Frequently Asked Questions about Unix - with Answers. [Monthly posting]

[Steve Hayman]

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?
	What does {awk,grep,fgrep,egrep,biff,cat} stand for?


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 at 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)
    
    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% "'



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

	main()
	{
		int c;

		printf("Hit any character to continue\n");
		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 $<.

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 and pipes.  In System V Release 3, you
	can use poll(2), but that only works on streams.

	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?  Well, the shell expands
	*.foo and *.bar before the mv command ever sees the arguments,
	so since there probably aren't any *.bar files already, mv will
	be invoked as "mv a.foo b.foo c.foo" which doesn't do what you want..

	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. 


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

    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?

    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 can run it
    in the background.  Also, the input/output redirections ensure
    that rsh thinks the session can be terminated (there's no
    data flow any more.)

    Various parts of these complicated commands aren't necessary in all cases.

13) What does {awk,grep,fgrep,egrep,biff,cat} 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".

    egrep = "Extended Grep"

	egrep uses fancier regular expressions than grep.

    cat = "catenate"

	catenate is an obscure word meaning "to connect in a series".
	Yes, "catenate" is a real word.

    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.
    


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