Help, I've Fallen

by Gary Kline, David Leonard and Dirk Myers

Perl! Text Editors! Df! Shells! And more... Thrills and chills as the Answermen show you around a land that isn't Kansas any more.

• How do I configure the perl5 locale settings so that perl behaves?
• How do I change my default text editor so that all programs use my editor of choice?
• What is the correct way to initialize shell variables?
• Is there a pre-processor symbols that works for all versions of BSD?
• The mysteries of df revealed.
• How can I mount a floppy msdos floppy drive as myself rather than as root?
• Help with some tape-drive utilities.

Q1: Why does perl show these errors? How can I stop them?

 $ perl -e 'print "hello!\n"'
 perl: warning: Setting locale failed.
 perl: warning: Please check that your locale settings:
         LC_ALL = (unset),
         LC_CTYPE = "en_US",
         LANG = (unset)
     are supported and installed on your system.
 perl: warning: Falling back to the standard locale ("C").
 hello!
 $

These errors are generated because perl is trying to set up for a locale setting of 'en_US', and failing. The locale environment variables are used to tell the system how it should behave when doing things which are common, but which vary in different parts of the world -- for example, formatting a date. Perl attempts to determine the locale it should use by looking at the environment when it starts up. Perl will complain when any of the environment variables used to indicate your locale are set to values which don't exactly match the locale information stored on your system. The solution is to set these environment variables to the precise values perl expects to see.

Assuming that you intended for the locale to be English, in the United States variant, add the following lines to your initialization file (.login, .profile, .bashrc, .zlogin). (The lines shown are for csh, and the E.)

setenv LC_ALL   en_US.ISO_8859-1
setenv LC_CTYPE en_US.ISO_8859-1
setenv LANG en_US.ISO_8859-1
setenv LESSCHARSET latin1

For more information on perl's locale support, see the perllocale documentation included with the perl distribution. You can read this by typing 'man perllocale' at the shell prompt.

Q2: How do I change the default text editor?

$ EDITOR=emacs ; export EDITOR

and for the csh-like shells:

% setenv EDITOR emacs

Q3: What's the correct way to set environment variables?

   export VARIABLE=value

   VARIABLE=value
   export VARIABLE 

Which is the "correct" way?

If you want to make your shell set-up files anywhere near portable, it is probably better to use

   VARIABLE=value
   export VARIABLE 

  export VARNAME=value 

If you are writing a shell script to be widely used (say for teaching or illustrative purposed), it makes sense to use the slightly longer syntax that is sure to work in all Bourne or Bourne-like shells.

On the other hand, if you don't care about portability and are certain that your script will only be used on bash, ksh, zsh, then, sure, use the shorter notation.

There's another variant on this question, of course: csh, and shells derived from it, use a completely different syntax:

% setenv VARNAME value

To make your life even simpler if you're cutting-and-pasting the setenv/unsetenv syntax of the csh, putting the following functions in the initialization files for a Bourne-like shell will let the csh syntax work for you there, too.

setenv() { export "$1=$2"; }
unsetenv() { unset "$1"; export "$1"; }

Q4: I'm writing a C program, and I want to make sure it's portable to the BSD family. I think I've got a handle on the actual code I need to include, but I'm not sure how to make sure it's included. Is there a common way among FreeBSD, NetBSD, and OpenBSD to #define BSD ?

Without the C compiler (usually gcc) should only automatically define preprocessor symbols such as `__unix__' and `__GNUC__'. Non-ANSI C compilers will often define the symbol `unix'. For some operating systems, the compiler is also configured to predefine an operating system specific symbol (such as `__FreeBSD__'.) If you are really interested, you can find out what symbols are automatically defined with this command:

gcc -x c /dev/null -E -dM

#if defined(__unix__) || defined(unix)
#  include 
#endif

#if defined(BSD)
/* BSD-specific things */
#endif

46:#ifdef        __linux__
65:# if (BSD >= 199103) || defined(__linux) || defined(ultrix) ||
defined(AIX) ||\
79:#if !defined(BSD) && !defined(__linux) && !defined(UNICOS)
#if (defined(BSD)) && (BSD >= 199103) || defined(__linux) ||
defined(AIX)

Q5: Someone said that the df utility was especially useful? Can you explain how? I thought that df only tells how much diskspace is free.

df does indeed report on the free space of mounted disks. It also reports on some other statistics of disk usage which you may or may not be interested in.

