Files. Unix Way. Unix approach to. Files are central to UNIX. Files Processes Devices

(1)

1

Unix Way

Unix approach to

Files

Processes

Devices

Files

Files are central to UNIX

Commands

System privileges and permission control

Device I/O

Inter-process communication

(2)

3

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

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

(3)

5

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

File permission

File Access Types

Access classes

User access (u)

Group access (g)

Other access (o)

Make it your current directory (cd to it) Run executable

file x

Alter directory contents ( e.g. delete files in it).

Alter file contents.

w

Search directory contents (e.g. use ls, ls *.txt).

View file contents.

r

Meaning on Directory Meaning on File

Access

(4)

7

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

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.

(5)

9

CASE STUDY

New employee Sandy joined the

department and needs to share the access to some files owned by John.

Possible ways:

1. Copy all the files from John to Sandy

2. Make all the files permission 777.

3.

File

Special Purpose Access Modes

Set mandatory file locking on read/writes.

File locking l

Set process group ID on execution SGID

s

Set process user ID on execution SUID

s

Keep executable in memory after exit

Sticky bit t

Meaning Name

Code

(6)

11

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

Files

Numerical equivalents for special access modes

Additional octal digit is prepended to the mode

Setuid, setgid, sticky bit

Example

4755: setuid

6755 : setuid and setgid

1777: stick bit

(7)

13

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.

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?

(8)

15

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

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

(9)

17

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

File

Example:

Show file types with -l

[ruihong@dafinn filetypes]$ lslslsls ----ld * /dev/log /dev/hda1ld * /dev/log /dev/hda1ld * /dev/log /dev/hda1ld * /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]$ lslslsls ----ldFldFldF * /dev/log /dev/hda1ldF* /dev/log /dev/hda1* /dev/log /dev/hda1* /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]$ lslslsls ----ldFildFildFi *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/file /etc/file /etc/file /etc/passwdpasswdpasswd /bin/passwd/bin//bin//bin/lslsls /dev/log /dev/hda1ls/dev/log /dev/hda1/dev/log /dev/hda1/dev/log /dev/hda1

(10)

19

Processes

What is a process?

A single program that is running in its own virtual address space.

Process type

Interactive program

Associated with a terminal session

Control on foreground and background

Batch processes

Not associated with any terminal

Submitted to a queue and run it later

UNIX provide the command batch, at

Others: Network Queuing System to support load balancing among workstations in the network.

Processes

Daemons

Server processes

Often initiated at boot time

Run continuously while the system is up

Idle until a request arrives

Examples:

init

syslogd

sendmail

lpd, lpsche, ..

crond,

Gettty

Pagedaemon

Inetd

Named

Nfsd

Ntpd

Portmap, rpcbind

.. …

(11)

21

Process

Process attributes

Process id (PID)

Parent process IP (PPID)

Nice number– different from priority

TTY

Real and Effective User ID (RUID, EUID)

Real and Effective Group ID (RGID, EGID)

SUID and SGID File access and Process Execution

Allow ordinary users to perform tasks requiring

privileges and access rights which are ordinarily denied to them

Example:

[ruihong@dafinn ~]$ ls -l /usr/bin/passwd

-r-s--x--x 1 root root 18992 Jun 5 2003 /usr/bin/passwd

Example:

$ps –eo pid,ppid,tty,ruser,euser,rgroup,egroup,cmd

Process Life Cycle

The life cycle of a process

Fork-and-exec

Inherit the env values from parent process

Example:

Run a grep _Init

fork Init

Pid 1 Pid 424 exec

Getty

Pid 424 exec login

Pid 424 exec

sh fork sh

(12)

23

Processes Controlling

$kill %2 kill

$bg Restart stopped background process bg

$fg %?em Refers to the background job

command containing the characters in str.

%?str

$fg %1 bring background process to

foreground fg

Refers to background job number n

%n

$jobs List background processes

jobs

^Z stop foreground process

^Z

$long-cmd&

Run command in the background

&

Processes and files

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,

(13)

25

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.

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

(14)

27

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

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

(15)

29

Devices: Naming conventions

Example: Special files for Other devices

Null devices.

/dev/null

Swap device /dev/swap

Map of kernel virtual memory /dev/kmem

Console device /dev/console

Virtual terminal console

Serial line (hardwired terminal/modem) /dev/tty1

/dev/ttyS1 /dev/ttyn

/dev/ttySn

Cdrom device /dev/cdrom

Non rewind tape device /dev/nrmt1

/dev/nrmtn

Tape devices /dev/rmt1

/dev/rmtn

Floppy disk /dev/fd0

/dev/[r]fdn*

Device/Use Example

Special file form

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

(16)

31

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

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

(17)

33

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 header files

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.