp (133.210.4.4), where it's
    "pub/unix-faq/Unix-C-Booklist".
    If you can't use anonymous ftp, email the line "help" to
    "[e-mail]" for instructions on
    retrieving things via email.

    Send additions or suggestions to [e-mail].

6)  What happened to the pronunciation list that used to be part of this
    document?

    From its inception in 1989, this FAQ document included a comprehensive
    pronunciation list maintained by Maarten Litmaath (thanks, Maarten!).
    (Does anyone know who *created* it?)

    It has been retired, since it is not really relevant to the topic of
    "Unix questions".  You can still find it as part of the
    widely-distributed "Jargon" file (maintained by Eric S. Raymond,
    [e-mail]) which seems like a much more appropriate
    forum for the topic of "How do you pronounce  /* ?"

    If you'd like a copy, you can ftp one from ftp.wg.omron.co.jp
    (133.210.4.4), it's "pub/unix-faq/Pronunciation-Guide".
--
Ted Timar - [e-mail]
Omron Corporation, Shimokaiinji, Nagaokakyo-city, Kyoto 617,  Japan


..
Hope you know what you're doing ...
Data directory is /big/gd
Port is 70
Logging to File /usr/adm/gopherd.l

>From news.cso.uiuc.edu!ux1.cso.uiuc.edu!uwm.edu!cs.utexas.edu!sun-barr!olivea!uunet!snorkelwacker.mit.edu!bloom-picayune.mit.edu!news Mon Jul  6 13:59:33 CDT 1992
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>From: [e-mail]
Newsgroups: comp.unix.questions,comp.unix.shell,news.answers
Subject: Frequently Asked Questions about Unix (2/4) [Monthly posting]
Message-ID: <unix-faq/[e-mail]>
Date: 3 Jul 92 05:01:38 GMT
References: <unix-faq/[e-mail]>
Sender: [e-mail] (News system)
Followup-To: comp.unix.questions
Organization: Omron Corporation
Lines: 828
Approved: [e-mail]
Supersedes: <unix-faq/[e-mail]>
X-Last-Updated: 1992/05/04
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Status: RO

Archive-name: unix-faq/part2
Version: $Id: part2,v 1.5 1992/04/26 20:14:45 tmatimar Exp $

These four 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.

This article includes answers to:

      2.1)  How do I remove a file whose name begins with a "-" ?
      2.2)  How do I remove a file with funny characters in the filename ?
      2.3)  How do I get a recursive directory listing?
      2.4)  How do I get the current directory into my prompt?
      2.5)  How do I read characters from the terminal in a shell script?
      2.6)  How do I rename "*.foo" to "*.bar", or change file names
              to lowercase?
      2.7)  Why do I get [some strange error message] when I
              "rsh host command" ?
      2.8)  How do I {set an environment variable, change directory} inside a
              program or shell script and have that change affect my
              current shell?
      2.9)  How do I redirect stdout and stderr separately in csh?
      2.10) How do I tell inside .cshrc if I'm a login shell?
      2.11) How do I construct a shell glob-pattern that matches all files
            except "." and ".." ?
      2.12) How do I find the last argument in a Bourne shell script?
      2.13) What's wrong with having '.' in your $PATH ?

    If you're looking for the answer to, say, question 2.5, and want to skip
    everything else, you can search ahead for the regular expression "^5)".

While these are all legitimate questions, they seem to crop up in
comp.unix.questions 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].

1)  How do I remove a file whose name begins with a "-" ?

    Figure out some way to name the file so that it doesn't
    begin with a dash.  The simplest answer is to use

        rm ./-filename

    (assuming "-filename" is in the current directory, of course.)
    This method of avoiding the interpretation of the "-" works
    with other commands too.

    Many commands, particularly those that have been written to use
    the "getopt(3)" argument parsing routine, accept a "--" argument
    which means "this is the last option, anything after this is not
    an option", so your version of rm might handle "rm -- -filename".
    Some versions of rm that don't use getopt() treat a single "-"
    in the same way, so you can also try "rm - -filename".

2)  How do I remove a file with funny characters in the filename ?

    If the 'funny character' is a '/', skip to the last part of
    this answer.  If the funny character is something else,
    such as a ' ' or control character or character with
    the 8th bit set, keep reading.

    The classic answers are

    rm -i some*pattern*that*matches*only*the*file*you*want

    which asks you whether you want to remove each file matching
    the indicated pattern;  depending on your shell, this may
    not work if the filename has a character with the 8th bit set
    (the shell may strip that off);

    and

    rm -ri .

    which asks you whether to remove each file in the directory.
    Answer "y" to the problem file and "n" to everything else.
    Unfortunately this doesn't work with many versions of rm.
    Also unfortunately, this will walk through every subdirectory
    of ".", so you might want to "chmod a-x" those directories
    temporarily to make them unsearchable.

    Always take a deep breath and think about what you're doing
    and double check what you typed when you use rm's "-r" flag
    or a wildcard on the command line;

    and

    find . -type f ... -ok rm '{}' \;

    where "..." is a group of predicates that uniquely identify the
    file.  One possibility is to figure out the inode number
    of the problem file (use "ls -i .") and then use

    find . -inum 12345 -ok rm '{}' \;

    or
    find . -inum 12345 -ok mv '{}' new-file-name \;

    "-ok" is a safety check - it will prompt you for confirmation of the
    command it's about to execute.  You can use "-exec" instead to avoid
    the prompting, if you want to live dangerously, or if you suspect
    that the filename may contain a funny character sequence that will mess
    up your screen when printed.

    What if the filename has a '/' in it?

    These files really are special cases, and can only be created
    by buggy kernel code (typically by implementations of NFS
    that don't filter out illegal characters in file names from
    remote machines.)  The first thing to do is to try to
    understand exactly why this problem is so strange.

    Recall that Unix directories are simply pairs of
    filenames and inode numbers.  A directory essentially
    contains information like this:

    filename  inode

    file1      12345
    file2.c      12349
    file3     12347

    Theoretically, '/' and '\0' are the only two characters that
    cannot appear in a filename - '/' because it's used to separate
    directories and files, and '\0' because it terminates a filename.

