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Unix - Frequently Asked Questions (2/7) [Frequent posting]


Archive-name: unix-faq/faq/part2
Version: $Id: part2,v 2.9 1996/06/11 13:07:56 tmatimar Exp $

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

This collection of documents is Copyright (c) 1994, Ted Timar, except
Part 6, which is Copyright (c) 1994, Pierre Lewis and Ted Timar.
All rights reserved.  Permission to distribute the collection is
hereby granted providing that distribution is electronic, no money
is involved, reasonable attempts are made to use the latest version
and all credits and this copyright notice are maintained.
Other requests for distribution will be considered.  All reasonable
requests will be granted.

All information here has been contributed with good intentions, but
none of it is guaranteed either by the contributors or myself to be
accurate.  The users of this information take all responsibility for
any damage that may occur.

Many FAQs, including this one, are available on the archive site
rtfm.mit.edu in the directory pub/usenet/news.answers.
The name under which a FAQ is archived appears in the "Archive-Name:"
line at the top of the article.  This FAQ is archived as
"unix-faq/faq/part[1-7]".

These articles are divided approximately as follows:

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

This article includes answers to:

      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 ?
      2.14) How do I ring the terminal bell during a shell script?
      2.15) Why can't I use "talk" to talk with my friend on machine X?
      2.16) Why does calendar produce the wrong output?

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 "^2.5)".

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

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

----------------------------------------------------------------------

Subject: How do I remove a file whose name begins with a "-" ?
Date: Thu Mar 18 17:16:55 EST 1993

2.1)  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".

------------------------------

Subject: How do I remove a file with funny characters in the filename ?
Date: Thu Mar 18 17:16:55 EST 1993

2.2)  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.

------------------------------

Subject: How do I get a recursive directory listing?
Date: Thu Mar 18 17:16:55 EST 1993

2.3)  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.

------------------------------

Subject: How do I get the current directory into my prompt?
Date: Thu Mar 18 17:16:55 EST 1993

2.4)  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 a signal that's not 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
            %/          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 $ '

------------------------------

Subject: How do I read characters from the terminal in a shell script?
Date: Thu Mar 18 17:16:55 EST 1993

2.5)  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 ."

------------------------------

Subject: How do I rename "*.foo" to "*.bar", or change file names to lowercase?
Date: Thu Mar 18 17:16:55 EST 1993

2.6)  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 $_;
        }

------------------------------

Subject: Why do I get [some strange error message] when I "rsh host command" ?
Date: Thu Mar 18 17:16:55 EST 1993

2.7)  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" or "remote"; 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.

------------------------------

Subject: How do I ... and have that change affect my current shell?
Date: Thu Mar 18 17:16:55 EST 1993

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?

      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.

      A much more detailed answer prepared by
      xtm@telelogic.se (Thomas Michanek) can be found at
      ftp.wg.omron.co.jp in /pub/unix-faq/docs/script-vs-env.

------------------------------

Subject: How do I redirect stdout and stderr separately in csh?
>From: msb@sq.com (Mark Brader)
Date: Mon, 26 Oct 1992 20:15:00 -0500

2.9)  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.

      If what you want is to avoid redirecting stdout at all, let sh
      do it for you.

        sh -c 'command 2>stderr_file'

------------------------------

Subject: How do I tell inside .cshrc if I'm a login shell?
Date: Thu Mar 18 17:16:55 EST 1993

2.10) 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

------------------------------

Subject: How do I construct a ... matches all files except "." and ".." ?
Date: Thu Mar 18 17:16:55 EST 1993

2.11) 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.

      In ksh, you can use:  .!(.|) *

------------------------------

Subject: How do I find the last argument in a Bourne shell script?
Date: Thu Mar 18 17:16:55 EST 1993

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

      Answer by:
        Martin Weitzel <@mikros.systemware.de:martin@mwtech.uucp>
        Maarten Litmaath <maart@nat.vu.nl>

      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

------------------------------

Subject: What's wrong with having '.' in your $PATH ?
Date: Thu Mar 18 17:16:55 EST 1993

2.13) 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.

------------------------------

Subject: How do I ring the terminal bell during a shell script?
>From: uwe@mpi-sb.mpg.de (Uwe Waldmann)
Date: Fri, 30 Apr 93 16:33:00 +0200

2.14) How do I ring the terminal bell during a shell script?

      The answer depends on your Unix version (or rather on the kind of
      "echo" program that is available on your machine).

      A BSD-like "echo" uses the "-n" option for suppressing the final
      newline and does not understand the octal \nnn notation.  Thus
      the command is

        echo -n '^G'

      where ^G means a _literal_ BEL-character (you can produce this in
      emacs using "Ctrl-Q Ctrl-G" and in vi using "Ctrl-V Ctrl-G").

      A SysV-like "echo" understands the \nnn notation and uses \c to
      suppress the final newline, so the answer is:

        echo '\007\c'

------------------------------

Subject: Why can't I use "talk" to talk with my friend on machine X?
>From: tmatimar@isgtec.com (Ted Timar)
Date: Thu Mar 18 17:16:55 EST 1993

2.15) Why can't I use "talk" to talk with my friend on machine X?

      Unix has three common "talk" programs, none of which can talk with
      any of the others.  The "old" talk accounts for the first two types.
      This version (often called otalk) did not take "endian" order into
      account when talking to other machines.  As a consequence, the Vax
      version of otalk cannot talk with the Sun version of otalk.
      These versions of talk use port 517.

      Around 1987, most vendors (except Sun, who took 6 years longer than
      any of their competitors) standardized on a new talk (often called
      ntalk) which knows about network byte order.  This talk works between
      all machines that have it.  This version of talk uses port 518.

      There are now a few talk programs that speak both ntalk and one
      version of otalk.  The most common of these is called "ytalk".

------------------------------

Subject: Why does calendar produce the wrong output?
>From: tmatimar@isgtec.com (Ted Timar)
Date: Thu Sep  8 09:45:46 EDT 1994

2.16) Why does calendar produce the wrong output?

      Frequently, people find that the output for the Unix calendar
      program, 'cal' produces output that they do not expect.

      The calendar for September 1752 is very odd:

               September 1752
             S  M Tu  W Th  F  S
                   1  2 14 15 16
            17 18 19 20 21 22 23
            24 25 26 27 28 29 30

      This is the month in which the US (the entire British Empire actually)
      switched from the Julian to the Gregorian calendar.

      The other common problem people have with the calendar program is
      that they pass it arguments like 'cal 9 94'.  This gives the calendar
      for September of AD 94, NOT 1994.

------------------------------

End of unix/faq Digest part 2 of 7
**********************************

-- 
Ted Timar - tmatimar@isgtec.com
ISG Technologies Inc., 6509 Airport Road, Mississauga, Ontario, Canada L4V 1S7



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