Chapter 4: File Security & Permissions Also: Hard and Soft Links, see p77-80 &

File Permissions (1) u With respect to a particular file, Unix divides the set of all users on a system into three categories: –user v The owner of the file. –group users v Most of you are in the group Domain Users v Used for easier administration of access control. v Normally only the superuser can set up groups. v Users can be in more than one group. –others v Everyone else.

File Permissions (2)  Permissions can be viewed with the ls -l command mlc104[1] > ls -l total rw csnow 1117 Jul 23 15:49 bad.cpp drwx--x--x 2 csnow 2048 Jul 17 10:13 bibd/ drwxr-xr-x 2 csnow 512 Aug 27 23:18 cache/ -rw csnow 2081 Jul 23 15:49 tst2.s -rw-r-xr-- 1 csnow 1275 Jul 23 15:49 vecexpr.cpp -rw-r-xr-- File type - = file d = directory l=symbolic link User Permissions Group Permissions Other Permissions r read permission w write permission x execute permission

File Permissions (3) u Permissions are changed with the chmod command. u There are two syntaxes you can use: chmod DDD file [file...] –DDD are 3 octal digits representing bits of protection –rwx rwx rwx can be thought of as in binary rw- r-- r chmod 644 file

File Permissions (4) u chmod [ugoa][+-=][rwx] file [...] –This is the “symbolic” method. –chmod u+rwx file gives the User Read, Write, and eXecute –chmod g+rx file gives the Group Read and eXecute –chmod o-rwx file removes R, W, and X from Others –chmod a+x file gives All eXecute permission –chmod g=r file gives Group Read permission and makes sure it has nothing else u Symbolic modes can be appended with commas –chmod u=rwx,g-w,o-rwx file for instance

The umask command u umask sets the default permissions for any file you will create u Format is backwards to the chmod command –tells you which permissions will NOT be given v umask 077 means don't let anyone but the User do anything with my files by default u Generally set umask once in your.cshrc file and never set it again

Directory Permissions (1) u Directory permissions are different from the file permissions –Requires execute permission to access files in the directory and its subdirectories, (read/write) –Requires read permission to list the contents of the directory (does not affect the subdirectory) –Requires write permission to create files in the directory (does not affect the subdirectory)

Directory Permissions (2) mlc104[1] > ls -l drwx--x Jul 17 10:13 bibd/ mlc104[2] > ls -l bibd -r--r--rwx 173 Jul 17 10:13 readme u Files in bibd/ are accessible to user u Files in bibd/ are accessible by name (if you know the name) for group users u Files in bibd/ and subdirectories are not accessible to others.

Directory Permissions (3) u The -R option to chmod is useful when working with directories. –It recursively changes the mode for each chmod operand that is a directory. –All files and directories would receive those permissions. –chmod -R a+rw dir gives everyone read and write permission to each file under dir (not execute though!!!) –chmod -R a+rwx dir gives the executable access to allow people to actually access the files under dir v Makes all files executable though... –chmod -R a+rwX dir gives the executable access only to those files already executable (programs, directories, …)

Exercise – File permission u Create a directory dir1 in your home directory. u Edit a file test.txt in dir1. u Remove your own read permission of test.txt. u Try to display the content of test.txt by cat. u Restore your own read permission for test.txt u Remove your own write permission of test.txt u Use joe to edit the file u Make some changes to test.txt and try to save. u Try to delete the file test.txt

Exercise – Directory Permission u Create a directory dir2. –What is the permission of dir2? –What argument is provided to umask in your.cshrc file? u Copy test.txt to dir2/test2.txt u Remove your own ‘r’ permission of dir2. –Try to ls dir2. –cat dir2/test2.txt –cd dir2 –ls –cd.. u Set your own permission of dir2 to be r-x –cp test.txt dir2/test3.txt –rm dir2/test2.txt –edit the file dir2/test2.txt using an editor and save the changes u Set your own permission of dir2 to be rw- –cd dir2 –cat dir2/test2.txt –cp test.txt dir2/test3.txt –‘ls’ dir2 –ls dir2

Hard Links u When a file is created, there is one link to it. u Additional links can be added to a file using the command ln. These are called hard links. u Each hard link acts like a pointer to the file and are indistinguishable from the original. mlc104[1] > ls readme.txt mlc104[2] > ln readme.txt unix_is_easy mlc104[3] > ls -l -rw-r—r Jul 17 10:13 readme -rw-r—r Jul 17 10:13 unix_is_easy There is only one copy of the file contents on the hard disk, but now two distinct names! Note: identical information.

Figure 3-8 A Hard Link Each file on the disk has an inode which keeps information about the file, it’s address, etc

Figure 3-11 Multiple Hard Links to One File A directory simply connects each filename to its inode Multiple hard links simply duplicate the index to a file

Symbolic Links u A symbolic link is an indirect pointer to another file or directory. u It is a directory entry containing the pathname of the pointed to file. mlc104[1] > cd mlc104[2] > ln -s /usr/local/bin bin mlc104[3] > ls -l Lrwxr-xr-x bin -> /usr/local/bin …… mlc104[4] > cd bin mlc104[5] > pwd /usr/local/bin Notice the letter L before permissions Indicates soft link

Figure 3-09 A Symbolic Link Each symbolic link is another file in the system which points to the target file being linked to… More overhead than hard link

Hard vs Symbolic Links u Two hard links have the same authority to a file –Removing any one of them will NOT remove the contents of the file –Removing all of the hard links will remove the contents of the file from the hard disk. u A symbolic link is just an entry to the real name –Removing the symbolic link does not affect the file –Removing the original name will remove the contents of the file u Soft links can point to directories, hard links can’t u Soft links can point to other file systems, but hard links must point to files in the same file system