    Unfortunately some implementations of NFS will blithely create
    filenames with embedded slashes in response to requests from remote
    machines.  For instance, this could happen when someone on a Mac or
    other non-Unix machine decides to create a remote NFS file on
    your Unix machine with the date in the filename.  Your Unix
    directory then has this in it:

    filename  inode

    91/02/07  12357

    No amount of messing around with 'find' or 'rm' as described above
    will delete this file, since those utilities and all other Unix
    programs, are forced to interpret the '/' in the normal way.

    Any ordinary program will eventually try to do unlink("91/02/07"),
    which as far as the kernel is concerned means "unlink the file 07
    in the subdirectory 02 of directory 91", but that's not what we
    have - we have a *FILE* named "91/02/07" in the current directory.
    This is a subtle but crucial distinction.

    What can you do in this case?
    The first thing to try is to return to the Mac that created this
    crummy entry, and see if you can convince it and your local NFS
    daemon to rename the file to something without slashes.

    If that doesn't work or isn't possible, you'll need help from your
    system manager, who will have to try the one of the following.
    Use "ls -i" to find the inode number of this bogus file, then
    unmount the file system and use "clri" to clear the inode, and
    "fsck" the file system with your fingers crossed.  This destroys
    the information in the file.  If you want to keep it, you can try:

    create a new directory in the same parent directory as the one
    containing the bad file name;

    move everything you can (i.e. everything but the file with
    the bad name) from the old directory to the new one;

    do "ls -id" on the directory containing the file with the
    bad name to get its inumber;

    umount the file system;

    "clri" the directory containing the file with the bad name;

    "fsck" the file system.

    Then, to find the file,

    remount the file system;

    rename the directory you created to have the name of
    the old directory (since the old directory should have
    been blown away by "fsck")

    move the file out of "lost+found" into the directory
    with a better name.

    Alternatively, you can patch the directory the hard way
    by crawling around in the raw file system.
    Use "fsdb", if you have it.

3)  How do I get a recursive directory listing?

    One of the following may do what you want:

    ls -R             (not all versions of "ls" have -R)
    find . -print        (should work everywhere)
    du -a .            (shows you both the name and size)

    If you're looking for a wildcard pattern that will match
    all ".c" files in this directory and below, you won't find one,
    but you can use

    % some-command `find . -name '*.c' -print`

    "find" is a powerful program.  Learn about it.

4)  How do I get the current directory into my prompt?

    It depends which shell you are using.  It's easy with some shells,
    hard or impossible with others.

    C Shell (csh):
    Put this in your .cshrc - customize the prompt variable
    the way you want.

        alias setprompt 'set prompt="${cwd}% "'
        setprompt        # to set the initial prompt
        alias cd 'chdir \!* && setprompt'

    If you use pushd and popd, you'll also need

        alias pushd 'pushd \!* && setprompt'
        alias popd  'popd  \!* && setprompt'

    Some C shells don't keep a $cwd variable - you can use
    `pwd` instead.

    If you just want the last component of the current directory
    in your prompt ("mail% " instead of "/usr/spool/mail% ")
    you can use

        alias setprompt 'set prompt="$cwd:t% "'

    Some older csh's get the meaning of && and || reversed.
    Try doing:

        false && echo bug

    If it prints "bug", you need to switch && and || (and get
    a better version of csh.)

    Bourne Shell (sh):

    If you have a newer version of the Bourne Shell (SVR2 or newer)
    you can use a shell function to make your own command, "xcd" say:

        xcd() { cd $* ; PS1="`pwd` $ "; }

    If you have an older Bourne shell, it's complicated but not impossible.
    Here's one way.  Add this to your .profile file:

        LOGIN_SHELL=$$ export LOGIN_SHELL
        CMDFILE=/tmp/cd.$$ export CMDFILE
        # 16 is SIGURG, pick some signal that isn't likely to be used
        PROMPTSIG=16 export PROMPTSIG
        trap '. $CMDFILE' $PROMPTSIG

    and then put this executable script (without the indentation!),
    let's call it "xcd", somewhere in your PATH

        : xcd directory - change directory and set prompt
        : by signalling the login shell to read a command file
        cat >${CMDFILE?"not set"} <<EOF
        cd $1
        PS1="\`pwd\`$ "
        EOF
        kill -${PROMPTSIG?"not set"} ${LOGIN_SHELL?"not set"}

    Now change directories with "xcd /some/dir".

    Korn Shell (ksh):

    Put this in your .profile file:
        PS1='$PWD $ '

    If you just want the last component of the directory, use
        PS1='${PWD##*/} $ '

    T C shell (tcsh)

    Tcsh is a popular enhanced version of csh with some extra
    builtin variables (and many other features):

        %~        the current directory, using ~ for $HOME
        %d or %/    the full pathname of the current directory
        %c or %.    the trailing component of the current directory

    so you can do

        set prompt='%~ '

    BASH (FSF's "Bourne Again SHell")

    \w in $PS1 gives the full pathname of the current directory,
    with ~ expansion for $HOME;  \W gives the basename of
    the current directory.  So, in addition to the above sh and
    ksh solutions, you could use

        PS1='\w $ '
    or
        PS1='\W $ '

5)  How do I read characters from the terminal in a shell script?

    In sh, use read.  It is most common to use a loop like

        while read line
        do
            ...
        done

    In csh, use $< like this:

        while ( 1 )
        set line = "$<"
        if ( "$line" == "" ) break
        ...
        end

    Unfortunately csh has no way of distinguishing between
    a blank line and an end-of-file.

    If you're using sh and want to read a *single* character from
    the terminal, you can try something like

        echo -n "Enter a character: "
        stty cbreak        # or  stty raw
        readchar=`dd if=/dev/tty bs=1 count=1 2>/dev/null`
        stty -cbreak

        echo "Thank you for typing a $readchar ."

6) How do I rename "*.foo" to "*.bar", or change file names to lowercase?

