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Getting the ip address of eth0

Convert seconds to [DD:][HH:]MM:SS
Converts any number of seconds into days, hours, minutes and seconds. sec2dhms() { declare -i SS="$1" D=$(( SS / 86400 )) H=$(( SS % 86400 / 3600 )) M=$(( SS % 3600 / 60 )) S=$(( SS % 60 )) [ "$D" -gt 0 ] && echo -n "${D}:" [ "$H" -gt 0 ] && printf "%02g:" "$H" printf "%02g:%02g\n" "$M" "$S" }

tcpdump top 10 talkers
capture 2000 packets and print the top 10 talkers

Test how well a web server handles concurrent connections and big load.
This command will execute 1000 requests to the http URL http://127.0.0.1:8000 handlink 100 concurent connections at a time. Then it will display statistics about the time that have been taken.

Remove everything except that file
Remove everything except that file with shell tricks inside a subshell to avoid changes in the environment. $ help shopt

IP addresses connected to port 80
IP addresses and number of connections connected to port 80.

Get your external IP address without curl
Curl is not installed by default on many common distros anymore. wget always is :) $ wget -qO- ifconfig.me/ip

Superfast portscanner

check open ports without netstat or lsof

save date and time for each command in history
Date-time format: YYYY-MM-DD HH:MM:SS


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