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Terms in this set (124)
id
display user identity
chmod
change a files node
umask
set the default file permissions
su
run a shell as another user
sudo
execute a command as another user
chown
change a file's owner
chgrp
change a file group ownership
passwd
change a user's password
a user
may own files and directories
a user
has control of their files and directories access to others
group
consisting of one or more users who are given access to files and directories of their owners
world
owner grants some access rights to everybody
when user accounts are created
users are assigned a number called a user id (uid)
after a new user is assigned a uid
then mapped to a username
primary group id
gid and may belong to additional groups
where are user accounts defined in?
/etc/passwd
where are user groups defined in?
/etc/group
what is another location that holds information about a user password?
/etc/shadow
what does the /etc/passwd define?
user(login)name,the uid, the gid, the accounts real name, the home directory, and the login shell
the first 10 characters of a file listing are called?
file attributes
the first character of a file listing is called?
file type
remaining 9 characters of a file listing is called
file mode
file mode is defined as
representing the read,write and execute permissions for the file owner, the files group owner, and everybody else
file type -
a regular file
file type d
a directory
file type l
a symbolic link, the remaining file attributes are always rwxrwxrwx and are dummy values, the real file attributes are those of the file the symbolic link point to
file type c
a character special file, refers to a device that handles data as a stream of bytes, like a terminal or modem
file type b
a block special file, refers to a device that handles data in blocks, like a hard drive or cd-rom drive
example of typing in id command
aadrin2002@aadrin2002-MS-7850:~$ id
uid=1000(aadrin2002) gid=1000(aadrin2002)
groups=1000(aadrin2002),4(adm),24(cdrom),27(sudo),30(dip),46(plugdev),108(lpadmin),124(sambashare)
file permission attribute r
allows a file to be opened and read
directory attribute r
allows a directory's contents to be listed if the execute attribute is also set
file attribute w
allows a file to be written or truncated, does not allow files to be renamed or deleted
the ability to delete or rename files is determined by?
directory attributes
directory attribute w
allows files within a directory to be created, deleted and renamed if the execute attribute is set
file attribute x
allows a file to be treated as a program and executed. program files written in scripting languages must also be set as readable to be executed
directory attribute x
allows a directory to be entered; e.g., cd directory
-rwx------
a regular file that is readable, writable and executable by the file's owner. no one else has any access
-rw-------
a regular file that is readable and writable by the file's owner. no one else has any access
-rw-r--r--
a regular file that is readable and writable by the file's owner. Members of the file's owner may read the file. the file is world readable.
-rwxr-xr-x
a regular file that is readable, writable, and executable by the file's owner. The file may be read and executed by everybody else
-rw-rw----
a regular file that is readable and writable by the file's owner and members of the file's owner group only
Lrwxrwxrwx
a symbolic link. All symbolic links have "dummy" permissions. The real permissions are kept with the actual file pointed to by the symbolic link.
drwxrwx---
A directory. The owner and the members of the owner group may enter the directory and create,rename, and remove files within the directory
drwxr-x---
A directory. The owner may enter the directory and create, rename, and delete files within the directory. Members of the owner group may enter the directory but cannot create, delete, or rename files
who can change the the mode of a file or directory?
only the file's owner or the superuser
i.e chmod
what are the two distinct ways that chmod supports to specify mode changes?