    Why doesn't "mv *.foo *.bar" work?  Think about how the shell
    expands wildcards.   "*.foo" and "*.bar" are expanded before the mv
    command ever sees the arguments.  Depending on your shell, this
    can fail in a couple of ways.  CSH prints "No match." because
    it can't match "*.bar".  SH executes "mv a.foo b.foo c.foo *.bar",
    which will only succeed if you happen to have a single
    directory named "*.bar", which is very unlikely and almost
    certainly not what you had in mind.

    Depending on your shell, you can do it with a loop to "mv" each
    file individually.  If your system has "basename", you can use:

    C Shell:
    foreach f ( *.foo )
        set base=`basename $f .foo`
        mv $f $base.bar
    end

    Bourne Shell:
    for f in *.foo; do
        base=`basename $f .foo`
        mv $f $base.bar
    done

    Some shells have their own variable substitution features, so instead
    of using "basename", you can use simpler loops like:

    C Shell:

    foreach f ( *.foo )
        mv $f $f:r.bar
    end

    Korn Shell:

    for f in *.foo; do
        mv $f ${f%foo}bar
    done

    If you don't have "basename" or want to do something like
    renaming foo.* to bar.*, you can use something like "sed" to
    strip apart the original file name in other ways, but
    the general looping idea is the same.  You can also convert
    file names into "mv" commands with 'sed', and hand the commands
    off to "sh" for execution.  Try

    ls -d *.foo | sed -e 's/.*/mv & &/' -e 's/foo$/bar/' | sh

    A program by Vladimir Lanin called "mmv" that does this job nicely
    was posted to comp.sources.unix (Volume 21, issues 87 and 88) in
    April 1990.  It lets you use

    mmv '*.foo' '=1.bar'

    Shell loops like the above can also be used to translate
    file names from upper to lower case or vice versa.  You could use
    something like this to rename uppercase files to lowercase:

    C Shell:
        foreach f ( * )
        mv $f `echo $f | tr '[A-Z]' '[a-z]'`
        end
    Bourne Shell:
        for f in *; do
        mv $f `echo $f | tr '[A-Z]' '[a-z]'`
        done
    Korn Shell:
        typeset -l l
        for f in *; do
        l="$f"
        mv $f $l
        done

    If you wanted to be really thorough and handle files with
    `funny' names (embedded blanks or whatever) you'd need to use

    Bourne Shell:

        for f in *; do
              g=`expr "xxx$f" : 'xxx\(.*\)' | tr '[A-Z]' '[a-z]'`
              mv "$f" "$g"
        done

    The `expr' command will always print the filename, even if it equals
    `-n' or if it contains a System V escape sequence like `\c'.

    Some versions of "tr" require the [ and ], some don't.  It happens
    to be harmless to include them in this particular example; versions of
    tr that don't want the [] will conveniently think they are supposed
    to translate '[' to '[' and ']' to ']'.

    If you have the "perl" language installed, you may find this rename
    script by Larry Wall very useful.  It can be used to accomplish a
    wide variety of filename changes.

    #!/usr/bin/perl
    #
    # rename script examples from lwall:
    #       rename 's/\.orig$//' *.orig
    #       rename 'y/A-Z/a-z/ unless /^Make/' *
    #       rename '$_ .= ".bad"' *.f
    #       rename 'print "$_: "; s/foo/bar/ if <stdin> =~ /^y/i' *

    $op = shift;
    for (@ARGV) {
        $was = $_;
        eval $op;
        die $@ if $@;
        rename($was,$_) unless $was eq $_;
    }

7)  Why do I get [some strange error message] when I "rsh host command" ?

    (We're talking about the remote shell program "rsh" or sometimes "remsh";
     on some machines, there is a restricted shell called "rsh", which
     is a different thing.)

    If your remote account uses the C shell, the remote host will
    fire up a C shell to execute 'command' for you, and that shell
    will read your remote .cshrc file.  Perhaps your .cshrc contains
    a "stty", "biff" or some other command that isn't appropriate
    for a non-interactive shell.  The unexpected output or error
    message from these commands can screw up your rsh in odd ways.

    Here's an example.  Suppose you have

    stty erase ^H
    biff y

    in your .cshrc file.  You'll get some odd messages like this.

    % rsh some-machine date
    stty: : Can't assign requested address
    Where are you?
    Tue Oct  1 09:24:45 EST 1991

    You might also get similar errors when running certain "at" or
    "cron" jobs that also read your .cshrc file.

    Fortunately, the fix is simple.  There are, quite possibly, a whole
    *bunch* of operations in your ".cshrc" (e.g., "set history=N") that are
    simply not worth doing except in interactive shells.  What you do is
    surround them in your ".cshrc" with:

        if ( $?prompt ) then
            operations....
        endif

    and, since in a non-interactive shell "prompt" won't be set, the
    operations in question will only be done in interactive shells.

    You may also wish to move some commands to your .login file; if
    those commands only need to be done when a login session starts up
    (checking for new mail, unread news and so on) it's better
    to have them in the .login file.

8)  How do I {set an environment variable, change directory} inside a
      program or shell script and have that change affect my
      current shell?

    In general, you can't, at least not without making special
    arrangements.  When a child process is created, it inherits a copy
    of its parent's variables (and current directory).  The child can
    change these values all it wants but the changes won't affect the
    parent shell, since the child is changing a copy of the
    original data.

    Some special arrangements are possible.  Your child process could
    write out the changed variables, if the parent was prepared to read
    the output and interpret it as commands to set its own variables.

    Also, shells can arrange to run other shell scripts in the context
    of the current shell, rather than in a child process, so that
    changes will affect the original shell.

    For instance, if you have a C shell script named "myscript":

    cd /very/long/path
    setenv PATH /something:/something-else

    or the equivalent Bourne or Korn shell script

    cd /very/long/path
    PATH=/something:/something-else export PATH

    and try to run "myscript" from your shell, your shell will fork and run
    the shell script in a subprocess.  The subprocess is also
    running the shell; when it sees the "cd" command it changes
    *its* current directory, and when it sees the "setenv" command
    it changes *its* environment, but neither has any effect on the current
    directory of the shell at which you're typing (your login shell,
    let's say).

