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bash-quine
this command prints itself out. it doesn't need to be stored in a file and it isn't as easy as $ echo $BASH_COMMAND for information on quines see http://en.wikipedia.org/wiki/Quine_(computing)

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" }

Watch Network Service Activity in Real-time

Monitor all DNS queries made by Firefox

Block an IP address from connecting to a server
This appends (-A) a new rule to the INPUT chain, which specifies to drop all packets from a source (-s) IP address.

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"

Simplified video file renaming
I used this when I had a directory of movies from a camera. I wanted to watch a little of each movie, then rename it depending on what was in the movie. This did the trick for me.

create an uncompressed tar file of each child directory of the current working directory
First, use find to find directories exactly one level below current directory, then create a tar file using the directory as the basename.

list folders containing less than 2 MB of data
Just shortened the awk a bit and removed sed. Edit: I'm assuming there are no spaces in the path. To support white space in pathname try: $ awk '($1 < 2048) {sub(/^[0-9]+[ \t]+/,""); print $0}'

Testing reading speed with dd


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