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Path: utzoo!attcan!utgpu!utstat!jarvis.csri.toronto.edu!mailrus!iuvax!sahayman
From: sahayman@iuvax.cs.indiana.edu (Steve Hayman)
Newsgroups: comp.unix.questions,comp.unix.wizards
Subject: Frequently Asked Questions about Unix - with Answers [Monthly posting]
Message-ID: <FAQ2.8@iuvax.cs.indiana.edu>
Date: 1 Nov 89 20:28:42 GMT
Expires: 1 Dec 89 05:00:00 GMT
Followup-To: comp.unix.questions
Organization: Computer Science Department, Indiana University
Lines: 643
Supersedes: <FAQ2.7@iuvax.cs.indiana.edu>

This article contains the answers to some Frequently Asked Questions
often seen in comp.unix.questions and comp.unix.wizards.  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.

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?
	How do I rename "*.foo" to "*.bar"?
	Why do I get [some strange error message] when I "rsh host command" ?
	How do I find out the creation time of a file?
	How do I use "rsh" without having the rsh hang around
	    until the remote command has completed?
	How do I truncate a file?
	How do I {set an environment variable, change directory} inside a
	    shell script and have that change affect my current shell?
	Why doesn't find's "{}" symbol do what I want?
	What does {awk,grep,fgrep,egrep,biff,cat,gecos,nroff,troff,tee,bss}
	    stand for?
	How do I pronounce "vi"?


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
sahayman@iuvax.cs.indiana.edu or iuvax!sahayman.

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 ?

    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.,
	and which, unfortunately, doesn't work with many versions of rm;
	(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)

    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.

    If none of these work, find your system manager.

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 cd 'chdir \!* && set prompt="${cwd}% "'
	
	If you use pushd and popd, you'll also need

	    alias pushd 'pushd \!* && set prompt="${cwd}% "'
	    alias popd  'popd  \!* && set prompt="${cwd}% "'

	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 do
	    alias cd 'chdir \!* && 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
		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##*/} $ '


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 slow and inefficient, and you
    should change the code to do it right some time:

    main()
    {
	    int c;

	    printf("Hit any character to continue\n");
	    /*
	     * ioctl() would be better here; only lazy
	     * programmers do it this way:
	     */
	    system("/bin/stty cbreak");
	    c = getchar();
	    system("/bin/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 $< 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
	    readchar=`dd if=/dev/tty bs=1 count=1 2>/dev/null`
	    stty -cbreak

	    echo "Thank you for typing a $readchar ."

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

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

    There is no way to check whether characters are available to be
    read from a FILE pointer.  (Well, there is no *good* way.  You could
    poke around inside stdio data structures to see if the input buffer
    is nonempty but this is a bad idea, forget about it.)

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


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.


9) How do I rename "*.foo" to "*.bar"?
	
    Why doesn't "mv *.foo *.bar" work?  Think about how the shell
    expands wildcards.   "*.foo" "*.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. 

    A program called "ren" that does this job nicely was posted
    to comp.sources.unix some time ago.  It lets you use

	ren '*.foo' '#1.bar'


10) 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.

    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.

11) 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 the time the file's status was last changed,
    either by writing or changing the inode (via mv or chmod, etc...).

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

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

    (See note in question 10 about what "rsh" we're talking about.)

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

    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: on the remote machine, you needn't redirect to/from
    /dev/null; any ordinary file will do.

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

13) 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).

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

    You can't, unless you use a special command to run the script in
    the context of the current shell rather than in a child program.
    The process environment (including environment variables and
    current directory) is inherited by child programs but cannot be
    passed back to parent programs.

    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.

15) 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 {} \;


    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

	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.


16) What does {awk,grep,fgrep,egrep,biff,cat,gecos,nroff,troff,tee,bss}
    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 ROFF"
	
	These are descendants of "roff", which was a re-implementation
	of the Multics "runoff" program.
	
    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 allegedly named after someone's dog that barked whenever
	the postman arrived.  Or so the story goes.
    
   
    Don Libes' book "Life with Unix" contains lots more of
    these tidbits.

17) How do I pronounce "vi" ?

    You can start a very long and pointless discussion by wondering
    about this topic on the net.  Some people say "vye", some say
    "vee-eye" (the vi manual suggests this) and some Roman numerologists
    say "six".  How you pronounce "vi" has nothing to do with whether
    or not you are a true Unix wizard.

    Similarly, you'll find that some people pronounce "char" as "care",
    and that there are lots of ways to say "#" or "/*" or "!" or
    "tty" or "/etc".  No one pronunciation is correct - enjoy the regional
    dialects and accents.  


-- 
Steve Hayman    Workstation Manager    Computer Science Department   Indiana U.
sahayman@iuvax.cs.indiana.edu     iuvax!sahayman                 (812) 855-6984