Files

1. Files • Files are central to UNIX • Commands • System privileges and permission control • Device I/O • Inter-process communication

2. File ownership • File Ownership • User owner • Group owner • The group owner is independent of user owner • Example: Displaying file ownership “ls –l” drwxrwxr-x 2 pop 403 512 Sep 24 11:47 acm drwxr-xr-x 9 pop csdept 1024 Oct 13 15:54 acm.contest -rw-r----- 1 john csdept 261 Mar 11 1999 address

3. More about file ownership • Who owns new files? • Owner is the one who create it • Group owner • On System V, current group of the user • On BSD-based systems, the group owner of the directory • Changing file ownership • Use chown , chgrp • Traditional System V, both superuser and user owner are allowed • POSIX-compliant SystemV and BSD, only superuser can change

4. More about file ownership • More about changing file ownership • Use recursive option –R #chown –R harvey /home/xyz • Use find command # find /home/xyz –print | xargs chown test • Change owner and group using chown #chown new-owner:new-group files • Change group owner #chgrp newgroup file BSD: non-root users of chgrp must be • The owner of the file and • A member of the new group to change

5. File permission • File Access Types • Access classes • User access (u) • Group access (g) • Other access (o) • Example • -rw-rw-r--

6. File permission • Setting file protection • chmod accessstring_list files • Access string has three parts: • Access_classes operator access_type • Access classes u, g, o, a • Operator: +, -, = • Access type: any combination of r,w,x • For example: • #chmod g+w testfile • Recursive option: -R • #chmod –R go-rwx /home/xyz • Read the mannual of chmod for more options

7. File permission • Specifying numeric file modes • Example: r-xr—r– is 544 • Specifying the default file mode • umask • Specify the inhibited access permission with a three digit numeric mode. • Example: if umask is 077 • 077 is 000 111 111 • No r,w,x for group and others. Possible rwx for owners.

8. CASE STUDY • New employeeSandy joined the department and needs to share the access to some files owned by John. • Possible ways: • Copy all the files from John to Sandy • Make all the files permission 777.

9. File • Special Purpose Access Modes

10. File • Sticky bit on directories • If set, a user can only delete files that she owns or she has explicit write permission. • Example: /tmp [ruihong@dafinn ~]$ ls -ld /tmp drwxrwxrwt 27 root root 4096 Jan 13 14:58 /tmp • Set it with User access class • #chmod u+t /tmp • Setgid access on directories • Files created in this directory will have the same group ownership as the directory itself. • Example: when groups of users need to share a lot of files, with s bit, correct group ownership will be set for new files. • #chmod g+s files

11. Files • Numerical equivalents for special access modes • Additional octal digit is prepended to the mode • Setuid, setgid, sticky bit • Chmod u+s • Chmod g+s • Chmod o+s • Example • 4755: setuid • 6755 : setuid and setgid • 1777: stick bit

12. File • How to recognize a File access problem • File ownership or protection problem • If root can do it, then … • Example • The temporary files with the same name created by a different users still exist • Application switched the group ownership behind the scene. • A administrator decided to protect /dev/null, which caused some editor stopping working.

13. inode • Mapping Files to Disks - inode • Data structure • Created when initialize the disk – Max number • Typically, one inode for every 2 – 8Kbytes file storage • Information stored • User owner and group owner ID’s. • File type • Access modes • File creation, access and modification times • Inode modification time • Number of Links to the file • Size of the file • Disk addresses specifying/leading to • No file name in inode, • Where is file name stored? Where is the mapping between name and inode?

14. File • File Types • Regular files • Directories • A directory is a binary file. • Directory entries are filename-inode pairs • Special files • Two types of special files • Character special files • Block special files • Located under /dev • Links • Several filenames to refer to a single file on disk • Hardlink and softlinks

15. File • More file types • Unix domain sockets • Communication connection points • Printing system • Syslog (/dev/log) • Named pipes • Also known as FIFO • Communication between programs • Commonly used to avoid writing temporary files • like regular pipe

16. File • Using “ls –l “to identify file types • - Plain file (hard link) • d Directory • l Symbolic link • b Block specific file • c Character special file • s socket • p Named pipe

17. File • Example: • Show file types with -l [ruihong@dafinn filetypes]$ ls -ld * /dev/log /dev/hda1 brw-rw---- 1 root disk 3, 1 Sep 15 2003 /dev/hda1 srw-rw-rw- 1 root root 0 Nov 30 10:08 /dev/log -rw------- 2 ruihong csdept 0 Jan 4 09:43 gold.dat -rw------- 2 ruihong csdept 0 Jan 4 09:43 hlink lrwxrwxrwx 1 ruihong csdept 8 Jan 4 09:43 slink -> gold.dat drwx------ 2 ruihong csdept 512 Jan 4 09:48 testdir • Show types with -F [ruihong@dafinn filetypes]$ ls -ldF * /dev/log /dev/hda1 brw-rw---- 1 root disk 3, 1 Sep 15 2003 /dev/hda1 srw-rw-rw- 1 root root 0 Nov 30 10:08 /dev/log= -rw------- 2 ruihong csdept 0 Jan 4 09:43 gold.dat -rw------- 2 ruihong csdept 0 Jan 4 09:43 hlink lrwxrwxrwx 1 ruihong csdept 8 Jan 4 09:43 slink -> gold.dat drwx------ 2 ruihong csdept 512 Jan 4 09:48 testdir/ • Show hard link –i [ruihong@dafinn filetypes]$ ls -ldFi * 2624881 -rw------- 2 ruihong csdept 0 Jan 4 09:43 gold.dat 2624881 -rw------- 2 ruihong csdept 0 Jan 4 09:43 hlink 2624882 lrwxrwxrwx 1 ruihong csdept 8 Jan 4 09:43 slink -> gold.dat 2675566 drwx------ 2 ruihong csdept 512 Jan 4 09:48 testdir/ • Check file type using file command, which uses /etc/magic or /usr/share/file/magic [ruihong@dafinn filetypes]$ file /etc/passwd /bin/ls /dev/log /dev/hda1 /etc/passwd: ASCII text /bin/ls: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), for GNU/Linux 2.2.5, dynamically linked (uses shared libs), stripped /dev/log: socket /dev/hda1: block special (3/1)

