UNIX file systems

1. UNIX file systems Learning Objectives: To understand the basics of file systems To understand the hierarchical structure in Unix file system To learn the Unix commands for files / directories’ manipulations To learn the security & access permission for files / directories in Unix System

2. / bin homes lib . . . horner jbond . . . .mailrc 111 top10 . . . File Systems • What is a file system? A means of organizing information on the computer. A file system is a logical view, not necessarily a physical view. • What does the file system provide: • Ways to create, move, and remove files • Ways to order files • Security • Examples of file systems: • DOS, Macintosh, CD-ROM, UNIX, NFS (networked file system)

3. Directory Shorthands • “.” is the directory itself • “..” is the parent directory • In most shells “~” means your home directory) • ~user means user’s home directory, so: $ more ~jbond/.plan • looks at the file .plan in /home/jbond, which is jbond’s home directory.

4. Special Directories • “/” (pronounced “slash” and also called “the “root”) is the ancestor of all files in the file system • /bin and /usr/bin contain UNIX utilities (e.g., cat) • /dev contains special files which describe “devices” such as terminals and printers • /etc has administrative programs like password files • /tmp is for temporary files; periodically deleted • Every directory has at least two entries: “.” is the directory itself, and “..” is the directory’s parent

5. Simple filenames Can only be used if files are in working directory Relative pathname A string of directory references, beginning with the working directory. Examples: ./secret1 ../007/names top10/LG7soBad Absolute pathname A pathname beginning at the root. e.g.,: /homes/jbond/.plan /etc/passwd mkdir makes a new directory (if you have permission to do so). With a simple pathname, rmdir removes a directory Pathnames Directory Commands

6. Directory Commands • You can copy a directory tree to another tree $ cp -R secret secret2 • You can delete a whole directory tree (Be careful!) $ rm -R secret2 • You may want to use the -i option for prompting: $ rm -Ri secret2 rm: examine files in directory secret2 (yes/no)? y rm: remove secret2/letter1 (yes/no)? y rm: remove secret2/letter2 (yes/no)? y rm: remove secret2: (yes/no)? y

7. Security and Access Permissions (1) • There are three types of users: • The owner of the file (user) • The group of the file (group) • Anyone else (other) • There are three types of permission (independent of each other): • Read permission • Write permission • Execute permission

8. Security and Access Permissions (2) • Use ls -l to see file permissions -rw-r--r-- 1 jbond cs 154 Feb 4 15:00 letter3 • There are four sets of items in the permissions: -rw-r--r-- • The type is: “-” regular files, “d” directories , “l” symbolic links. • The next nine characters indicate if the file is readable, writable, or executable for the file owner, the file group, or other users, respectively. Byte size Name #links Group Last modification Permissions User type user group other

9. Security and Access Permissions (3) • ls -l $ ls -l total 34 -r-xr-xr-x 1 jbond cs 9388 Feb 4 16:31 cat* -rw-r--r-- 1 jbond cs 154 Feb 4 15:00 letter1 -rw------- 1 jbond cs 64 Feb 4 15:00 names drwxr-xr-x 2 jbond cs 512 Feb 4 15:41 newdir/ drwxr-xr-x 2 jbond cs 512 Feb 4 16:38 secret/ d--------- 2 jbond cs 512 Feb 4 16:39 secret1/ dr--r--r-- 2 jbond cs 512 Feb 4 16:39 secret2/ d--x--x--x 2 jbond cs 512 Feb 4 16:38 secret3/

10. Directory Permissions (1) • Can use ls -ld to lists a directory’s information (instead of its contents): $ ls -l secret total 4 -rw-r--r-- 1 jbond cs 154 Feb 4 16:38 letter1 -rw-r--r-- 1 jbond cs 34 Feb 4 15:00 letter4 $ ls -ld secret drwxr-xr-x 2 jbond cs 512 Feb 4 16:38 secret/ $ ls -ld secret* drwxr-xr-x 2 jbond cs 512 Feb 4 16:38 secret/ d--------- 2 jbond cs 512 Feb 4 16:39 secret1/ dr--r--r-- 2 jbond cs 512 Feb 4 16:39 secret2/ d--x--x--x 2 jbond cs 512 Feb 4 16:38 secret3/

11. Directory Permissions (2)

12. Changing Permissions (1) • The chmod command is used to modify permissions. • chmod can only be used by the owner of a file/dir (or the administrator root). • The arguments are: chmod [ugoa] [+-=] [rwxdd] [file/dir] In other words: • Optionally, one of the characters: u (user/owner), g (group), o (other), or a (all). • Optionally, one of the characters: + (add permission), - (remove permission), or = (set permission). • Any combination of the characters r (read), w (write), or x (execute).

13. Changing Permissions (2) • Another way to change permission is to use numbers representing the permissions. The arguments are: chmod nungno [file/dir] Code table (3 bits) --- 0 r-- 4 --x 1 r-x 5 -w- 2 rw- 6 -wx 3 rwx 7 nu - user’s permission code ng - group’s permission code no - other’s permission code -rwxrwxrwx

14. Permission Example • To let everybody read or write the file letter1 $ chmod a+rw letter1 (chmod 666 letter1) $ ls -l letter1 -rw-rw-rw- 1 jbond cs 154 Feb 4 15:00 letter1 • To allow user to execute file letter1 $ chmod u+x letter1 (chmod 766 letter1) $ ls -l letter1 -rwxrw-rw- 1 jbond cs 154 Feb 4 15:00 letter1* • To not let “other” to read or write file letter1 $ chmod o-rw letter1 (chmod 760 letter1) $ ls -l letter1 -rwxrw---- 1 jbond cs 154 Feb 4 15:00 letter1* • To let “group” only read the file letter1 $ chmod g=r letter1 (chmod 740 letter1) $ ls -l letter1 -rwxr----- 1 jbond cs 154 Feb 4 15:00 letter1*