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Generate trigonometric/log data easily

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"

list processes with established tcp connections (without netstat)
Uses lsof to list open network connections (file descriptors), grepping for only those in an established state

Convert files from DOS line endings to UNIX line endings
The old dos2unix from sysutils has been deprecated on Debian systems to this tool.

Print current runlevel
Prints current runlevel and system start time. On older systems it also shows the last init state. Pretty useful on remote systems, pretty useless on local ones :)

Check if a machine is online with better UI
I have used single packet, and in a silent mode with no display of ping stats. This is with color and UI improvement to the http://www.commandlinefu.com/commands/view/10220/check-if-a-machine-is-online. It is as per the enhancements suggested.

Suppress output of loud commands you don't want to hear from
Suppresses all output to /dev/null. This could be expanded to check for a -l command line option to log the stderr to a file maybe -l file or -l to log to default quietly.log. I'm finding that I use it more often than one would think.

Search manpages for a keyword
Search manpages for a keyword. Very useful when you don't know where to find the information.

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" }

Recursively remove 0kb files from a directory


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