Useful Linux Commands

export HISTTIMEFORMAT="%d/%m/%y  %H:%M:%S " 
export HISTSIZE=10000000 
export HISTFILESIZE=10000000 
shopt -s histappend histreedit histverify 
export HISTCONTROL=ignoreboth 
export HISTIGNORE="ls:ps:history:cd" 
export PS1="\[\e[32m\][\[\e[m\]\[\e[31m\]\u\[\e[m\]\[\e[33m\]@\[\e[m\]\[\e[32m\]\h\[\e[m\]:\[\e[36m\]\w\[\e[m\]\[\e[32m\]]\[\e[m\]\[\e[32;47m\]\[\e[m\] " 
alias vi='vim'
PROMPT_COMMAND="history -a"
shopt -s no_empty_cmd_completion # bash>=2.04 only

Then source your ~/.bash_profile

. ~/.bash_profile

inline text editing

# changes 0.2 to 1.61 globally in the resolv.conf file 
sudo sed -i 's/0\.2/1\.61/g' /etc/resolv.conf 

# Grep removing comments 
grep -v '^\s*$\|^\s*\#' <filename>

useful firewall commands

flushes the firewall opens port 22 | 443 saves the configuration restarts the firewall

iptables -F 
iptables -A INPUT -p tcp -m tcp --dport 22 -j ACCEPT 
iptables -A INPUT -p tcp -m tcp --dport 443 -j ACCEPT 
iptables -A INPUT -p tcp --tcp-flags ALL NONE -j DROP 
iptables -A INPUT -p tcp ! --syn -m state --state NEW -j DROP 
iptables -A INPUT -p tcp --tcp-flags ALL ALL -j DROP 
iptables -A INPUT -i lo -j ACCEPT 
iptables-save > /etc/sysconfig/iptables 
iptables -L 
service iptables restart