Question

I want to empty the file which is more than 30 MB size. I tried to solve that using find command.

   find . -maxdepth 1 -size +32M -exec cat /dev/null > {} \;

and

   find . -maxdepth 1 -size +32M -exec echo -n > '{}' \;

But the both commands doesn’t empty the file.Instead of that ,it creates a {} file.

why it doesn’t empty the file? and why it creates a file called {}?.
what is wrong with the find command?

Answer 1

Solution

You must use sh -c in this case:

 -exec sh -c "> {}" \;

Example of usage

Preparing files:

$ echo hello > a1
$ cp a1 a2; cp a1 b1; cp a1 b2
$ ls -l
total 16
-rw-r--r-- 1 ic ic 6  6. Aug 14:45 a1
-rw-r--r-- 1 ic ic 6  6. Aug 14:45 a2
-rw-r--r-- 1 ic ic 6  6. Aug 14:45 b1
-rw-r--r-- 1 ic ic 6  6. Aug 14:45 b2

Executing the command:

$ find . -name 'a*' -exec sh -c '> {}' \;

Checking the results:

$ ls -l
total 8
-rw-r--r-- 1 ic ic 0  6. Aug 14:45 a1
-rw-r--r-- 1 ic ic 0  6. Aug 14:45 a2
-rw-r--r-- 1 ic ic 6  6. Aug 14:45 b1
-rw-r--r-- 1 ic ic 6  6. Aug 14:45 b2
$ cat a1
$ cat b1
hello

As you can see, files that are starting with a are now empty; and other files are not.

Explanation

When you make something like

find . -maxdepth 1 -size +32M -exec cat /dev/null > {} \;

you make this redirection not inside the exec command, but outside of it.

( find . -maxdepth 1 -size +32M -exec cat /dev/null )  ===> ( {} \; )

Of course it is incorrect. You must make this redirection inside the command that is executed by find. To do that, you must run a new shell inside the exec command. You make this using sh -c.