Proposed monthly comp.unix.{questions,wizards} article
Steve Hayman
sahayman at iuvax.cs.indiana.edu
Thu Jun 1 07:23:36 AEST 1989
Here is version 0.9 of an article I'd like to post every month to
comp.unix.questions and comp.unix.wizards. I welcome any comments.
Thanks to everyone who either mailed or posted suggestions,
including David Elliott, John Cowan, Paul English, Don Libes
(whose "Life with Unix" really is a great book), Mike Liveright,
Jimmy Aitken, Giles Lean, Jim Kingdon, Steve Kinzler, Brad Templeton
and perhaps some others who I have inadvertently overlooked.
This wound up being longer than I thought it would be. Perhaps it
should be split into two articles - "Guidelines" and "Frequently
Asked Questions". Anyway, I welcome any and all comments,
and especially corrections if there are any blatant goofs in here.
Steve Hayman
sahayman at iuvax.cs.indiana.edu
Subject: Welcome to comp.unix.questions and comp.unix.wizards [Monthly posting]
Newsgroups: comp.unix.questions,comp.unix.wizards
Followup-To: comp.unix.questions
Comp.unix.questions and comp.unix.wizards are two 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 each
of the two newsgroups. If you would like to make any suggestions about
the content of this article, please contact sahayman at iuvax.cs.indiana.edu.
Later on in this article, you'll see the answers to some Frequently
Asked Questions. Please don't ask these questions again, they've
been answered plenty of times already. Thank you. This article
includes answers to:
How do I remove a file whose name begins with a "-" ?
How do I remove a file with funny characters in the filename ?
How do I get a recursive directory listing?
How do I get the current directory into my prompt?
How do I read characters from a terminal without requiring the user
to hit RETURN?
How do I read characters from the terminal in a shell script?
How do I check to see if there are characters to be read without
actually reading?
How do I find the name of an open file?
Also, if you have not already read the overall Usenet introductory material
posted to "news.announce.newusers", please do. Again, thank you.
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. Most 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 "-".
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.
Please remember that comp.unix.questions and comp.unix.wizards are
read by over 50,000 people around the world, and that posting a question
to either of these groups will cost a lot of time and money by the
time your article is distributed to Japan, Australia, Norway, Israel,
and all corners of North America.
Should I Post to "comp.unix.questions" or "comp.unix.wizards" ?
Comp.unix.wizards is intended for advanced discussion of Unix features - the
sort of topics the average user never thinks about. Simple questions
about using normal commands should never go there. Unfortunately, it's
often hard to tell whether your question is simple until you know
what the answer is. A good rule of thumb is -
Don't post to comp.unix.wizards unless *you* *yourself* are
a Unix wizard.
Don't post to comp.unix.wizards just because you want to get the
attention of a unix wizard. Many unix wizards read comp.unix.questions
also and will be happy to help you out if they see your question there.
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 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
UnAX 9.3 on a Vax 68086502"
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 responsed 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.
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.
If you're posting a shell script for others to use, remember that
file names can contain blanks and characters such as '?', so try
to make sure you script can handle those properly.
What About Those People Who Continue to Ask Stupid or Frequently Asked Questions
In Spite of This 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.
FREQUENTLY ASKED QUESTIONS
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. Suggestions or corrections
should be sent to sahayman at iuvax.cs.indiana.edu .
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
2) How do I remove a file with funny characters in the filename ?
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)
and
rm -ir .
(which asks you whether to remove each file in the directory,
answer "y" to the problem file and "n" to everything else.,
and which, unfortunately, doesn't work with many versions of rm)
and
find . -type f ... -exec 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 -exec rm '{}' \;
or
find . -inum 12345 -exec mv '{}' new-file-name \;
If none of these work, find your system manager.
3) How do I get a recursive directory listing?
There are a couple of ways.
The "ls" command has a "-R" option that makes it
recursively descend through directories, so you
could use "ls -lR /some/directory".
If you just need a list of files, you can use "find".
find /some/directory -print
or if you want to get fancy, there is a lot you can do with
this powerful program:
# Ordinary files only
find /some/directory -type f -print
# Directories only
find /some/directory -type d -print
# Everything beneath a couple of different directories
find /some/directory /another/directory -print
# "ls -l" all of my files that are bigger than 300 blocks
# if your machine has "xargs", you can use
find /my/home/directory -type f -size +300 -print | xargs ls -l
# or later BSD versions of find will let you use
find /my/home/directory -type f -size +300 -ls
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 cd 'chdir \!* && set prompt="${cwd}% "'
If you just want the last component of the current directory
in your prompt ("mail% " instead of "/usr/spool/mail% ")
you can do
alias cd 'chdir \!* && set prompt="$cwd:t% "'
Bourne Shell (sh):
Complicated but not impossible. Here's one way.
Add this to your .profile file:
LOGIN_SHELL=$$ export LOGIN_SHELL
CMDFILE=/tmp/cd.$$ export CMDFILE
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 ${LOGIN_SHELL?"not set"}
Now change directories with "xcd /some/dir".
Korn Shell (ksh):
Put this in your .profile file:
PS1='$PWD $ '
5) 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 inefficient,
and you should change the code to do it right some time:
main()
{
int c;
printf("Hit any character to continue\n");
system("stty cbreak");
c = getchar();
system("stty -cbreak");
printf("Thank you for typing %c.\n", c);
exit(0);
}
6) 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 $<.
7) How do I check to see if there are characters to be read without
actually reading?
In BSD, you can use select(2). In later versions of System V,
poll(2) will work. The most common "portable" way is to time
out a read(2) using alarm(3) and longjmp(3)/
8) 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.
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