Understanding Users and File Systems

1
Understanding Users and File Systems

• Chapter Four

2
Linux Users and Groups

• Must first log in using valid user account name
  and password
• More user accounts on Linux system
• More work required to keep them running smoothly

3
/etc/login.defs

• /etc/login.defs - Configuration control
  definitions for the login package. Three
  items must be defined MAIL_DIR, ENV_SUPATH, and
  ENV_PATH. If unspecified, some arbitrary (and
  possibly incorrect) value will be assumed. All
  other items are optional - if not specified
  then the described action or option will be
  inhibited. Comment lines (lines beginning
  with "") and blank lines are ignored.
  Modified for Linux. --marekm

4
Policy Settings

• PASS_MAX_DAYS Maximum number of days a
  password may be used. PASS_MIN_DAYS Minimum
  number of days allowed between password
  changes. PASS_WARN_AGE Number of days warning
  given before a password expires.PASS_MAX_DAYS
  99999PASS_MIN_DAYS 0PASS_WARN_AGE 7
  Min/max values for automatic uid selection in
  useraddUID_MIN 1000UID_MAX 60000 Min/max
  values for automatic gid selection in
  groupaddGID_MIN 100GID_MAX 60000

5
Types of User Accounts

• Preconfigured user accounts
• Root
• Administrative account
• Also called superuser
• Can perform any operation on Linux system
• Do not log in as root for normal work
• Change temporarily to root user
• Not present as a default on Ubuntu

6
Types of User Accounts (continued)

• su command
• Temporarily changes access rights to those of
  another user
• Without any parameters
• Change to root account
• Hyphen after su command
• su - tsmith
• Causes command to run login scripts
• Places you in login directory of user

7
Types of User Accounts (continued)

• Preconfigured user accounts
• Regular user accounts
• Users who log in at keyboard and use Linux system
• Commonly associated with named individuals
• Special user account
• Used by Linux programs
• Created during installation of Linux
• Vary depending on services installed

8
Linux Groups

• Group
• Collection of user accounts
• Can be collectively granted access to files and
  directories
• Each user in Linux is assigned to primary group
• /etc/group file
• Lists members of each defined group

9
Linux Groups (continued)
10
User and Group Files

• /etc/passwd
• Stores user account information
• Password information not stored in this file
• Contents include
• User account name
• Password
• User ID number (UID)
• Group ID number (GID)
• Users real name
• Home directory
• Default shell

11
User and Group Files (continued)

• /etc/group file
• Defines groups on Linux system
• vigr command
• Edit this file
• Contents include
• Name of group
• Group password
• GID number
• Members of group

12
Shadow Passwords

• /etc/shadow
• Stores encrypted passwords
• Can only be read by
• Root user
• Certain utilities
• Shadow password suite
• Collection of password-related programs
• Have been modified to recognize /etc/shadow file

13
Changing User Passwords

• Can change password of any user account on system
• When logged in as root
• Use passwd command
• Standard procedure
• System administrator assigns initial password to
  new account
• User immediately selects new password

14
Changing User Passwords (continued)

• To change users password
• Use passwd without any parameters
• Must enter current password

15
Changing User Passwords (continued)

• Good password characteristics
• At least eight characters long
• Include digits or punctuation marks
• Mix uppercase and lowercase letters
• Easy for account owner to remember
• Hard for anyone else to guess
• Not created from simple manipulation of
• Word found in dictionary
• Name of person or place

16
User Information Commands

• id command
• Shows effective UID
• logname command
• View user name that you used to log in
• whoami command
• Shows user name of currently effective UID
• groups command
• Lists all groups you are a member of

17
User Information Commands (continued)

• who command
• Lists all regular users on system
• Shows location where they are logged in
• w command
• Information from who command, plus
• Time person logged in
• Program they are running
• Information about system resources user is
  consuming

