bash

disable bell (also for other programs that use readline):
~/.inputrc: set bell-style none

.bashrc:
export PAGER="less -X"
export EDITOR=vi
export HISTFILE=
export PS1="\h \w\\$ "

alias more="less -X"
alias vi="vim -v"

using sudo instead of su on gnome apps (debian)
root# update-alternatives --set libgksu-gconf-defaults \
 /usr/share/libgksu/debian/gconf-defaults.libgksu-sudo
root# update-gconf-defaults

user$ gconftool-2 --type bool --set /apps/gksu/sudo-mode true
building a kernel.org the debian way
# needed packages
aptitude install linux-headers-2.6-amd64 kernel-package libncurses5-dev

#get the src into /usr/src and then
cd /usr/src/2.6.xx
# because I can :-)
export CONCURRENCY_LEVEL=4
# build it
make-kpkg clean
fakeroot make-kpkg --initrd --revision=mykernel.0.1 kernel_image
# install 
dpkg -i ../linux-image...
 
man 5 kernel-pkg.conf
man 5 kernel-img.conf
man 1 make-kpkg
mount ffs
# -ro
mount -t ufs -o ro,ufstype=44bsd  /dev/sda3 /mnt
reinstall grub
To reinstall  simply boot to your Debian install disk, live CD or 
whatever else you have, switch to the other console (Alt-F2), then mount 
your root filesystem (mount -t ext2 /dev/whatever /target ; mount --bind 
/dev /target/dev), then chroot into it (chroot /target) then run "mount 
/proc ; update-grub && grub-install /dev/hda".  --dpkg in #debian
iptables.rules
cat <<'EOF'> iptables.rules

*filter

# Reject all other inbound - default deny unless explicitly allowed policy:
 -P INPUT DROP
 -P FORWARD DROP
 -P OUTPUT DROP

# Reject packets that are out-of-sequence are silently dropped
-A INPUT -m state --state INVALID -j DROP

# Allows all loopback (lo0) traffic and drop all traffic to 127/8 that doesn't use lo0
 -A INPUT -i lo -j ACCEPT
 -A INPUT ! -i lo -d 127.0.0.0/8 -j REJECT

# Accepts all established inbound connections
 -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT

# Allows all outbound traffic
 -A OUTPUT -j ACCEPT

# Allows SSH connections from our LAN
 -A INPUT -p tcp -m state --state NEW -s 192.168.1.0/24 --dport 22 -j ACCEPT
# Allow ping
 -A INPUT -p icmp -m icmp --icmp-type 8 -j ACCEPT

# log iptables denied calls (access via 'dmesg' command)
 -A INPUT -m limit --limit 5/min -j LOG --log-prefix "iptables input denied: " --log-level 7

COMMIT
EOF
ip6tables.rules
cat <<'EOF'> ip6tables.rules
*filter

# Allows all loopback (lo0) traffic
 -A INPUT -i lo -j ACCEPT
 -A OUTPUT -o lo -j ACCEPT

# Reject all other inbound - default deny unless explicitly allowed policy:
 -P INPUT DROP
 -P FORWARD DROP
 -P OUTPUT DROP
COMMIT
EOF
# iptables-restore <  iptables.rules
# iptables -n -v -L
# iptables-save > /etc/iptables.rules
# ip6tables-restore <  ip6tables.rules
# ip6tables -n -v -L
# ip6tables-save > /etc/ip6tables.rules
# create /etc/network/if-pre-up.d/iptables:
#!/bin/bash
/sbin/iptables-restore  < /etc/iptables.rules
/sbin/ip6tables-restore < /etc/ip6tables.rules
# chmod +x /etc/network/if-pre-up.d/iptables
disable iptables
iptables -F
iptables -X
iptables -t nat -F
iptables -t nat -X
iptables -t mangle -F
iptables -t mangle -X
iptables -P INPUT ACCEPT
iptables -P OUTPUT ACCEPT
iptables -P FORWARD ACCEPT

Logging in debian with rsyslog:

#cat /etc/rsyslog.d/iptables.conf
:msg, contains, "iptables denied" -/var/log/iptables.log
& ~
# cat /etc/logrotate.d/iptables 
/var/log/iptables.log
{
        rotate 7
        daily
        missingok
        notifempty
        delaycompress
        compress
        postrotate
        invoke-rc.d rsyslog rotate > /dev/null
        endscript
}
# service rsyslog restart
http://wiki.debian.org/iptables

update-alternatives --display libgksu-gconf-defaults

disable gdm on centos /etc/inittab: id:3:initdefault: