1. Chapter 4: Understanding Users and File Systems
The Complete Guide to Linux System Administration

2. Objectives Create and manage user and group accounts
View and set access permissions on files and directories
Understand how file systems are configured and accessed
Perform basic file compression and archiving tasks
The Complete Guide to Linux System Administration

3. 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
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4. 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
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5. Types of User Accounts (continued)
System  Administration Users & Groups
Then the Query for root password will appear.
(eg or linux5)
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6. Type of users
The Complete Guide to Linux System Administration

7. 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
Causes command to run login scripts
Places you in login directory of user
$ su -
Password: #
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8. 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
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9. Linux Groups Group Each user in Linux is assigned to primary 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
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10. Linux Groups (continued)
The Complete Guide to Linux System Administration

11. 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)
User’s real name
Home directory
Default shell
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12. 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
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13. Shadow Passwords /etc/shadow Shadow password suite
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
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14. 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
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15. Changing User Passwords (continued)
To change user’s password
Use passwd without any parameters
Must enter current password
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16. Change password The Complete Guide to Linux System Administration
Suppose your system includes a user account Jizw.
To change her password, do the following:
1. Make sure you're logged in as root (typically by using the su command).
2. Enter the command passwd lizw.
Changing password for user lizw
New password:
3. Type the new password for the Linux user account and press Enter. Nothing
appears on screen as you type, so work carefully.
The following text appears after you press Enter:
Retype new password:
4. Type the new password a second time, exactly as you typed it the first time.
This verifies that the password was entered as you intended to type it. When
you press Enter the second time, the following text appears:
passwd: all authentication tokens updated successfully
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17. Changing User Passwords (continued)
Good password characteristics
At least five 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
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18. User Information Commands
id command
Shows effective UID id
uid=500(nwells) gid=500(nwells) groups=500 (nwells)
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
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19. User Information Commands (continued)
who command
Lists all regular users on system
Shows location where they are logged in
$ who
Root tty3 Mar 26 10:33
Nwells : Mar 26 10:29
Nwells pts/1 Mar 26 10:30 (:0.0)
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20. User Information Commands (continued
w command
Information from who command, plus:
Time person logged in
Program they are running
Information about system resources user is consuming
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21. File Permissions
Each file and each directory in Linux is assigned an owner
the owner is one of the users who has an account on the system.
Each file and directory also has a group assigned to it
the group must be one of the groups named in the /etc/groups file
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
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22. File Permissions (continued)
Access mode
Permissions
Read permission (r)
Write permission (w)
Execute permission (x)
Permissions can be assigned by:
User permissions (u)
Group permissions (g)
Other permissions (o)
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23. File Permissions (continued)
The Complete Guide to Linux System Administration

24. 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
# chown jtaylor *
If you add the -R option to the chown command, the command is recursive, meaning that it
changes the owner on any files in subdirectories of the current directory that match the file name given.
# chown –R pascal (pascal is a directory)
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25. Changing Ownership
Can use graphical file manager to change owner of file
chgrp command
Change group assigned to file or directory
Example: chgrp managers report.doc
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26. Changing File Permissions
chmod (change mode) command
To use the chmod command, include the type of permissions you want to change
(user u , group g , or other o),
followed by a + to add , - to remove or = to set all permissions,
followed by the permissions you want to set (r for read , W for write , or x for execute ).
Change file permissions
Regular users can alter permissions assigned to any file or directory that you own
Example: chmod o+w reportDoc
System administrators normally use shortcut syntax
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27. The following command changes those permissions to rwx:
For example, suppose the report.doc file has r-x permissions set for the group.
The following command changes those permissions to rwx:
$ chmod g+w report .doc
But the following command changes the group permissions to be only rw, removing the x permission
$ chmod g=rw report.doc
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28. Changing File Permissions (continued)
Alternate syntax
chmod -R ### <filename or directory>
-R is optional and when used with directories will traverse all the sub- directories of the target directory changing ALL the permissions to ###.
Each of sets of three permissions is represented by number from 0 to 7
for (r w x )
Example: chmod 640 report.doc
6 for u user (110) (r w -)
4 for g group (010) (r - -)
0 for o other (000) (- - -)
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29. The #'s can be:
0 = Nothing 1 = Execute 2 = Write 3 = Execute & Write  (2 + 1) 4 = Read 5 = Execute & Read (4 + 1) 6 = Read & Write (4 + 2) 7 = Execute & Read & Write ( )
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30. Eg.
$ chmod 640 report.doc
$ ls -l report.doc
-rw-r nwells nwells May 27 14:01 report.doc
$ chmod 755 program_file
$ ls -l program_file
-rwxr-xr-x 1 root root 2177 May 29 14:01 program_file
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31. Changing File Permissions (continued)
The Complete Guide to Linux System Administration