    In order to get your login shell to execute the script (without forking)
    you have to use the "." command (for the Bourne or Korn shells)
    or the "source" command (for the C shell).  I.e. you type

    . myscript

    to the Bourne or Korn shells, or

    source myscript

    to the C shell.

    If all you are trying to do is change directory or set an
    environment variable, it will probably be simpler to use a
    C shell alias or Bourne/Korn shell function.  See the "how do
    I get the current directory into my prompt" section
    of this article for some examples.

9)  How do I redirect stdout and stderr separately in csh?

    In csh, you can redirect stdout with ">", or stdout and stderr
    together with ">&" but there is no direct way to redirect
    stderr only.  The best you can do is

        ( command >stdout_file ) >&stderr_file

    which runs "command" in a subshell;  stdout is redirected inside
    the subshell to stdout_file, and both stdout and stderr from the
    subshell are redirected to stderr_file, but by this point stdout
    has already been redirected so only stderr actually winds up in
    stderr_file.

    Sometimes it's easier to let sh do the work for you.

    sh -c 'command >stdout_file 2>stderr_file'

10) How do I tell inside .cshrc if I'm a login shell?

    When people ask this, they usually mean either

    How can I tell if it's an interactive shell?  or
    How can I tell if it's a top-level shell?

    You could perhaps determine if your shell truly is a login shell
    (i.e. is going to source ".login" after it is done with ".cshrc")
    by fooling around with "ps" and "$$".  Login shells generally have
    names that begin with a '-'.  If you're really interested in the
    other two questions, here's one way you can organize your .cshrc to
    find out.

    if (! $?CSHLEVEL) then
        #
        # This is a "top-level" shell,
        # perhaps a login shell, perhaps a shell started up by
        # 'rsh machine some-command'
        # This is where we should set PATH and anything else we
        # want to apply to every one of our shells.
        #
        setenv      CSHLEVEL        0
        set home = ~username        # just to be sure
        source ~/.env               # environment stuff we always want
    else
        #
        # This shell is a child of one of our other shells so
        # we don't need to set all the environment variables again.
        #
        set tmp = $CSHLEVEL
        @ tmp++
        setenv      CSHLEVEL        $tmp
    endif

    # Exit from .cshrc if not interactive, e.g. under rsh
    if (! $?prompt) exit

    # Here we could set the prompt or aliases that would be useful
    # for interactive shells only.

    source ~/.aliases

11) How do I construct a shell glob-pattern that matches all files
    except "." and ".." ?

    You'd think this would be easy.

    *        Matches all files that don't begin with a ".";

    .*         Matches all files that do begin with a ".", but
         this includes the special entries "." and "..",
         which often you don't want;

    .[!.]*   (Newer shells only; some shells use a "^" instead of
         the "!"; POSIX shells must accept the "!", but may
         accept a "^" as well; all portable applications shall
         not use an unquoted "^" immediately following the "[")

         Matches all files that begin with a "." and are
         followed by a non-"."; unfortunately this will miss
         "..foo";

    .??*     Matches files that begin with a "." and which are
         at least 3 characters long.  This neatly avoids
         "." and "..", but also misses ".a" .

    So to match all files except "." and ".." safely you have to use
    3 patterns (if you don't have filenames like ".a" you can leave out
    the first):

    .[!.]* .??* *

    Alternatively you could employ an external program or two and use
    backquote substitution.  This is pretty good:

    `ls -a | sed -e '/^\.$/d' -e '/^\.\.$/d'`

    (or `ls -A` in some Unix versions)

    but even it will mess up on files with newlines, IFS characters
    or wildcards in their names.

12) How do I find the last argument in a Bourne shell script?

    Answer by:
    Martin Weitzel <@mikros.systemware.de:[e-mail]>
    Maarten Litmaath <[e-mail]>

    If you are sure the number of arguments is at most 9, you can use:

    eval last=\${$#}

    In POSIX-compatible shells it works for ANY number of arguments.
    The following works always too:

    for last
    do
        :
    done

    This can be generalized as follows:

    for i
    do
        third_last=$second_last
        second_last=$last
        last=$i
    done

    Now suppose you want to REMOVE the last argument from the list,
    or REVERSE the argument list, or ACCESS the N-th argument directly,
    whatever N may be.  Here is a basis of how to do it, using only
    built-in shell constructs, without creating subprocesses:

    t0= u0= rest='1 2 3 4 5 6 7 8 9' argv=

    for h in '' $rest
    do
        for t in "$t0" $rest
        do
            for u in $u0 $rest
            do
                case $# in
                0)
                    break 3
                esac
                eval argv$h$t$u=\$1
                argv="$argv \"\$argv$h$t$u\""    # (1)
                shift
            done
            u0=0
        done
        t0=0
    done

    # now restore the arguments
    eval set x "$argv"                    # (2)
    shift

    This example works for the first 999 arguments.  Enough?
    Take a good look at the lines marked (1) and (2) and convince yourself
    that the original arguments are restored indeed, no matter what funny
    characters they contain!

    To find the N-th argument now you can use this:

    eval argN=\$argv$N

    To reverse the arguments the line marked (1) must be changed to:

    argv="\"\$argv$h$t$u\" $argv"

    How to remove the last argument is left as an exercise.

    If you allow subprocesses as well, possibly executing nonbuilt-in
    commands, the `argvN' variables can be set up more easily:

    N=1

    for i
    do
        eval argv$N=\$i
        N=`expr $N + 1`
    done

    To reverse the arguments there is still a simpler method, that even does
    not create subprocesses.  This approach can also be taken if you want
    to delete e.g. the last argument, but in that case you cannot refer
    directly to the N-th argument any more, because the `argvN' variables are
    set up in reverse order:

    argv=

    for i
    do
        eval argv$#=\$i
        argv="\"\$argv$#\" $argv"
        shift
    done

    eval set x "$argv"
    shift

13) What's wrong with having '.' in your $PATH ?

