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How to clear the contents of a file from the command line?
You can use the user command : truncate
truncate -s 0 test.txt
("-s 0" to specify the size)
http://www.commandlinefu.com/commands/view/12/empty-a-file
https://superuser.com/questions/90008/how-to-clear-the-contents-of-a-file-from-the-command-line