octal number representation and symbolic representation
octal numbers are used to
set the pattern of desired permissions
each digit of octal number represents
three binary digits
octal 0, binary 000
file mode ---
octal 1, binary 001
file mode --x
octal 2, binary 010
file mode -w-
octal 3, binary 011
file mode -wx
octal 4, binary 100
file mode r--
octal 5, binary 100
file mode r-x
octal 6, binary 110
rw-
octal 7, binary 111
rwx
Octal (base 8)
counting is done with numerals 0 to 7
hexadecimal (base 16)
counting down using numerals 0 through 9 plus letters A through F
binary (base2)
number system only has two numerals, o and 1
bit patterns
small portions of data represented on computers
example RBG color displays
most displays each pixel is composed of three color components, 8 bits red, 8 bits green, and 8 bits blue
a medium blue would be a 24 digit number
010000110110111111001101
each digit in a hexadecimal number
represents four digits in binary
each digit in Octal represents
3 binary numbers
a 24 digit medium blue can be condensed to
a 6 digit hexadecimal number 436FCD
the digits of in the hexadecimal number line up with the
the bits in the binary number, the red component is 43, the green 64 and the blue CD
three octal digits
set the file mode for owner, group owner, and the world
most common octal to binary mappings
only five to memorize
7
rxw
6
rw-
5
r-x
4
r--
0
---
symbolic representation in the form of
symbolic notation for specifying file nodes
symbolic notation divided in 3 parts
whom the change will affect, which operation will be preformed, and which permission will be set
to specify who will be affected
a combination of characters u, g, o and a is used
u
short for user but means the file or directory owner
g
group owner
a
short for all; the combination of u, g, and 0
if no character specified
then all will be assumed
+
permission added
-
permission taken away
a=
only the specified permissions are to be applied and that all others are to be removed
advantage of using symbolic notation over octal notation
symbolic notation allows one to set a single attribute without disturbing any of the others
symbolic notation permissions are specified with
r, w, and x characters
u+x
add execute permission for the owner
u-x
remove execute permission for the owner
+x
add execute permission for the owner, group, and world. equivalent to a+x
0-rw
remove the read and write permissions from anyone besides the owner and group owner
go=rw
set the group owner and anyone besides the owner to have read and write permission. If either the group owner or world previously had execute permissions, remove them
u+x, go=rx
add execute permission for the owner and set permissions for the group and others to read and execute. multiple specifications may be separated by commas
chmod recursive option warning
--recursive option acts on both files and directories, rarely want files and directories to have the same permissions
umask command controls
the default permissions given to a file when it is created
umask command uses octal notation to
express a mask of bits to be removed from a file's mode attributes
changing identities
taking on the identity of another user, often for testing an account
How to change
identities
3 ways
log out and back in as the alternative user
use the su command
use the sudo command
from within our own session the su command allows?
you to assume the identity of another user
the sudo command allows
an administrator to setup a configuration file called /etc/sudoers
/ect/sudoers
configuration file that defines specific commands that particular users are permitted to exectute under an assumed identity
run a shell with substitute user and group ids
su command is used to start a shell of another user
command syntax of a login shell for a specified user
su [-[1]] [user]
using the -l option with su
the resulting shell session is a login shell for the specified user. user environment is loaded and the working directory is changed to the user's home directory
if the user is not specified with su
the superuser is assumed
-l can be abbreviated
-
how to shell for the superuser?
[me@linuxbox ~]$ su -
Password:
[root@linuxbox ~]#
trailing #
indicates this shell has superuser privileges and current working directory is now the home directory for the superuser /root
pressing exit with superuser
returns to the previous shell
[root@linuxbox ~]# exit
[me@linuxbox ~]$
how to just execute a single command with su?
su -c 'command'
using quotes to enclose the command so its expansion occurs in then new shell instead of our own
sudo allows allows an ordinary user to execute commands in a very controlled way
user may be restricted to more or more specific commands and no others
sudo does not require
access to the superuser's password
to authenticate using sudo, the user uses his own password
important difference between su and sudo
sudo by default does not start a new shell, nor does it load another's user environment
sudo default behavior can be
modified with specifying various options, see sudo man page for the details
to see what privileges are granted by sudo what option is used?
sudo -l
Ubuntu disables by default?
disables logins to root account(by failing to set a password for the account)
How does Ubuntu grant superuser privileges by default?
by using the sudo command to grant superuser privileges
Why does Ubuntu use sudo instead of root privileges by default?
to keep people from running their operating system with su (-) root permissions at all times to avoid permission denied messages
chown
change file owner and group
what privileges are needed to use the chown command?
Superuser privileges are required
chown [owner][:[group]] file...
changing the file owner and/or the file group owner depending on the first argument of the command
chown
argument: bob
-changes the ownership of the file from its current owner to user bob
bob:users
changes ownership of the file from its current owner to user bob and changes the file group owner to users
:admins
changes the group owner to admins. the file owner is unchanged
bob:
change the file owner from the current owner to user bob and changes the group owner to the login group of user bob
to set or change a password
passwd [user]
passwd command enforces
strong passwords, will refuse to accept ones that are too short, too similar to previous passwords or dictionary words, or are too easily guessed
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