Tar overwrites read only files

Question

I created directory test, created file 1.txt in test, wrote 'Before' in this file. Then I went

cd ..

and used the command:

tar -cvzf ./test.tgz ./test

Then I entered the test dir again. Opened the 1.txt file again. Changed content to "After". I saved the file and changed chmod to read only by executing:

chmod -w ./1.txt

So for now my 1.txt is read only. Then I go up

cd ..

and extract the test.tgz archive.

tar -xvzf ./test.tgz

Then I go again to test dir, do

cat 1.txt

and get "Before".

It is not logical that this happens, since the file was set to be read-only. Why does it happen?

Answer 1

Tar didn't overwrite the existing read-only file, it removed it and then created a new file with the same name. This is a consequence of the way -x works; it replaces existing versions of a file by design in order to accommodate the old incremental backup method of appending files to an existing archive. A tar archive might have multiple versions of a file in it; only the last one will appear on disk after extraction is completed. A side effect of this is that tar also removes existing copies of files even if they appear only once in the archive.

Answer 2

The fact that a file cannot be removed does not depend on a file's permissions being set to read-only, but on the permissions of the parent directory.

When you have write access to a directory, you can remove read-only files in that directory even if you do not own those files yourself (link).

# file removal test
(
mkdir -p test
echo hello > test/file.txt
chmod -w test
chmod -w test/file.txt
sudo chown root:wheel test/file.txt
#sudo chflags uchg test/file.txt
ls -ld test test/file.txt
rm -fv test/file.txt
echo
chmod 700 test  # set write permission
ls -ld test test/file.txt
rm -fv test/file.txt
)

If you remove the write access to the test directory in your example above, gnutar (on Mac OS X) cannot remove test/1.txt and complains:

# gnutar: ./test/1.txt: Cannot open: File exists
# gnutar: Exiting with failure status due to previous errors
# After

tartest() {
   dirperms="$1"
   mkdir -p test
   [[ -f test/1.txt ]] && chmod +w test/1.txt
   echo "Before" > test/1.txt
   gnutar -czf ./test.tgz ./test
   echo "After" > test/1.txt
   chmod -w test/1.txt
   chmod "${dirperms}" test
   gnutar -xzf ./test.tgz
   cat test/1.txt
   return 0
}

tartest 755
tartest 555