Data permission: what if adding only write permision to a file?

Question

What if a user got only write access to a file? Is he able to read the file or only to "write"? However, if you can write, you can also read, so this doesn't make sense?

The question is for Unix-like system also for windows system, I'm sure there is a difference right? Thank you

Answer 1

*nix:

For *nix systems you can simply set "write-only" which works as expected. There is no matter why it should not. I don't understand your assumtion of automatically get read access... it's wrong. You can e.g. append to the file with no problem.

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Windows:

It is possible to modify the permissions to only have "write"-access but my tests show that even it should work from what the properties show, it doesn't and always gives "Access denied". I think it is by design not meant to set only write on NTFS.

Answer 2

Linux

You could append text to the file even if you can't read it. Check this example:

Create a file with one line

nine@nine-laptop:~$ echo "Some text" >/tmp/testfile.txt

Show that file

nine@nine-laptop:~$ cat /tmp/testfile.txt
Some text

Make it write only

nine@nine-laptop:~$ chmod 200 /tmp/testfile.txt

Show file with permissions

nine@nine-laptop:~$ ls -l /tmp/testfile.txt  
--w------- 1 nine nine 10 nov 14 10:54 /tmp/testfile.txt 

Try to read it (which won't work)

nine@nine-laptop:~$ cat /tmp/testfile.txt  
cat: /tmp/testfile.txt: Access denied

Append text to end of file

nine@nine-laptop:~$ echo "Appended text" >>/tmp/testfile.txt 

Set permissions to read and write

nine@nine-laptop:~$ chmod 600 /tmp/testfile.txt  

Show the file (which now has one more line)

nine@nine-laptop:~$ cat /tmp/testfile.txt
Some text
Appended text

BSD

Works precisely the same as on Linux.

Windows

Adding weeks comment: It works the same in Windows. Exept there are several rights in write permission, like "Write data", "Append data", "Write Attributes", "Write Extended Attributes"..