18
File Permissions

• Files and directories have
• Owner
• Group
• Linux determines who can access file or directory
  based on
• Who owner is
• Which group is assigned to object
• File permissions define access granted to file or
  directory
• Permissions are cumulative for user and group

19
File Permissions (continued)

• Access mode
• Permissions
• Read permission (r)
• Write permission (w)
• Execute permission (x)
• Permissions can be assigned by
• User permissions
• Group permissions
• Other permissions

20
File Permissions (continued)
21
Changing Ownership

• chown command
• Change user and group assigned to file or
  directory
• Can only use when logged in as root
• Example chown jtaylorManagers report.doc
• Can use graphical file manager to change owner of
  file
• chgrp command
• Change group assigned to file or directory
• Example chgrp managers report.doc
• newgrp project
• User is a member of the group project
• Change group membership to project for all files
  or folders created during a session

22
Changing File Permissions

• chmod (change mode) command
• Change file permissions
• Regular users can alter permissions assigned to
  any file or directory that you own
• Example chmod ow reportDoc
• Adds write permission to other
• Example chmod urwx,grx,o reportDoc
• Explicitly sets permissions for user, group, and
  other
• System administrators normally use shortcut syntax

23
Changing File Permissions (continued)

• Alternate syntax
• Each of sets of three permissions is represented
  by number from 0 to 7
• 24 22 20
• 110 100 000
• rw- r-- ---
• Example chmod 640 report.doc
• Graphical environment provides easy method of
  setting file permissions
• Properties dialog box
• Permissions tab

24
Changing File Permissions (continued)
25
Changing File Permissions (continued)
26
Special Permissions

• SUID Set User ID
• Allows user to temporarily become the owner of
  the file
• SGID Set Group ID
• Allows user to temporarily become a member of the
  group owner of the file
• Sticky Bit
• User with write permission may modify file, but
  not delete files that they do not own.
• May delete their own files.

27
Default File Permissions

• umask command
• Defines mask to stop certain permissions from
  being granted by default when files created
• Executed automatically when you log in to Linux
• Uses same three-digit permission codes as chmod
  command
• 24 22 20
• 000 010 111
• umask 027
• Removes write permissions from group and all
  permissions from other, subtract from 777
• rw-r----

28
Partitions and File Systems

• Partition
• Distinct area of hard disk
• Has been prepared to store particular type of
  data
• File system
• Arrangement of information on device such as hard
  disk
• df command
• See status of all currently accessible file
  systems

29
Partitions and File Systems (continued)

• Linux can access a number of file system types
• Ext2 permissions
• Ext3 permissions and journaling
• Reiserfs many small files
• Vfat - DOS
• ISO-9660 CD ROM read only
• Udf - DVD, CD-RW, CD-R
• Ntfs read-only

30
Partitions and File Systems (continued)
31
Inodes and Links

• inode
• Hold information about files
• Within ext2 or ext3 file system
• Has associated number
• Controls file to which it points
• Does not contain file name
• File record contains
• File name
• Inode number for file

32
Inodes and Links (continued)

• Directory record contains list of files with
  corresponding inode numbers
• Link allows two or more file records to refer to
  same physical data stored in file system
• Symbolic link
• File that refers to another file or directory,
  rather than containing data itself
• Used when same data must be accessed from two
  locations in directory structure
• Takes only a few bytes of hard disk space

33
Inodes and Links (continued)

• ln command
• Create symbolic link
• Use -s option
• Syntax ln -s be created
• Hard link
• File record that includes file name and inode
• Just like regular file record
• Refers to inode that already has file record
  pointing to it

34
Inodes and Links (continued)
35
Inodes and Links (continued)
36
File Types

• Different types of files are used to perform
  different functions
• stat command
• Ties together file name with inode
• Shows information about inode

37
File Types (continued)
38
Accessing Removable Media

• To use any file system
• Must be mounted
• mount command
• Instruct Linux how to access file system
• Mount point
• Path in directory structure where you access data
  in file system
• umount command
• unmount media
• umount /dev/fd0

