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kos
Asked: 2015-11-04 11:14:59 +0800 CST

Why does Tar preserve the original files' permissions even when not passing the `-p`, `--preserve-permissions` or `--same-permissions` switch?

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From man 1 tar:

[...]
     -p, --preserve-permissions, --same-permissions
           extract information about file permissions (default for superuser)
[...]
     --no-same-permissions
           apply the user's umask when extracting permissions from the archive
           (default for ordinary users)
[...]

From this I understand that by default extracted files' permissions are set based on the user's umask, unless the user is root:

% umask
002

So files extracted by me should have permissions 664 (666-002).

However:

% touch foo
% chmod +x foo
% ls -l
total 0
-rwxrwxr-x 1 user user 0 nov  3 19:36 foo
% tar cf foo.tar foo
% rm foo
% tar xf foo.tar 
% ls -l
total 12
-rwxrwxr-x 1 user user     0 nov  3 19:36 foo
-rw-rw-r-- 1 user user 10240 nov  3 19:36 foo.tar

I.e., Tar is preserving the permissions of the original file even though I didn't pass the -p, --preserve-permissions or --same-permissions switch.

Nonetheless if I pass the --no-same-permissions switch:

% tar xf foo.tar --no-same-permissions
% ls -l
total 12
-rwxrwxr-x 1 user user     0 nov  3 19:36 foo
-rw-rw-r-- 1 user user 10240 nov  3 19:36 foo.tar

Still Tar is preserving the permissions of the original file.

Can someone please explain why is that?

tar

1 Answers

  1. Best Answer

    Since adonis (who spotted the problem) didn't post an answer yet, I'll post an answer myself.

    [...]
     --no-same-permissions
           apply the user's umask when extracting permissions from the archive
           (default for ordinary users)
[...]

    This, contrarily to what I thought, means that the umask is applied to the permissions of the folders / files in the archive, and not to the conventional permissions of newly created folders / files (777 / 666) like I thought.

    I.e. a folder / file extracted without passing the -p, --preserve-permissions or --same-permissions switch won't have the permissions set to 777 & ~umask / 666 & ~umask, but to folder's/file's_archived_permissions & ~umask.

    In this specific case I was additionally fooled by the fact that applying an umask of 002 to a file with permissions 775 doesn't change anything, since 775 & ~002 = 775.

    So, in short, extracting a file with permissions 775 with an user umask of 002 produces, correctly, a file with permissions 775, result of 775 & ~002.

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