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Using commandoutput as a file descriptor
Description is moved to "Sample output" because the html sanitizer for commandlinefu breaks the examples..

Number of CPU's in a system
/proc/cpuinfo contains information about the CPU. Search for "processor" in the /proc/cpuinfo file wc -l, counts the number of lines.

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"

Display duplicated lines in a file
Displays the duplicated lines in a file and their occuring frequency.

Compress a series of png pictures to an avi movie.
cd /path/to/your/png_pics mencoder "mf://*.png" -mf fps=2 -o output.avi -ovc lavc -lavcopts vcodec=mpeg4 then you can get a movie named output.avi.

For a $FILE, extracts the path, filename, filename without extension and extension.
Useful for use in other scripts for renaming, testing for extensions, etc.

generate iso

start vim in diff mode
vim will open both files side by side and show colored diffs

Getting started with tcpdump
At some point you want to know what packets are flowing on your network. Use tcpdump for this. The man page is obtuse, to say the least, so here are some simple commands to get you started. -n means show IP numbers and don't try to translate them to names. -l means write a line as soon as it is ready. -i eth0 means trace the packets flowing through the first ethernet interface. src or dst w.x.y.z traces only packets going to or from IP address w.x.y.z. port 80 traces only packets for HTTP. proto udp traces only packets for UDP protocol. Once you are happy with each option combine them with 'and' 'or' 'not' to get the effects you want.

Display a block of text: multi-line grep with perl
-n reads input, line by line, in a loop sending to $_ Equivalent to while () { mycode } -e execute the following quoted string (i.e. do the following on the same line as the perl command) the elipses .. operator behaves like a range, remembering the state from line to line.


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