    A bit of background: the PATH environment variable is a list of
    directories separated by colons.  When you type a command name
    without giving an explicit path (e.g. you type "ls", rather than
    "/bin/ls") your shell searches each directory in the PATH list in
    order, looking for an executable file by that name, and the
    shell will run the first matching program it finds.

    One of the directories in the PATH list can be the
    current directory "." .  It is also permissible to use
    an empty directory name in the PATH list to indicate
    the current directory.  Both of these are equivalent

    for csh users:

    setenv PATH :/usr/ucb:/bin:/usr/bin
    setenv PATH .:/usr/ucb:/bin:/usr/bin

    for sh or ksh users

    PATH=:/usr/ucb:/bin:/usr/bin export PATH
    PATH=.:/usr/ucb:/bin:/usr/bin export PATH

    Having "." somewhere in the PATH is convenient - you can type
    "a.out" instead of "./a.out" to run programs in the current
    directory.  But there's a catch.

    Consider what happens in the case  where "." is the
    first entry in the PATH.  Suppose your current directory is a
    publically-writable one, such as "/tmp".  If there just happens to
    be a program named "/tmp/ls" left there by some other user, and you
    type "ls" (intending, of course, to run the normal "/bin/ls"
    program), your shell will instead run "./ls", the other
    user's program.  Needless to say, the results of running
    an unknown program like this might surprise you.

    It's slightly better to have "." at the end of the PATH:

    setenv PATH /usr/ucb:/bin:/usr/bin:.

    Now if you're in /tmp and you type "ls", the shell will
    search /usr/ucb, /bin and /usr/bin for a program named
    "ls" before it gets around to looking in ".", and there
    is less risk of inadvertently running some other user's
    "ls" program.  This isn't 100% secure though - if you're
    a clumsy typist and some day type "sl -l" instead of "ls -l",
    you run the risk of running "./sl", if there is one.
    Some "clever" programmer could anticipate common typing
    mistakes and leave programs by those names scattered
    throughout public directories.  Beware.

    Many seasoned Unix users get by just fine without having
    "." in the PATH at all:

    setenv PATH /usr/ucb:/bin:/usr/bin

    If you do this, you'll need to type "./program" instead
    of "program" to run programs in the current directory, but
    the increase in security is probably worth it.
--
Ted Timar - [e-mail]
Omron Corporation, Shimokaiinji, Nagaokakyo-city, Kyoto 617,  Japan


..
Hope you know what you're doing ...
Data directory is /big/gd
Port is 70
Logging to File /usr/adm/gopherd.l

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Archive-name: unix-faq/part3
Version: $Id: part3,v 1.5 1992/04/26 20:14:45 tmatimar Exp $

These four 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.

This article includes answers to:

      3.1)  How do I find out the creation time of a file?
      3.2)  How do I use "rsh" without having the rsh hang around
              until the remote command has completed?
      3.3)  How do I truncate a file?
      3.4)  Why doesn't find's "{}" symbol do what I want?
      3.5)  How do I set the permissions on a symbolic link?
      3.6)  How do I "undelete" a file?
      3.7)  How can a process detect if it's running in the background?
      3.8)  Why doesn't redirecting a loop work as intended?  (Bourne shell)
      3.9)  How do I run 'passwd', 'ftp', 'telnet', 'tip' and other interactive
              programs from a shell script or in the background?
      3.10) How do I find out the process ID of a program with a particular
            name from inside a shell script or C program?
      3.11) How do I check the exit status of a remote command
            executed via "rsh" ?
      3.12) Is it possible to pass shell variable settings into an awk program?
      3.13) How do I get rid of zombie processes that persevere?
      3.14) How do I get lines from a pipe as they are written instead of
            only in larger blocks.

    If you're looking for the answer to, say, question 3.5, and want to skip
    everything else, you can search ahead for the regular expression "^5)".

While these are all legitimate questions, they seem to crop up in
comp.unix.questions 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].

1)  How do I find out the creation time of a file?

    You can't - it isn't stored anywhere.  Files have a last-modified
    time (shown by "ls -l"), a last-accessed time (shown by "ls -lu")
    and an inode change time (shown by "ls -lc"). The latter is often
    referred to as the "creation time" - even in some man pages -  but
    that's wrong; it's also set by such operations as mv, ln,
    chmod, chown and chgrp.

    The man page for "stat(2)" discusses this.

2)  How do I use "rsh" without having the rsh hang around until the
    remote command has completed?

    (See note in question 2.7 about what "rsh" we're talking about.)

    The obvious answers fail:
            rsh machine command &
    or      rsh machine 'command &'

    For instance, try doing   rsh machine 'sleep 60 &'
    and you'll see that the 'rsh' won't exit right away.
    It will wait 60 seconds until the remote 'sleep' command
    finishes, even though that command was started in the
    background on the remote machine.  So how do you get
    the 'rsh' to exit immediately after the 'sleep' is started?

    The solution - if you use csh on the remote machine:

        rsh machine -n 'command >&/dev/null </dev/null &'

    If you use sh on the remote machine:

        rsh machine -n 'command >/dev/null 2>&1 </dev/null &'

    Why?  "-n" attaches rsh's stdin to /dev/null so you could run the
    complete rsh command in the background on the LOCAL machine.
    Thus "-n" is equivalent to another specific "< /dev/null".
    Furthermore, the input/output redirections on the REMOTE machine
    (inside the single quotes) ensure that rsh thinks the session can
    be terminated (there's no data flow any more.)

    Note: The file that you redirect to/from on the remote machine
    doesn't have to be /dev/null; any ordinary file will do.

    In many cases, various parts of these complicated commands
    aren't necessary.

3)  How do I truncate a file?

    The BSD function ftruncate() sets the length of a file.  Xenix -
    and therefore SysV r3.2 and later - has the chsize() system call.
    For other systems, the only kind of truncation you can do is
    truncation to length zero with creat() or open(..., O_TRUNC).

4)  Why doesn't find's "{}" symbol do what I want?

    "find" has a -exec option that will execute a particular
    command on all the selected files. Find will replace any "{}"
    it sees with the name of the file currently under consideration.

