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List all directories only.
Undocumented syntax, but should work on every shell. It'll list all directories in the current one. Change `*/` into globbing `**/` for recursivity.

Find files that are older than x days
Find files that are older than x days in the working directory and list them. This will recurse all the sub-directories inside the working directory. By changing the value for -mtime, you can adjust the time and by replacing the ls command with, say, rm, you can remove those files if you wish to.

Sort lines on clipboard
Does the same thing in environments where you have "xclip" instead of "pbpaste"/"pbpate" and "pbcopy".

Kill all processes that listen to ports begin with 50 (50, 50x, 50xxx,...)
Run netstat as root (via sudo) to get the ID of the process listening on the desired socket. Use awk to 1) match the entry that is the listening socket, 2) matching the exact port (bounded by leading colon and end of column), 3) remove the trailing slash and process name from the last column, and finally 4) use the system(…) command to call kill to terminate the process. Two direct commands, netstat & awk, and one forked call to kill. This does kill the specific port instead of any port that starts with 50. I consider this to be safer.

Remove all unused kernels with apt-get
Note the double space: "...^ii␣␣linux-image-2..." Like 5813, but fixes two bugs: [1]This leaves the meta-packages 'linux-headers-generic' and 'linux-image-generic' alone so that automatic upgrades work correctly in the future. [2]Kernels newer than the currently running one are left alone (this can happen if you didn't reboot after installing a new kernel). I'm bummed that this took 228 characters. I'd like to see a simpler version.

Find usb device in realtime
Using this command you can track a moment when usb device was attached.

Remove spaces from filenames - through a whole directory tree.
Sometimes, you don't want to just replace the spaces in the current folder, but through the whole folder tree - such as your whole music collection, perhaps. Or maybe you want to do some other renaming operation throughout a tree - this command's useful for that, too. To rename stuff through a whole directory tree, you might expect this to work: for a in `find . -name '* *'`;do mv -i "$a" ${a// /_};done No such luck. The "for" command will split its parameters on spaces unless the spaces are escaped, so given a file "foo bar", the above would not try to move the file "foo bar" to "foo_bar" but rather the file "foo" to "foo", and the file "bar" to "bar". Instead, find's -execdir and -depth arguments need to be used, to set a variable to the filename, and rename files within the directory before we rename the directory. It has to be -execdir and won't work with just -exec - that would try to rename "foo bar/baz quux" to "foo_bar/baz_quux" in one step, rather than going into "foo bar/", changing "baz quux" to "baz_quux", then stepping out and changing "foo bar/" into "foo_bar/". To rename just files, or just directories, you can put "-type f" or "-type d" after the "-depth" param. You could probably safely replace the "mv" part of the line with a "rename" command, like rename 'y/ /_/' *, but I haven't tried, since that's way less portable.

Install pip with Proxy
Installs pip packages defining a proxy

Display connections histogram
Displays a connection histogram of active tcp connections. Works even better under an alias. Thanks @Areis1 for sharing this one.

Exclude svn directories with grep
exclude-dir option requires grep 2.5.3


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