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watch iptables counters
Watch the number of packets/bytes coming through the firewall. Useful in setting up new iptables rules or chains. Use this output to reorder rules for efficiency.

Find the biggest files
Also: * find . -type f -exec ls -s {} \; | sort -n -r | head -5 * find . -type f -exec ls -l {} \; | awk '{print $5 "\t" $9}' | sort -n -r | head -5

guitar synthesizer in one line of C
outputs a f=220Hz guitar string sound (fifth string A) needs ALSA

dstat - a mix of vmstat, iostat, netstat, ps, sar...
This is a very powerful command line tool to gather statistics for a Linux system. http://dag.wieers.com/home-made/dstat/

Perl One Liner to Generate a Random IP Address

repeat a command every one second
Short method of "while x=0; do foo ; sleep 1 ; done"

GREP a PDF file.
PDF files are simultaneously wonderful and heinous. They are wonderful in being ubiquitous and mostly being cross platform. They are heinous in being very difficult to work with from the command line, search, grep, use only the text inside the PDF, or use outside of proprietary products. xpdf is a wonderful set of PDF tools. It is on many linux distros and can be installed on OS X. While primarily an open PDF viewer for X, xpdf has the tool "pdftotext" that can extract formated or unformatted text from inside a PDF that has text. This text stream can then be further processed by grep or other tool. The '-' after the file name directs output to stdout rather than to a text file the same name as the PDF. Make sure you use version 3.02 of pdftotext or later; earlier versions clipped lines. The lines extracted from a PDF without the "-layout" option are very long. More paragraphs. Use just to test that a pattern exists in the file. With "-layout" the output resembles the lines, but it is not perfect. xpdf is available open source at http://www.foolabs.com/xpdf/

See entire packet payload using tcpdump.

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" }

Scan a gz file for non-printable characters and display each line number and line that contains them.
Scans the file once to build a list of line numbers that contain non-printable characters Scans the file again, passing those line numbers to sed as two commands to print the line number and the line itself. Also passes the output through a tr to replace the characters with a ?


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