    So, some day you might try to use "find" to run a command on every
    file, one directory at a time.  You might try this:

    find /path -type d -exec command {}/\* \;

    hoping that find will execute, in turn

    command directory1/*
    command directory2/*
    ...

    Unfortunately, find only expands the "{}" token when it appears
    by itself.  Find will leave anything else like "{}/*" alone, so
    instead of doing what you want, it will do

    command {}/*
    command {}/*
    ...

    once for each directory.  This might be a bug, it might be a feature,
    but we're stuck with the current behaviour.

    So how do you get around this?  One way would be to write a
    trivial little shell script, let's say "./doit", that
    consists of

    command "$1"/*

    You could then use

    find /path -type d -exec ./doit {} \;

    Or if you want to avoid the "./doit" shell script, you can use

    find /path -type d -exec sh -c 'command $0/*' {} \;

    (This works because within the 'command' of "sh -c 'command' A B C ...",
     $0 expands to A, $1 to B, and so on.)

    or you can use the construct-a-command-with-sed trick

    find /path -type d -print | sed 's:.*:command &/*:' | sh

    If all you're trying to do is cut down on the number of times
    that "command" is executed, you should see if your system
    has the "xargs" command.  Xargs reads arguments one line at a time
    from the standard input and assembles as many of them as will fit into
    one command line.  You could use

    find /path -print | xargs command

    which would result in one or more executions of

    command file1 file2 file3 file4 dir1/file1 dir1/file2

    Unfortunately this is not a perfectly robust or secure solution.
    Xargs expects its input lines to be terminated with newlines, so it
    will be confused by files with odd characters such as newlines
    in their names.

5)  How do I set the permissions on a symbolic link?

    Permissions on a symbolic link don't really mean anything.  The
    only permissions that count are the permissions on the file that
    the link points to.

6)  How do I "undelete" a file?

    Someday, you are going to accidentally type something like "rm * .foo",
    and find you just deleted "*" instead of "*.foo".  Consider it a rite
    of passage.

    Of course, any decent systems administrator should be doing regular
    backups.  Check with your sysadmin to see if a recent backup copy
    of your file is available.  But if it isn't, read on.

    For all intents and purposes, when you delete a file with "rm" it is
    gone.  Once you "rm" a file, the system totally forgets which blocks
    scattered around the disk comprised your file.  Even worse, the blocks
    from the file you just deleted are going to be the first ones taken
    and scribbled upon when the system needs more disk space.  However,
    never say never.  It is theoretically possible *if* you shut down
    the system immediately after the "rm" to recover portions of the data.
    However, you had better have a very wizardly type person at hand with
    hours or days to spare to get it all back.

    Your first reaction when you "rm" a file by mistake is why not make
    a shell alias or procedure which changes "rm" to move files into a
    trash bin rather than delete them?  That way you can recover them if
    you make a mistake, and periodically clean out your trash bin.  Two
    points:  first, this is generally accepted as a *bad* idea.  You will
    become dependent upon this behaviour of "rm", and you will find
    yourself someday on a normal system where "rm" is really "rm", and
    you will get yourself in trouble.  Second, you will eventually find
    that the hassle of dealing with the disk space and time involved in
    maintaining the trash bin, it might be easier just to be a bit more
    careful with "rm".  For starters, you should look up the "-i" option
    to "rm" in your manual.

    If you are still undaunted, then here is a possible simple answer.  You
    can create yourself a "can" command which moves files into a
    trashcan directory. In csh(1) you can place the following commands
    in the ".login" file in your home directory:

    alias can    'mv \!* ~/.trashcan'       # junk file(s) to trashcan
    alias mtcan    'rm -f ~/.trashcan/*'       # irretrievably empty trash
    if ( ! -d ~/.trashcan ) mkdir ~/.trashcan  # ensure trashcan exists

    You might also want to put a:

    rm -f ~/.trashcan/*

    in the ".logout" file in your home directory to automatically empty
    the trash when you log out.  (sh and ksh versions are left as an
    exercise for the reader.)

    MIT's Project Athena has produced a comprehensive
    delete/undelete/expunge/purge package, which can serve as a
    complete replacement for rm which allows file recovery.  This
    package was posted to comp.sources.misc (volume 17, issue 023-026)

7)  How can a process detect if it's running in the background?

    First of all: do you want to know if you're running in the background,
    or if you're running interactively? If you're deciding whether or
    not you should print prompts and the like, that's probably a better
    criterion. Check if standard input is a terminal:

        sh: if [ -t 0 ]; then ... fi
        C: if(isatty(0)) { ... }

    In general, you can't tell if you're running in the background.
    The fundamental problem is that different shells and different
    versions of UNIX have different notions of what "foreground" and
    "background" mean - and on the most common type of system with a
    better-defined notion of what they mean, programs can be moved
    arbitrarily between foreground and background!

    UNIX systems without job control typically put a process into the
    background by ignoring SIGINT and SIGQUIT and redirecting the standard
    input to "/dev/null"; this is done by the shell.

    Shells that support job control, on UNIX systems that support job
    control, put a process into the background by giving it a process group
    ID different from the process group to which the terminal belongs.  They
    move it back into the foreground by setting the terminal's process group
    ID to that of the process.  Shells that do *not* support job control, on
    UNIX systems that support job control, typically do what shells do on
    systems that don't support job control.

8)  Why doesn't redirecting a loop work as intended?  (Bourne shell)

    Take the following example:

    foo=bar

    while read line
    do
        # do something with $line
        foo=bletch
    done < /etc/passwd

    echo "foo is now: $foo"

    Despite the assignment ``foo=bletch'' this will print ``foo is now: bar''
    in many implementations of the Bourne shell.  Why?
    Because of the following, often undocumented, feature of historic
    Bourne shells: redirecting a control structure (such as a loop, or an
    ``if'' statement) causes a subshell to be created, in which the structure
    is executed; variables set in that subshell (like the ``foo=bletch''
    assignment) don't affect the current shell, of course.

    The POSIX 1003.2 Shell and Tools Interface standardization committee
    forbids the behaviour described above, i.e. in P1003.2 conformant
    Bourne shells the example will print ``foo is now: bletch''.

