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Unlock more space form your hard drive
This command changes the reserved space for privileged process on '/dev/sda' to 1 per cent.

Prepend a text to a file.
Using the sed -i (inline), you can replace the beginning of the first line of a file without redirecting the output to a temporary location.

diff two unsorted files without creating temporary files
bash/ksh subshell redirection (as file descriptors) used as input to diff

Convert diff output to HTML ins/del

Diff files over SSH
Sometimes you need to compare two config files on different servers. Put the file names into the above script and let 'er rip.

Calculate days on which Friday the 13th occurs (inspired from the work of the user justsomeguy)
Friday is the 5th day of the week, monday is the 1st. Output may be affected by locale.

Create a 5 MB blank file via a seek hole
Similar to the original, but is much faster since it only needs to write the last byte as zero. A diff on testfile and testfile.seek will return that they are the same.

Fibonacci numbers with awk
Another combination of seq and awk. Not very efficient, but sufficiently quick.

Find out how old a web page is
I used to use the Firefox "View page info" feature a lot to determine how stale the web page I was looking at was. Now that I use mostly Chrome I miss that feature, so here is a command line alternative using wget. The -S says to display the server response, the --spider says to not download any files/pages, just fetch the header. The output goes to stderr, so to grep it you use 2>&1 to combine the stderr stream with stdout, the pipe that to grep for Last-Modified. You can use curl instead if you have it installed, like this: $ curl --head -s http://osswin.sourceforge.net | grep Mod

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"


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