Bash attributes

Cyhaxor · 05-14-2008, 04:48 AM

hello all,

I was playing around with the following commands:

Code:

sort, >, >>

Firstly I created a test.txt file and I wrote the days in order. One day per line. Then I opened the terminal and I did:

Code:

$ sort test.txt
As it was expected I got:
Friday
Monday
Saturday
Sunday
Thursday
Tuesday
Wednesday

Then I did:

Code:

$ sort test.txt >> test.txt
Again as it was expected I got into the test.txt the following content:
Monday
Tuesday
Wednesday
Thursday
Friday
Saturday
Sunday
Friday
Monday
Saturday
Sunday
Thursday
Tuesday
Wednesday

Then I decided to try and overwrite the contend with only the sorted list:

Code:

$ sort test.txt > test.txt
Unexpectedly I got an empty file :p

I know I could transfer the sorted list to a new file and then overwrite the content of the test.txt file with the content of the new file.. And finally delete the new file.

But my question is why bash couldn't overwrite directly the content of the test.txt and as a result I got an empty file?

Thanks in advance

technosapien · 05-14-2008, 04:54 AM

Quote:

Originally Posted by Cyhaxor

hello all,

(snip)

Then I decided to try and overwrite the contend with only the sorted list:

Code:

$ sort test.txt > test.txt
Unexpectedly I got an empty file :p

I know I could transfer the sorted list to a new file and then overwrite the content of the test.txt file with the content of the new file.. And finally delete the new file.

But my question is why bash couldn't overwrite directly the content of the test.txt and as a result I got an empty file?

Thanks in advance

I'm not sure WHY you got the results you did, but try this:

Code:

$ sort -o text.txt test.txt

and see if that works for you. It tells sort to sort the file in-place.

- A

Cyhaxor · 05-14-2008, 05:01 AM

worked like a charm

But I still wondering why my method fail. Any technical explanation would be appreciated! Thanks again

ilikejam · 05-14-2008, 06:01 AM

Hi.

When you run 'sort test.txt > test.txt' , the first thing that happens is that test.txt is opened in preparation for writing. Since it's a single '>' redirection, the existing file is unlinked (rm) before this happens, so the original test.txt ceases to exist (this is called clobbering).

So in effect, you're attempting to sort an empty file.

Dave

Cyhaxor · 05-14-2008, 06:26 AM

Quote:

Originally Posted by ilikejam

Hi.

When you run 'sort test.txt > test.txt' , the first thing that happens is that test.txt is opened in preparation for writing. Since it's a single '>' redirection, the existing file is unlinked (rm) before this happens, so the original test.txt ceases to exist (this is called clobbering).

So in effect, you're attempting to sort an empty file.

Dave

Wow I thought that with redirection (both > & >>) the output of a command is taken (located temporary in memory) and then passed to the file. So I expected the content to be taken and then overwrite the previous..

Thanks for the info Dave!

ilikejam · 05-14-2008, 07:47 AM

Quote:

Originally Posted by Cyhaxor

Wow I thought that with redirection (both > & >>) the output of a command is taken (located temporary in memory) and then passed to the file. So I expected the content to be taken and then overwrite the previous..

Thanks for the info Dave!

Cool. In a command pipe (and redirection I would imagine), there's only a few kB buffer in memory (looks like 4kB in 2.6 kernels), so command output effectively 'streams' between inputs, outputs, and files.

Dave