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7.1.2.1. Emptying files

Sometimes the content of a file doesn't interest you, but you need the file name as a marker (for instance, you just need the timestamp of a file, a reminder that the file was there or should be there some time in the future). Redirecting the output of a null command is how this is done in the Bourne and Bash shells:


andy:~> cat wishlist > placeholder

andy:~> ls -la placeholder
-rw-rw-r--    1 andy   andy       200 Jun 12 13:34 placeholder

andy:~>  > placeholder

andy:~> ls -la placeholder

-rw-rw-r--    1 andy   andy         0 Jun 12 13:35 placeholder

The process of reducing an existing file to a file with the same name that is 0 bytes large is called truncating.

For creating a new empty file, the same effect is obtained with the touch command. On an existing file, touch will only update the timestamp. See the Info pages on touch for more details.

To "almost" empty a file, use the tail command. Suppose user andy's wishlist becomes rather long because he always adds stuff at the end but never deletes the things he actually gets. Now he only wants to keep the last five items:


andy:~> tail -5 wishlist > newlist

andy:~> cat newlist > wishlist

andy:~> rm newlist

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