6

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?

Improve this question

2 Answers 2

Reset to default
9

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.

Improve this answer
1
  • Thank you for your answer, Kyle Jones. So I will use --exclude option to exclude files I don't want to be overwritten. Commented Feb 28, 2012 at 1:30
3

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
Improve this answer

You must log in to answer this question.

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.