asked on

Clear contents of a file

Let's say that I have a Java program running, with its standard output directed to a log file:

java MyJavaProgram > stdout.log

This program will keep on running. To ensure that the log file does not grow very big, I am trying to write a script to remove the contents of the file. I tried two ways:

echo "" > stdout.log

cp /dev/null stdout.log

But in both cases, they are writting a lot of null characters into the log file. The file does not shrink at all. The contents are merely replaced with null characters.

How can I achive what I want? Thanks.

mdrichards

You could remove the file then "touch" it

rm -f stdout.log
touch stdout.log

This is probably not the most ideal scenario as you "may" need to reset user / group permissions.

Tintin

You can only reliably truncate a file when it's not being written to.

Do you have any control over the java program, ie: can you restart it?

Easiest way to truncate a file is

>stdout.log

integrosys

ASKER

Thanks guys. Actually I found that these two lines:

echo "" > stdout.log
cp /dev/null stdout.log

will work if the Java program is started this way instead:

java MyJavaProgram >> stdout.log

ASKER CERTIFIED SOLUTION

Brian Utterback

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yongsing

Hi blu, what is a sparse file? You said that it does not allocate storage blocks. But when I use the ls command, I notice that the file size never decreases.

Tintin

If you use

du -sh file

You'll see the real size of the file.

Brian Utterback

Or use "ls -ls"