    In historic (and P1003.2 conformant) implementations you can use the
    following `trick' to get around the redirection problem:

    foo=bar

    # make file descriptor 9 a duplicate of file descriptor 0 (stdin);
    # then connect stdin to /etc/passwd; the original stdin is now
    # `remembered' in file descriptor 9; see dup(2) and sh(1)
    exec 9<&0 < /etc/passwd

    while read line
    do
        # do something with $line
        foo=bletch
    done

    # make stdin a duplicate of file descriptor 9, i.e. reconnect it to
    # the original stdin; then close file descriptor 9
    exec 0<&9 9<&-

    echo "foo is now: $foo"

    This should always print ``foo is now: bletch''.
    Right, take the next example:

    foo=bar

    echo bletch | read foo

    echo "foo is now: $foo"

    This will print ``foo is now: bar'' in many implementations,
    ``foo is now: bletch'' in some others.  Why?
    Generally each part of a pipeline is run in a different subshell;
    in some implementations though, the last command in the pipeline is
    made an exception: if it is a builtin command like ``read'', the current
    shell will execute it, else another subshell is created.

    POSIX 1003.2 allows both behaviours so portable scripts cannot depend
    on any of them.

9)  How do I run 'passwd', 'ftp', 'telnet', 'tip' and other interactive
    programs from a shell script or in the background?

    These programs expect a terminal interface.  Shells makes no special
    provisions to provide one.  Hence, such programs cannot be automated
    in shell scripts.

    The 'expect' program provides a programmable terminal interface for
    automating interaction with such programs.  The following expect
    script is an example of a non-interactive version of passwd(1).

        # username is passed as 1st arg, password as 2nd
        set password [index $argv 2]
        spawn passwd [index $argv 1]
        expect "*password:"
        send "$password\r"
        expect "*password:"
        send "$password\r"
        expect eof

    expect can partially automate interaction which is especially
    useful for telnet, rlogin, debuggers or other programs that have no
    built-in command language.  The distribution provides an example
    script to rerun rogue until a good starting configuration appears.
    Then, control is given back to the user to enjoy the game.

    Fortunately some programs have been written to manage the connection
    to a pseudo-tty so that you can run these sorts of programs in a script.

    To get expect, email "send pub/expect/expect.shar.Z" to
    [e-mail] or anonymous ftp same from durer.cme.nist.gov.

    Another solution is provided by the pty 4.0 program, which runs a
    program under a pseudo-tty session and was posted to comp.sources.unix,
    volume 25.  A pty-based solution using named pipes to do the same as
    the above might look like this:

    #!/bin/sh
    /etc/mknod out.$$ p; exec 2>&1
    ( exec 4<out.$$; rm -f out.$$
    <&4 waitfor 'password:'
        echo "$2"
    <&4 waitfor 'password:'
        echo "$2"
    <&4 cat >/dev/null
    ) | ( pty passwd "$1" >out.$$ )

    Here, 'waitfor' is a simple C program that searches for
    its argument in the input, character by character.

    A simpler pty solution (which has the drawback of not
    synchronizing properly with the passwd program) is

    #!/bin/sh
    ( sleep 5; echo "$2"; sleep 5; echo "$2") | pty passwd "$1"

10) How do I find out the process ID of a program with a particular
    name from inside a shell script or C program?

    In a shell script:

    There is no utility specifically designed to map between program names
    and process IDs.  Furthermore, such mappings are often unreliable,
    since it's possible for more than one process to have the same name,
    and since it's possible for a process to change its name once it
    starts running.  However, a pipeline like this can often be used to
    get a list of processes (owned by you) with a particular name:

        ps ux | awk '/name/ && !/awk/ {print $2}'

    You replace "name" with the name of the process for which you are
    searching.

    The general idea is to parse the output of ps, using awk or grep or
    other utilities, to search for the lines with the specified name on
    them, and print the PID's for those lines.  Note that the "!/awk/"
    above prevents the awk process for being listed.

    You may have to change the arguments to ps, depending on what kind of
    Unix you are using.

    In a C program:

    Just as there is no utility specifically designed to map between
    program names and process IDs, there are no (portable) C library
    functions to do it either.

    However, some vendors provide functions for reading Kernel memory; for
    example, Sun provides the "kvm_" functions, and Data General provides
    the "dg_" functions.  It may be possible for any user to use these, or
    they may only be useable by the super-user (or a user in group "kmem")
    if read-access to kernel memory on your system is restricted.
    Furthermore, these functions are often not documented or documented
    badly, and might change from release to release.

    Some vendors provide a "/proc" filesystem, which appears as a
    directory with a bunch of filenames in it.  Each filename is a number,
    corresponding to a process ID, and you can open the file and read it
    to get information about the process.  Once again, access to this may
    be restricted, and the interface to it may change from system to
    system.

    If you can't use vendor-specific library functions, and you don't have
    /proc, and you still want to do this completely in C, you are going to
    have to do the grovelling through kernel memory yourself.  For a good
    example of how to do this on many systems, see the sources to
    "ofiles", available in the comp.sources.unix archives.
    (A package named "kstuff" to help with kernel grovelling was posted
    to alt.sources in May 1991 and is also available via anonymous ftp as
    usenet/alt.sources/articles/{329{6,7,8,9},330{0,1}}.Z from
    wuarchive.wustl.edu.)

11) How do I check the exit status of a remote command
    executed via "rsh" ?

    This doesn't work:

    rsh some-machine some-crummy-command || echo "Command failed"

    The exit status of 'rsh' is 0 (success) if the rsh program
    itself completed successfully, which probably isn't what
    you wanted.

    If you want to check on the exit status of the remote program, you
    can try using Maarten Litmaath's 'ersh' script, which was posted to
    alt.sources in January, 1991.  ersh is a shell script that
    calls rsh, arranges for the remote machine to echo the status
    of the command after it completes, and exits with that status.

12) Is it possible to pass shell variable settings into an awk program?

