5 Oct 2007 jnewbigin   » (Apprentice)

Making linux ‘user friendly’

For some years, there have been a few incompatability which turn users off linux. They are more configuration issues than anything else and probably windows is to blame, and specificly PuTTY. Let me explain the problems and then the solution.

Issue 1. UFT-8. UFT-8 is a Unicode encoding which uses variable length strings of 8 bit bytes to encode a character. The first 127 characters are the same as the ANSI characters which makes compatability easy. The problems start when the odd character above 127 gets thrown in but is not respected as the start of a unicode character. This is easily seen in man pages when ‘ turn into â. For the most part you can work around it but it is easy to fix.

The environment variable LANG indicates the current language. On modern linux distros which support unicode, it may well be set to something like ‘en_AU.UTF-8′. This indicates my language, country and unicode support. The simple fix is to set the language to ‘en_AU’ if my terminal does not support unicode.

Now this is all fine until you start using PuTTY to connect from windows, which I find most windows users do. Now your login script has to be able to turn of UFT-8, only if you are using PuTTY. Read on…

Issue 2. Colors. There are a number of factors which impact on the default color selection for programs like ls and vim. The biggest seems to be the terminal type as specified by the TERM environment variable. Common values are linux, vt100, vt220 and xterm. (There are hundreds of others). Now unfortunatly, assumptions are make like ‘the linux console background color is always black’ and ‘the xterm background color is always white’. This do not hold true, mostly because TERM indicated the terminal ‘emulation’, not the terminal it’s self. For example, by default, PuTTY reports that it does xterm emulation but it is clearly a windows program and is only emulating this. This issue is a bit harder to solve because there are many places were color information can come from (see /etc/profile.d/vim.sh). The TERM variable should match your terminal or your output will not be correct and expecting all users to correctly reconfigure there terminal is not realisting. A server side solution is required.

Solution Part 1. Anyone who has accidently viewed the contents of a binary file may well have ended up with PuTTY written all over the screen. This is a giveaway that you can query the terminal for information. (This is also a security issue, but for now it works in our favour.) (If you did view a binary file and now your text is broken, try the ‘reset’ command). Many terminals respond the the character 0×05 by sending the terminal type as a string, just as if the user had typed it on the keyboard. This is obviously quite an old feature (no one would design anything like that these days would they?) but luck for us it works by default with PuTTY.

A program which runs a login time can send an 0×05 and read back the string. This is not as easy as it should be because PuTTY does not ‘press enter’ when it is finished so you have to switch into noncanonical mode to read it. Still, it is possible to write a program which will report the terminal you are using.

Solution Part 2. If you find that your terminal is PuTTY and it is reporting xterm emulation then you can ‘correct’ some assumptions by altering the color ls configuration and the LANG environment variable.

Solution Part 2b. By correcting the COLORS environment variable (used by color ls) then you can make vim decide what color scheme to use. This is easly added to your .exrc

Implementation. Here are 3 code snippits. The first can be added to your ~/.bash_profile (or a global like /etc/profile.d/z_putty.sh). The next is for ~/.exrc or perhaps a gloabl vim rc file like /etc/vimrc. The third is the source to the answerback prorgram. See the code for compile instructions.

I will eventually package all this into a .rpm because we will be using it on our new servers. It will probably be available from http://www.chrysocome.net/

bash profile
eval `/usr/local/bin/answerback`
if [ "${ANSWERBACK}" ] ; then
if [ "${ANSWERBACK}" = PuTTY -a "${TERM}" = xterm ] ; then
echo "Terminal is ${ANSWERBACK}. Assuming dark background."
# Assume a dark background. This is not the assumes default for an xterm
# so we will reverse the default colorls setting
if [ "${COLORS}" = /etc/DIR_COLORS.xterm ] ; then
COLORS=/etc/DIR_COLORS
eval `dircolors --sh "$COLORS"`
fi
# PuTTY does not support UFT8 so we will remove that from our LANG
export LANG=${LANG/.*/}
else
echo "Terminal is ${ANSWERBACK}"
fi
fi

exrc
if $COLORS == "/etc/DIR_COLORS.xterm"
set background=light
else
set background=dark
endif

answerback.c

/* answerback.c version 0.1
Written by John Newbigin <jn@it.swin.edu.au>
GPL Version 2
Compile with 'gcc -Wall -o answerback answerback.c'
*/
#include <stdio.h>
#include <unistd.h>
#include <termios.h>
#include <signal.h>
#include <string.h>
int main(int argc, char **argv)
{
char *name = "ANSWERBACK";
char code = 5;
char buffer[16];
int r;
sigset_t sig, sigsave;
struct termios term, termsave;
FILE *fp;
if(argc == 2)
{
name = argv[1];
}
if((fp = fopen(ctermid(NULL), "r+")) == NULL)
{
perror("open");
return 0;
}
setbuf(fp, NULL);
sigemptyset(&sig);
sigaddset(&sig, SIGINT);
sigaddset(&sig, SIGTSTP);
sigprocmask(SIG_BLOCK, &sig, &sigsave);
tcgetattr(fileno(fp), &termsave);
term = termsave;
term.c_lflag &= ~(ECHO | ECHOE | ECHOK | ECHONL | ICANON);
term.c_cc[VMIN] = 0;
term.c_cc[VTIME] = 10;
tcsetattr(fileno(fp), TCSAFLUSH, &term);
write(fileno(fp), &code, 1);
r = read(fileno(fp), buffer, sizeof(buffer) - 1);
tcsetattr(fileno(fp), TCSAFLUSH, &termsave);
sigprocmask(SIG_SETMASK, &sigsave, NULL);
fclose(fp);
if(r > 0)
{
printf("export %s='%s'\n", name, buffer);
}
return 0;
}

Syndicated 2007-01-25 10:12:59 from John's test blog

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