32. Graphical environment provides easy method of setting file permissions
Properties dialog box
Permissions tab
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33. Changing File Permissions (continued)
The Complete Guide to Linux System Administration

34. Default File Permissions
When the bash shell creates a new file because of actions you take at the command line (such as using the touch command or redirecting command output to a file), it creates the file with
these permissions:
rw-rw-rw-
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
The Complete Guide to Linux System Administration

35. umask
The value you give as a parameter to the umask command disables one or more of the 4 existing default permissions. Using the -S option on umask is helpful because it shows which permissions are currently allowed on new files:
$ umask -s
If you enter this next command, all permissions are permitted on new files:
$ umask 000
If you enter the following command, no permissions at all are permitted for other on new files:
$ umask 007
In most cases, you have no need to change the default "mask setting.
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36. Introducing the File System
Managing file system is basic task for system administrators
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37. Introduction to Linux
UNIX File System

39. File System Introduction to Linux
The Unix file system looks like an inverted tree structure.
You start with the root directory, denoted by /, at the top and work down through sub-directories underneath it.

40. File System Introduction to Linux
Each node is either a file or a directory of files, where the latter can contain other files and directories.
You specify a file or directory by its path name, either the full, or absolute, path name or the one relative to a location.
The full path name starts with the root, /, and follows the branches of the file system, each separated by /, until you reach the desired file, e.g.:
/home/condron/source/xntp

41. File System Introduction to Linux
A relative path name specifies the path relative to another, usually the current working directory that you are at. Two special directories :
. the current directory
.. the parent of the current directory
So if I'm at /home/frank and wish to specify the path above in a relative fashion I could use:
../condron/source/xntp
This indicates that I should first go up one directory level, then come down through the condron directory, followed by the source directory and then to xntp.

42. Structure of Standard Directories in Unix/Linux
Introduction to Linux
Structure of Standard Directories in Unix/Linux
/ The ancestor of all directories on the system; all other directories are subdirectories of this directory, either directly or through other subdirectories.
/bin Essential tools and other programs (or binaries).
/dev Files representing the system's various hardware devices. For example, you use the file `/dev/cdrom' to access the CD−ROM drive.
/etc Miscellaneous system configuration files, startup files, etc.

43. Structure of Standard Directories in Unix/Linux
Introduction to Linux
Structure of Standard Directories in Unix/Linux
/home The home directories for all of the system's users.
/lib Essential system library files used by tools in `/bin'.
/proc Files that give information about current system processes.
/root The superuser's home directory, whose username is root. (In the past, the home directory for the superuser was simply `/'; later, `/root' was adopted for this purpose to reduce clutter in `/'.)

44. Structure of Standard Directories in Unix/Linux
Introduction to Linux
Structure of Standard Directories in Unix/Linux
/sbin Essential system administrator tools, or system binaries.
/tmp Temporary files.
/usr Subdirectories with files related to user tools and applications.

45. Directories, Files and Inodes
Introduction to Linux
Directories, Files and Inodes
Every directory and file is listed in its parent directory.
In the case of the root directory, that parent is itself.
A directory is a file that contains a table listing the files contained within it, giving file names to the inode numbers in the list.
The information about all the files and directories is maintained in INODE TABLE
An Inode (Index Nodes) is an entry in the table containing information about a file (metadata) including file permissions, UID, GID, size, time stamp, pointers to files data blocks on the disk etc.

46. Partitions and File Systems
Distinct area of hard disk
Has been prepared to store particular type of data
That partition is marked with a code indicating that the partition contains (Windows or linux) data.
File system
Arrangement of information on device such as hard disk
df command (display file system)
See status of all currently accessible file systems
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47. Partitions and File Systems (continued)
Linux typically uses one of two default file system types:
ext3
Reiserfs
Windows typically uses one of two default file system types:
FAT32
NTFS
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48. Partitions and File Systems (continued)
The Complete Guide to Linux System Administration

49. df command
You can see the status of all currently accessible file systems using the df command (display file systems) .
The output shows you the device on which the file system resides, the total storage space, the amount of free space, and other infornlation.
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50. Within an ext2 or ext3 file system, a fixed number of entries are defined to hold information about files.
Each of these entries is called an inode (pronounced eye-node).
A file system typically contains more than enough inode entries for all the files you might possibly create on that file system.
Each inode has an associated number and contains information such as
the ID number for the owner and group assigned to the file,
the file permissions of the file,
the file size, the time the file was created, and numeric pointers to the areas of the hard disk that contain the file's data.
The Complete Guide to Linux System Administration

51. 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 the file record is an indirect pointer to the file's data, through the inode
contains:
File name
Inode number for file
You can view the inode number associated with each file by using the Is command with the -i option $ ls -i
The Complete Guide to Linux System Administration

52. Inodes and Links (continued)
Directory record contains list of files with corresponding inode numbers
Every subdirectory is just a list of file records.
Link allows two or more file records to refer to same physical data stored in file system
Symbolic link (also called a soft 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
The Complete Guide to Linux System Administration

53. For example, suppose several employees in a company want to work on the same file.
The system administrator can place the file in a directory and then create a symbolic link in each user's home directory to access the real file.
If the real file is /tmp/report.doc,
the symbolic links might be
/home/nwells/report.doc,
/home/davis/report. doc,
/ home/ laura/report.doc.
When users make changes after opening the file in their home directory, they are all changing the same file.
Symbolic links are used when the same data must be accessed from two locations in the directory structure, or by two (or more) different names.
Symbolic links are commonly used in directories such as llib and /usr/lib.
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54. Inodes and Links (continued)
ln command
Create symbolic link
Use -s option
Syntax: ln -s <existing file> <symbolic link to be created>
For example, if you wanted to make a link from the /etc/profile file to your home directory under the name main_profile, use the following command:
$ ln -s /etc/profile /home/nwells/main-profile
In the command above, you might decide to use relative path names, depending on your current working directory.
In more technical terms, a symbolic link is a file record that includes a path and file name, but not an inode number. When a user refers to a symbolic link, Linux looks at the path and file name given in the symbolic link's file record. The file record for that path and file name
includes an inode, which is used to access the file data for the symbolic link.
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55. 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
The hard link is a second file record pointing to the same physical data
A single inode can have numerous file records (hard links) pointing to it.
Both symbolic and hard links are used often in a Linux file system.
Whenever you use the Is -I command, any symbolic links are indicated as extra file names in the right column of the output.
The Complete Guide to Linux System Administration

56. example
An example of a symbolic link in Red Hat Linux is the view command. This output illustrates how you can use the Is -I command to show the view command:
$ Is -l /bin/view
lrwxrwxrwx 1 root root 2 Aug 12 13:36 /bin/view -> vi
The arrow in the right column indicates that the file named view is a symbolic link to the file named vi .
The letter I in the far left column of the screen output also indicates that the file is a symbolic link.
The number in the second column from the left (in this case, 1) indicates the number of file records that refer to the same inode as this file record.
The Complete Guide to Linux System Administration

57. Example 2
The zcat file name (also in Red Hat Linux) is an example of a hard link. The zcat file record refers to an inode to which two other file records also refer.
The Is -l command again shows this:
$ Is -l /bin/zcat
-rwx-r-xr-x 3 root root Mar 25 13:28 /bin/zcat
the file record holding the file name zcat refers to an inode to which two other file records also refer (for a total of three).
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58. Inodes and Links (continued)
The Complete Guide to Linux System Administration