18. Files and processes • The Relationship between commands and files • Build-in shell commands • Executable files • Search path • $PATH $echo $PATH /usr/bin:/usr/ucb:/bin:/usr/local/bin:.:/$HOME/bin • The order is important in $PATH • Edit $HOME/.profile or $HOME/.login • Check $PATH for error message “Command not found” • Most administrative utilities are located under /etc, /usr/sbin, /sbin

19. Devices • Allow device I/O operations to look just like file I/O • CASE study • A junior SA was asked to backup all the files under /apg to a tape • The correct tape device name was /dev/rmt2 • The backup command should look like • #tar cvf /apg /dev/rmt2 • Well, the SA typed the command with a typo • #tar cvf /apg /dev/rnt2 • What will happen? • Will the command error out because of the wrong tape? • Will the command finish without error? Where did the stuff go? • As a matter of fact, in the middle of the backup, users started to get errors saying / was full. • Warning: Better to check the tape by reading it after the backup finish.

20. Devices • Example: disk drives • Disk partitions • Root partition/system disk • Disk special files (partitions, modes) • Mounting a disk partition (filesystem) into the Unix directory hierarchy • Mount /dev/disk0a /home

21. Devices: Naming conventions • On HP-UX, • /dev/rdsk/c2t4d0s0 • Where • c2: the controller number • t4: driver number on the controller • d0: logical unit number for SCSI devices Will be 0 only if more than one disk per target, say storage array. • S0: partition number on that drive

22. Devices: Naming conventions • Solaris Device Naming Conventions • Referenced in three ways • Physical device name - /devices • Instance name /etc/path_to_inst • Logical device name - /dev (links to /devices) • Linux Device Naming • Default is device type plus an incrementing value based on the order in which devices are found • Ethernet devices: eth0, eth1 • SCSI and Serial ATA disks: /dev/sda, dev/sdb • IDE devices: /dev/hda, /dev/hdb • Linux 2.6 kernels and “udev” tool enable administrators to assign arbitrary names to devices

23. Devices: Naming conventions • Example: Special files for Other devices

24. Devices • List the devices on a system • HP-UX: ioscan • Tru64 UNIX: hwmgr • Linux: • List pci devices “lspci” • List SCSI devices “scsiinfo–l” • List hard disk parameters “ hdparm” • Solaris: • Print system configuration: prtconf • Lists devices: getdev

25. The Unix Filesystem Layout • Common directories • / • Root directory • /bin • Executables for user commands and utilities. • Some files are links to file under /usr/bin • /dev • Device directory, may includes sub-dirs such as dsk, mnt, pts, etc. • /etc and /sbin • System configuration files and executables • Boot scripts • /etc/default • hold default parameter values for various commands • /home or /usr/users • /lost+found • Files marked as in use on disk, but not listed in any directory – found by fsck • There is a lost+found on every disk partition

26. The Unix Filesystem Layout • More about common directories • /mnt • Temporary mount directory • /proc • Designed to enable processes to be manipulated using UNIX file accessing system calls. • Linux puts more files about system configuration • /tmp • Available to all users as a scratch directory. • Normally, one of the UNIX startup script will clear /tmp. • /usr • Subdirs for programs, share libraries, administrative commands • /var • Spooling and other volatile directories. • Print spooling, mail system, cron facility • Optional softwares • Log files • /stand on HP – kernel image • /kernel on Solaris – kernel image

27. The Unix Filesystem Layout • The /usr directory • /usr/bin • Command binary files and shell scripts • X system: /usr/bin/X11 • /usr/include • Include files *.h, C-language headerfiles • Operating system include file /usr/include/sys • /usr/lib • Standard C libraries for mathematics and I/O • /usr/local • By convention, /usr/local/bin holds file that were developed locally or retrieved from other sources.

28. The Unix Filesystem Layout • More about /usr directory • /usr/share • Shared among a group of networked systems for static data files, … • /usr/share/man • Manual pages • Subdir /usr/share/man/man# for every man section • /usr/ucb • Contains standard UNIX commands originally developed under BSD.

29. Filesystem mounting/unmounting • Mount a file system to the point of tree • #mount /dev/sda4 /users • Umount • #umount /users • Check what process hold references • #fuser –c /users

30. Access Control List • Traditional UNIX 9bit permission • Windows has a much more complicated way – Access control lists • POSIX started ACL for Unix • Extension to support multiple user/groups • NSFv4 ACL • A union of all (UNIX + Windows) ACLs

31. ACL in Linux • Entries: • User::perms • User:username:perms • Group::perm • Group:groupname:perm • Other::perms • Mask::perms • Implemented at the file system level • Disabled by default • Enable by mounting as –o acl • Try to match the single most appropriate entry • Set/Get access control list – setfacl,getfacl

32. Example: grant write to a user $ ls –l -rw-r--r-- 1 test test 0 Sep 21 14:57 file $ setfacl -m user:ruihong:w file $ ls -l file -rw-rw-r--+ 1 test test 0 Sep 21 14:57 file $ getfacl file # file: file # owner: test # group: test user::rw- user:ruihong:-w- group::r-- mask::rw- other::r--

33. NFSv4 • Linux does not support it • Can map POSIX acl to NFSv4 acl • Solaris does support it • See the textbook for more info