    There are two different ways to do this.  The first involves simply
    expanding the variable where it is needed in the program.  For
    example, to get a list of all ttys you're using:

     who | awk '/^'"$USER"'/ { print $2 }'                (1)

    Single quotes are usually used to enclose awk programs because the
    character '$' is often used in them, and '$' will be interpreted by
    the shell if enclosed inside double quotes, but not if enclosed
    inside single quotes.  In this case, we *want* the '$' in "$USER"
    to be interpreted by the shell, so we close the single quotes and
    then put the "$USER" inside double quotes.  Note that there are no
    spaces in any of that, so the shell will see it all as one
    argument.  Note, further, that the double quotes probably aren't
    necessary in this particular case (i.e. we could have done

     who | awk '/^'$USER'/ { print $2 }'                (2)

    ), but they should be included nevertheless because they are
    necessary when the shell variable in question contains special
    characters or spaces.

    The second way to pass variable settings into awk is to use an
    often undocumented feature of awk which allows variable settings to
    be specified as "fake file names" on the command line.  For
    example:

     who | awk '$1 == user { print $2 }' user="$USER" -            (3)

    Variable settings take effect when they are encountered on the
    command line, so, for example, you could instruct awk on how to
    behave for different files using this technique.  For example:

     awk '{ program that depends on s }' s=1 file1 s=0 file2        (4)

    Note that some versions of awk will cause variable settings
    encountered before any real filenames to take effect before the
    BEGIN block is executed, but some won't so neither way should be
    relied upon.

    Note, further, that when you specify a variable setting, awk won't
    automatically read from stdin if no real files are specified, so
    you need to add a "-" argument to the end of your command, as I did
    at (3) above.

13) How do I get rid of zombie processes that persevere?

    From: [e-mail] (Jonathan I. Kamens)
    Date: Fri, 17 Jan 92 14:40:09 -0500

    Unfortunately, it's impossible to generalize how the death of child
    processes should behave, because the exact mechanism varies over
    the various flavors of Unix.

    First of all, by default, you have to do a wait() for child
    processes under ALL flavors of Unix.  That is, there is no flavor
    of Unix that I know of that will automatically flush child
    processes that exit, even if you don't do anything to tell it to do
    so.

    Second, under some SysV-derived systems, if you do "signal(SIGCHLD,
    SIG_IGN)" (well, actually, it may be SIGCLD instead of SIGCHLD, but
    most of the newer SysV systems have "#define SIGCHLD SIGCLD" in the
    header files), then child processes will be cleaned up
    automatically, with no further effort in your part.  The best way
    to find out if it works at your site is to try it, although if you
    are trying to write portable code, it's a bad idea to rely on this
    in any case.  Unfortunately, POSIX doesn't allow you to do this;
    the behavior of setting the SIGCHLD to SIG_IGN under POSIX is
    undefined, so you can't do it if your program is supposed to be
    POSIX-compliant.

    If you can't use SIG_IGN to force automatic clean-up, then you've
    got to write a signal handler to do it.  It isn't easy at all to
    write a signal handler that does things right on all flavors of
    Unix, because of the following inconsistencies:

    On some flavors of Unix, the SIGCHLD signal handler is called if
    one *or more* children have died.  This means that if your signal
    handler only does one wait() call, then it won't clean up all of
    the children.  Fortunately, I believe that all Unix flavors for
    which this is the case have available to the programmer the wait3()
    call, which allows the WNOHANG option to check whether or not there
    are any children waiting to be cleaned up.  Therefore, on any
    system that has wait3(), your signal handler should call wait3()
    over and over again with the WNOHANG option until there are no
    children left to clean up.

    On SysV-derived systems, SIGCHLD signals are regenerated if there
    are child processes still waiting to be cleaned up after you exit
    the SIGCHLD signal handler.  Therefore, it's safe on most SysV
    systems to assume when the signal handler gets called that you only
    have to clean up one signal, and assume that the handler will get
    called again if there are more to clean up after it exits.

    On older systems, signal handlers are automatically reset to
    SIG_DFL when the signal handler gets called.  On such systems, you
    have to put "signal(SIGCHILD, catcher_func)" (where "catcher_func"
    is the name of the handler function) as the first thing in the
    signal handler, so that it gets reset.  Unfortunately, there is a
    race condition which may cause you to get a SIGCHLD signal and have
    it ignored between the time your handler gets called and the time
    you reset the signal.  Fortunately, newer implementations of
    signal() don't reset the handler to SIG_DFL when the handler
    function is called.  To get around this problem, on systems that do
    not have wait3() but do have SIGCLD, you need to reset the signal
    handler with a call to signal() after doing at least one wait()
    within the handler, each time it is called.

    The summary of all this is that on systems that have wait3(), you
    should use that and your signal handler should loop, and on systems
    that don't, you should have one call to wait() per invocation of
    the signal handler.

    One more thing -- if you don't want to go through all of this
    trouble, there is a portable way to avoid this problem, although it
    is somewhat less efficient.  Your parent process should fork, and
    then wait right there and then for the child process to terminate.
    The child process then forks again, giving you a child and a
    grandchild.  The child exits immediately (and hence the parent
    waiting for it notices its death and continues to work), and the
    grandchild does whatever the child was originally supposed to.
    Since its parent died, it is inherited by init, which will do
    whatever waiting is needed.  This method is inefficient because it
    requires an extra fork, but is pretty much completely portable.

14) How do I get lines from a pipe as they are written instead of only in
    larger blocks.

    From: [e-mail] (Jonathan I. Kamens)
    Date: Sun, 16 Feb 92 20:59:28 -0500

    The stdio library does buffering differently depending on whether
    it thinks it's running on a tty.  If it thinks it's on a tty, it
    does buffering on a per-line basis; if not, it uses a larger buffer
    than one line.

    If you have the source code to the client whose buffering you want
    to disable, you can use setbuf() or setvbuf() to change the
    buffering.

    If not, the best you can do is try to convince the program that
    it's running on a tty by running it under a pty, e.g. by using the
    "pty" program mentioned in question 3.9.
--
Ted Timar - [e-mail]
Omron Corporation, Shimokaiinji, Nagaokakyo-city, Kyoto 617,  Japan


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