`df -i' will show you how many inodes remain on the filesystem. Inodes are basically file slots and a fixed number of them are allocated when the filesystem is first created. If you run out of inodes, you can't make any new files - even if you have free disk space! Some network news servers have been known to create huge numbers of tiny files and run out of inodes.

`df -n' is handy if you are having problems with filesystems - especially remote filesystems or experimental ones - that cause your processes to `lock up' whenever they access a disk. df without the -n flag may lock up in those cases too. When you include -n, df will show you whatever the most recently obtained statistics were, rather than ask a possibly dead filesystem for them.

The '-k' flag directs df to report numbers in kilobytes which some people find more useful than the default unit of `disk blocks' (usually half a kilobyte).

Q6: How can I mount a msdos floppy drive as me, not root?

You may know that the general way to mount a DOS floppy (as root) is with:

 # mount -t msdos /dev/fd0 /

But that command only works if you're root. Before telling you how to work around this, it's important to understand why this is the default. The answer, as you might suspect, boils down to security -- if you let arbitrary users mount file systems, the security on your machine is more easily compromised. There's also the potential for inadvertent problems, as BSD tends to panic when mounted filesystems suddenly disappear (for example, if the floppy is removed from the drive before the filesystem is unmounted), or if the filesystem is corrupted in certain ways.

Now that you understand some of the whys and wherefores about not letting anyone but root mount a floppy, here's how to let any user do it:

• As root, change the permissions on the floppy device (/dev/fd0 or whatever) to allow reads/writes by users.
• As root, set the sysctl vfs.usermount to 1

In other words,

    # chmod 0666 /dev/fd0
    # sysctl -w vfs.usermount=1
 

Then as yourself, create a mountpoint in your home directory. If you are usr `smith' and you want to call your mountpoint `floppy' type

    % mkdir /home/smith/floppy 

    % mount -t msdos /dev/fd0 /home/smith/floppy

If you run xdm on your console, then in /usr/X11R6/lib/X11/xdm there are usually scripts called GiveConsole and TakeConsole. They typically change ownership of /dev/console to the user logging in on ":0". It is trivial to extend these scripts to chown/chmod 0600 the local floppy devices as well. If you have come from a Macintosh background, you could even insert the right commands to umount and eject the floppy!

As usual, there's a lot more information on how to use the mount command in the man page for mount. We've only scratched the surface of the options available.

Q7: Can you help me with some of the tape drive utilities? I tried using mt and got error output that I didn't understand at all. Some concrete examples of the use of

 tar -, and mt, sa

would help!

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David Leonard is a PhD student in the Department of Computer Science and Electrical Engineering at the University of Queensland, Brisbane, Australia.

His area of research is QoS-adaptive component software architectures, and in his spare time is a developer for the OpenBSD project. That said, David enjoys living the quiet life with his wife, Kylie and cat, Mu. He especially enjoys frequenting Moreton Bay's many fabulous places to eat. Mmmmm!

[home| mail]

Dirk Myers works for a company that builds bar code scanners, making Unix systems at the company do things which are useful for document control. Much of his spare time is spent losing at Go, enjoying the central Oregon outdoors, and making Unix systems do things which are not useful for document control. He is married to Shanie, a scientist, and spends the rest of his spare time trying to understand the things his wife does with her time. Together, Dirk and his wife own large quantities of domestic animals, and they have recently acquired a house to put the animals in. Dirk is still looking for the man pages for the house.

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