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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" }

Calculate N!

journalctl -f
a tail -f variant of systemd journal. Follow the most recent updates or if events are appended to the journal

grep for minus (-) sign
Use flag "--" to stop switch parsing

useful tail on /var/log to avoid old logs or/and gzipped files
with discard wilcards in bash you can "tail" newer logs files to see what happen, any error, info, warn...

coloured shell prompt
This coloured prompt will show: username in green, grey "@" sign, hostname in red, current directory in yellow, typed commands in green.

a function to find the fastest DNS server
http://public-dns.info gives a list of online dns servers. you need to change the country in url (br in this url) with your country code. this command need some time to ping all IP in list.

Which processes are listening on a specific port (e.g. port 80)
swap out "80" for your port of interest. Can use port number or named ports e.g. "http"

old man's advice

Find the package that installed a command


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