Transcript Linux+ Guide to Linux Certification

Chapter 9 Part IV
Linux Advanced Command Line
• Filesystem Hierarchy Standard (FHS)
– Standard of outlining the location of set files
and directories on a Linux system
– Gives Linux software developers the ability
to locate files on a Linux system regardless
of the distribution
• This allows them to create software that is not
distribution specific
The Filesystem Hierarchy Standard
(FHS)
Table 5-1: Linux directories defined by FHS
The Filesystem Hierarchy Standard (FHS)
Table 5-1 (continued): Linux directories defined by FHS
Linking Files
• Files may be linked to another in one of
two ways:
– One file may simply be a pointer or a shortcut
to another file (known as a symbolic link or
symlink)
– The two files may share the same data
(known as a hard link)
Linking Files
• To better understand how files are linked, you must
understand how files are stored on a filesystem
• Structurally, a filesystem has three main sections:
– Superblock: section of info: # of inodes & data blocks
– inode table: each inode describes 1 file or directory and
contains a unique inode number for identification
• Other info: file size, data block locations, last date modified,
permissions, & ownership
• Since directories are files they have an unique inode also
– Data blocks: filename & file contents (data)
• Also called allocation units
• Directories data blocks contain a list of files located within it
Hard Link Files are direct copies of one another,
as they share the same inode & inode number.
The structure of hard linked files
Linking Files
• ln (link) command
– Command used to create hard and symbolic links
• To create hard link, you must use the ln command
and specify two arguments:
– The existing file to hard-link
– The target file that will be created as a hard link to the
existing file
Try This: create file1 with touch or one of the editors
$ ls -l i
(check parameters. i displays inode)
$ ln file1 file2
(file2 is direct copy)
$ ls –li
(have parameters changed?, what
about the inode numbers)
Linking Files
Symbolic links are
sometimes called soft links.
Symbolic links do not share
the same inode, they are
merely a pointer to the other,
thus will have different sizes.
Create file3
$ ln –s file3 file4
(-s symbolic option)
$ ls –li
(what’s different, what’s the same?)
of symbolically
linked
$ ls –FFigure 5-2: The structure
(what’s
different, what’s
thefiles
same?)
Listing the Contents of a Folder
• The ls command is used to list the contents of a folder and
information about files, by default the current directory.
# ls
–al
total 109
drwxr-xr-x 18 root
drwxr-xr-x 18 root
drwxr-xr-x 2 root
drwxr-xr-x 3 root
root
root
root
root
4096 Jun
4096 Jun
4096 Jun
1024 Jun
9 21:12 ./
9 21:12 ../
9 21:14 bin/
9 20:32 boot/
File type, permissions, hard link count, file owner, group owner, size, mod date,
directory name, filename or pointer.
File and Directory Permissions
• Recall that all users must successfully login with a
username and password to gain access to a Linux
system
• Once logged in, users are identified by their username
and group memberships
– All access to resources depends on whether their
username and group membership have the required
permission
• When a user creates a file or directory, that user’s name
and primary group become the owner and group owner
of the file, respectively
• Primary group
– Default group to which a user belongs
File and Directory Ownership
• chown (change owner) command
– Command used to change the owner and the group of a
file or directory
– Takes two arguments at a minimum:
• The new owner and the files or directories to change
$ chown userx file1
chgrp (change group) command
– Command to change the group owner of a file or directory
– Takes two arguments at a minimum:
• The new group owner and the files or directories to change
$ chgrp sys file1
Changing both with chown
$ chown userx.sys file1
Managing File and Directory
Permissions
Mode
– The section of the inode that stores permissions
– Divided into three sections based on the user(s) that
receive(s) the permission to that file or directory
• User (owner) permissions
• Group (group owner) permissions
• Other (everyone on the Linux system permissions
Managing File and Directory
Permissions
• There are three regular permissions that
you may assign to each of the user(s)
referenced on the previous slide:
– Read
– Write
– Execute
– And – permission is unavailable
Interpreting the Mode
Figure 5-3: The structure of a mode
Interpreting the Mode
• User or Owner
– When used in the mode of a certain file or directory, it
refers to the owner of that file or directory
• User
– User whose name appears in a long listing of a file or
directory and who has the ability to change
permissions on that file or directory
• Group
– Typically users in the same company department
• Other
– When used in the mode of a certain file or directory, it
refers to all users on the Linux system
Interpreting Permissions
Table 5-4: Linux permissions
Changing Permissions
• chmod (change mode) command
– Used to change the mode (permissions) of a file or dir
– Takes two arguments at a minimum:
• The first argument specifies the criteria used to change permissions
• The remaining arguments indicate filenames to change
-rw-r--r-$ chmod u=rwx,g=rw,o=rw
$ chmod u+x,g+w,o+w
$ chmod 766 file1
-rwxrw-rw-rwxrw-rw-rwxrw-rw-
Changing Permissions
Figure 5-4: Numeric
representation of the mode
Default Permissions
• Umask
– Used to alter the permissions on all new files and
directories by taking select default file and directory
permissions away
– Only applies to newly created files and directories
• Will never be used to modify the permissions of
existing files and directories
Default Permissions
Figure 5-5: Performing a umask 007 calculation
Special Permissions
• Read, write, and execute are the regular
file permissions used to assign security to
files
• Three more special permissions that you
may optionally use on file and directories:
– SUID (Set User ID)
– SGID (Set Group ID)
– Sticky bit
Defining Special Permissions
• The SUID has no special function when
set on a directory
– However, if the SUID is set on a file and that
file is executed, then the person who
executed the file temporarily becomes the
owner of the file while it is executing
• The SUID can only be applied to binary
compiled programs
Defining Special Permissions
• The SGID has a function when applied to both
files and directories
• The SGID allows regular users to execute a
binary compiled program and become a member
of the group that is attached to the file during
execution of the program
• The sticky bit was used on files in the past to
lock them in memory
– Today, the sticky bit performs a useful function only
on directories
Setting Special Permissions
• The mode of a file that is displayed using
the ls –l command does not have a section
for special permissions
• Special permissions require execute
– They mask the execute permission when
displayed using the ls –l command
Setting Special Permissions
Figure 5-7: Representing special permissions in the mode
Setting Special Permissions
Figure 5-8: Representing special permissions in
the absence of the execute permissions
Setting Special Permissions
Figure 5-9: Numeric representation of regular and special permissions
The grep Command
• grep
– Stands for Global Regular Expression Print
– Used to display lines in a text file that match a
certain common regular expression
– Search is case sensitive unless –i used
– -v reverse meaning of previous command
$ grep “ CIS 130” file5
• Use the egrep command to display lines of text that match
extended regular expressions
• The fgrep command does interpret any regular
expressions and consequently returns results much faster
than the egrep command
Viewing Processes
• There are several Linux utilities that can
view processes
• ps command
– The most versatile and common Linux utility
that can view processes
– Without arguments, the ps command simply
displays a list of processes that are running in
the current shell
Viewing Processes
• top command
– Most common command used to display
processes aside from ps
– Displays its interactive screen listing
processes organized by processor time
• Processes that use the most processor time are
listed at the top of the screen