59. Inodes and Links (continued)
The Complete Guide to Linux System Administration

60. File Types
Different types of files are used to perform different functions
Stat command
Ties together file name with inode
Shows information about inode number,the device on which the file is stored, the most recent access time, and the assigned file permissions.
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61. File Types (continued)
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62. 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
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63. mount $ mount /mnt/cdrom The /mnt/cdrom directory is a mount point
You use the mount command to make a new file system accessible via a mount point.
Using the mount command without any parameters displays a list of the currently mounted file systems.
Removable disks are not detected automatically. Before trying to access a 3.5-inch disk, you can mount it using this command:
$ mount /mnt/floppy
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64. unmount
After you have finished copying files to and from the floppy disk, or from a CD-ROM,
you should not eject the disk-you cannot eject the CD-ROM-until you
first unmount the media using the umount command
Before you can unmount a file system, you must change to a directory that is not in that file system;
if your working directory is /mnt/ cdrom or one of its subdirectories, you cannot unmount the CD-ROM.
These rwo examples show how you to unmount a CD-ROM or 3.5-inch
disk:
$ umount /mnt/floppy
$ umount /mnt/cdrom
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65. Accessing Removable Media (continued)
fdformat command
Format 3.5-inch disk
/dev/fd0
Device name for 3.5-inch disk drive
$ fdformat /dev/fd0
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66. Using find find command
Helps find objects matching exact search criteria
Doesn’t use prebuilt indexas as the slocate command does.
More slowly than slocate.
Provides many more options than slocate.
Example: find /home -name "report.doc" -print
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67. find
The simplest use of find is to search for files that match a specific pattern and display them onscreen.
In the following example, the path where the search should begin is /home;
the name of the file to search for is report. doc, and the action to take with each matching file name is to display (print) the name onscreen (displaying to the screen is the default action,
so the -print option shown here is not strictly necessary.
but it illustrates the format for more complex find operations):
# find /home –name “report.doc" -print
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68. Find
The find options let you perform complex searches for information on your Linux system.
For example, using a single (complex )find command, you can do any of the following tasks:
Create an archive file of all the files that have been modified in the last 24 hours.
Delete all files owned by a certain user on the Linux system.
Create a list of all files that are larger than a certain size.
Create a list of all files that have specific access permissions.
Create a list of all files that do not have a valid owner.
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69. find
The following command searches the /tmp directory (and all its subdirectories) for files owned by user wilsonr that have the jpg
file extension and are larger than 50 KB.
All matching file names are displayed onscreen and the files are immediately deleted.
# find /tmp -name "*jpg" –size 50k -user wilsonr -print -exec rm \{ \} \ ;
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70. Managing File Archives
System administrators often work with files that include:
Compressed data
Multiple files in archival format
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71. 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
For example, to compress the file large. doc, use this command:
$ gzip large.doc
The preceding command transforms the file large.doc into a compressed file called large.doc.gz
gunzip command
Uncompresses file compressed using gzip
$ gunzip large.doc gz
The resulting file is named large.doc
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72. Using tar and cpio for Archiving Files
tar 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
to extract the contents of an archive file named program.tgz, use this command (note that options for the tar command do not use a hyphen):
$ tar xvzf program.tgz
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73. When you use the tar command to extract the contents of a tar archive, the tar archive remains intact.
When you use the gzip or gunzip command to compress
or uncompress a file, the original file is altered (compressed or uncompressed), and its name is changed accordingly
The Complete Guide to Linux System Administration

74. 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
tar and cpio
Help create incremental or multilevel backup
The Complete Guide to Linux System Administration

75. example creating a full backup of the /home directory.
You can assume for this example that the device /dev/tape is configured as a tape drive.
# tar cf /dev/tape /home
If you had a small set of files to archive, you could send the archive to a 3.5-inch disk:
# tar cf /dev/fd0 /boot
c option of tar to create a new archive.
f option (for filename) followed by the device name indicates the location where the archived data will be stored. The last parameter, /home, indicates which files are to be archived
tar includes all files within that directory.
The Complete Guide to Linux System Administration

76. The Complete Guide to Linux System Administration
A cipo command equivalent to the above tar command is:
# find /home -print I cpio -0 > /dev/tape
This cpio command relies on the find command to generate a list of files (one file name per line) for cpio to back up.
Those file names are sent to cpio using a pipe symbol because cpio reads the file names in from STDIN.
The> redirection operator sends the archived files to the device /dev/tape.
The -0 option on cpio indicates that the archive is being output-that is, that data is being written out.
A simpler example of cpio can archive the contents of a single directory to a local file using the Is command to generate the list of files to archive:
# ls I cpio -o > /tmp/archive.cpio
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77. The v option is normally added to both tar and cpio so that the output of the command is verbose, meaning that the command prints details of what it is doing to the screen.
With that option added, the last example looks like this:
# ls I cpio -ov > /tmp/archive.cpio
The Complete Guide to Linux System Administration

78. Extract files # tar xvf /dev/fdo # cpio -idv < /dev/fd0
X for extract
The cpio command uses the -i option for input, again extracting the contents of the backup media into the current directory.
The -d option is also added here so that cpio creates subdirectories that existed in the data as required to re-create the original data organization.
When using the cpio command with the -i option, cpio reads the STDIN channel to get the archived data
so the < redirection operator is used with the file name or archive device name.
The Complete Guide to Linux System Administration

79. parameter with a value of 1: # find /home -mtime 1 –print
the following find command prints a list of all files in the /home directory (and its subdirectories) that have been modified in the last day by using the -mtime
parameter with a value of 1:
# find /home -mtime 1 –print
# find /home -mtime 1 -print I cpio -ov > /dev/tape
# tar cf /dev/tape ' find /home -mtime 1 -print'
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80. The Complete Guide to Linux System Administration

81. The Complete Guide to Linux System Administration

82. The Complete Guide to Linux System Administration

83. Summary User accounts form basis of file system security in Linux
Can change user that you are working as
Each user can belong to multiple Linux groups
Encrypted passwords are stored in /etc/shadow file
Linux file security is controlled by nine permissions
The Complete Guide to Linux System Administration

84. Summary (continued) File systems Hard and soft links
Organized collections of data
Created within partitions on hard disk
Hard and soft links
Refer to inode
Removable media must be mounted before it can be accessed
Linux includes several utilities for compressing files
The Complete Guide to Linux System Administration