39
Accessing Removable Media (continued)

• fdformat command
• Format 3.5-inch disk
• fdformat /dev/fd0
• /dev/fd0
• Device name for 3.5-inch disk drive
• sudo mkdir /media/floppy
• sudo mount t vfat /dev/fd0 /media/floppy
• cd /media/floppy to access disk
• sudo gedit /etc/fstab
• Modify mount points when logging on.
• Displays partition numbers to mount points.
• /dev/fd0 /media/floppy vfat rw,user,noauto 0 0

40
Accessing Removable Media (continued)

• Mounting a CD ROM
• mount -t iso9660 /dev/cdrom /media/cdrom
• Mounting a zip disk
• mkdir /media/zip
• Add to /etc/fstab
• /dev/sda1 /media/zip vfat noauto,rw,user,nosuid,sy
  nc
• mount /media/zip
• cd /media/zip

41
Mounting Windows Partition

• sudo mkdir /media/win1
• sudo gedit /etc/fstab
• /dev/hda1 /media/win1 vfat rw,user,umask000 0 0
• Partition number, mount point, file system,
  allows all users to read and write, 0 -
  filesystem does not need to be dumped, 0 - file
  system does not need to be checked (1 - reserved
  for root filesystem, 2 - all other drives to be
  checked sequentially)

42
Using find

• find command
• Helps find objects matching exact search criteria
• Doesnt use prebuilt index
• Recursively search from starting directory
• find /home -name "report.doc"
• Find file matching name
• find /home -user horvathe
• Find all files owned by horvathe
• find /home -size 2048M
• Find files larger than 2 Gbytes

43
Using find

• Other options
• mtime - modified time
• Example find /home atime -1
• -1 less than one day
• 3 more than three days
• mmin - modified minutes
• -15 less than fifteen minutes
• 30 more than thirty minutes
• atime - access time
• amin access minutes
• See http//www.ss64.com/bash/find.html

44
Managing File Archives

• System administrators often work with files that
  include
• Compressed data
• Multiple files in archival format

45
Compressing Files

• Compressing files
• Useful way to use less space for rarely accessed
  data
• Makes files smaller before transmitting them over
  network
• gzip command
• Compress any file best compression 60-70 of
  original size
• Lempel-Ziv Compression
• gzip v sample sample.gz
• -v verbose option
• zcat sample.gz to view compressed file contents
• gunzip command
• Uncompresses file compressed using gzip
• gunzip v sample.gz

46
Compressing Files

• bzip2 command
• Compress any file
• bzip2 v sample sample.bz2
• -v verbose option
• bzcat sample.bz2 to view compressed file contents
• bunzip2 command
• Uncompresses file compressed using gzip
• bunzip2 v sample.bz2

47
Using tar and cpio for Archiving Files

• tar archive
• Tape Archive
• Single file that can contain other files and
  directory structure
• tar command
• Create tar archive
• Specify files to be included in backup archive on
  command line
• Writes data to file name or device

48
Using tar for Archiving Files

• tar and compress an entire directorys contents
• tar zcvf backup.tar.gz
• View contents of archive
• tar ztvf backup.tar.gz
• tar to a tape device
• tar cvf /dev/st0
• Extract and uncompress
• tar zxvf backup.tar.gz
• Extract from a tape device
• tar xvf /dev/st0
• Tarball archive and compressed
• .tar.gz or .tgz

49
Using tar and cpio for Archiving Files (continued)

• cpio command
• Copy in and out
• Similar to tar
• Reads from STDIN channel for file names
• Writes data back to STDOUT
• Examples
• Archive
• find /home u horvathe cpio vocB O
  horvathe.cpio
• Extract
• cpio vicB I horvathe.cpio
• tar and cpio
• Help create incremental or multilevel backup