Chapter 3 Study Guide

CNT-2311-Chapter 3

Exploring Linux Filesystems

Objectives

Comparison of Windows and Linux filesystems
Navigate the directory structure using relative and absolute pathnames
Types of files
View filenames and file types
Use shell wildcards
Display contents of files
Search files for regular expressions using grep
Use vi to modify text files
Alternatives to vi
File Hierarchy

Comparison of Windows v. Linux filesystems

Navigate the directory structure using relative and absolute pathnames

start in home dir

use ~ to reference home directories

present dir = pwd

/home/randy/

change dir = cd

cd /etc - change directory to /etc

relative path name = path to the file in relation to the current working directory

".." references the next higher up dir

tab-completion!

Supported File Systems

Linux supports numerous file system types Ext2: This is like UNIX file system. It has the concepts of blocks, inodes and directories. Ext3: It is ext2 filesystem enhanced with journaling capabilities. Journaling allows fast file system recovery. Supports POSIX ACL (Access Control Lists). Isofs (iso9660): Used by CDROM file system. Sysfs: It is a ram-based filesystem initially based on ramfs. It is use to exporting kernel objects so that end user can use it easily. Procfs: The proc file system acts as an interface to internal data structures in the kernel. It can be used to obtain information about the system and to change certain kernel parameters at runtime using sysctl command. For example you can find out cpuinfo with following command:

Hierarchical File system

Often referred to as the "FHS" At the very top or the beginning is your / (forward slash) which represents your root directory. 1. / – Root Every single file and directory starts from the root directory. Only root user has write privilege under this directory. Please note that /root is root user’s home directory, which is not same as /.

2. /bin – User Binaries Contains binary executables. Common linux commands you need to use in single-user modes are located under this directory. Commands used by all the users of the system are located here. For example: ps, ls, ping, grep, cp.

3. /sbin – System Binaries Just like /bin, /sbin also contains binary executables. But, the linux commands located under this directory are used typically by system aministrator, for system maintenance purpose. For example: iptables, reboot, fdisk, ifconfig, swapon

4. /etc – Configuration Files Contains configuration files required by all programs. This also contains startup and shutdown shell scripts used to start/stop individual programs. For example: /etc/resolv.conf, /etc/logrotate.conf

5. /dev – Device Files Contains device files. These include terminal devices, usb, or any device attached to the system. For example: /dev/tty1, /dev/usbmon0

6. /proc – Process Information Contains information about system process. This is a pseudo filesystem contains information about running process. For example: /proc/{pid} directory contains information about the process with that particular pid. This is a virtual filesystem with text information about system resources. For example: /proc/uptime

7. /var – Variable Files var stands for variable files. Content of the files that are expected to grow can be found under this directory. This includes — system log files (/var/log); packages and database files (/var/lib); emails (/var/mail); print queues (/var/spool); lock files (/var/lock); temp files needed across reboots (/var/tmp);

8. /tmp – Temporary Files Directory that contains temporary files created by system and users. Files under this directory are deleted when system is rebooted.

9. /usr – User Programs Contains binaries, libraries, documentation, and source-code for second level programs. /usr/bin contains binary files for user programs. If you can’t find a user binary under /bin, look under /usr/bin. For example: at, awk, cc, less, scp /usr/sbin contains binary files for system administrators. If you can’t find a system binary under /sbin, look under /usr/sbin. For example: atd, cron, sshd, useradd, userdel /usr/lib contains libraries for /usr/bin and /usr/sbin /usr/local contains users programs that you install from source. For example, when you install apache from source, it goes under /usr/local/apache2 10. /home – Home Directories

Home directories for all users to store their personal files. For example: /home/john, /home/nikita

11. /boot – Boot Loader Files Contains boot loader related files. Kernel initrd, vmlinux, grub files are located under /boot For example: initrd.img-2.6.32-24-generic, vmlinuz-2.6.32-24-generic

12. /lib – System Libraries Contains library files that supports the binaries located under /bin and /sbin Library filenames are either ld* or lib*.so.* For example: ld-2.11.1.so, libncurses.so.5.7

13. /opt – Optional add-on Applications opt stands for optional. Contains add-on applications from individual vendors. add-on applications should be installed under either /opt/ or /opt/ sub-directory.

14. /mnt – Mount Directory Temporary mount directory where sysadmins can mount filesystems.

15. /media – Removable Media Devices

Temporary mount directory for removable devices. For examples, /media/cdrom for CD-ROM; /media/floppy for floppy drives; /media/cdrecorder for CD writer

16. /srv – Service Data srv stands for service. Contains server specific services related data.

Types of files used by Linux

Text, Binary, Executable

Directory = still just a file

Linked = reference other files, shortcut

Special device files = reference hard disks or ports, contained in /dev

Named pipes = communication between process in memory

Created using mkfifo or mknod One process is a reader other is a writer Sockets = named pipes between remote computers Filenames Filename facts 255 max characters alphanumeric, _ - . may or may not have extension to denote type Lots of different extensions

Links for Additional Info on File Extension

http://learn.clemsonlinux.org/wiki/File_extensions
http://www.debianhelp.co.uk/fileext.htm
http://filext.com/alphalist.php?extstart=%5EL (not limited to linux)

Commands to view filenames and file types

pwd - Show current directory

pwd - /home/randy

ls -ltr - Show listing and give color codes to information

listing including file permissions, ownership and date/time stamp.

Executable files: Green

Normal file : Normal

Directory: Blue

Symbolic link : Cyan

Pipe: Yellow

Socket: Magenta

Block device driver: Bold yellow foreground, with black background

Character device driver: Bold yellow foreground, with black background

Orphaned syminks : Blinking Bold white with red background

Missing links ( - and the files they point to) : Blinking Bold white with red background

Archives or compressed : Red (.tar, .gz, .zip, .rpm

Image files : Magenta (.jpg, gif, bmp, png, tif)

View filenames and file types

ls -F = gives file type, special character is listed on the left of the name = Metacharacters=

@ = linked file

= exe

/ = subdirectory

= is a socket

| = named pipe

Other files do not have special character appended to them - they can be anything not listed above

ls -l = ls long listing

When listing files/directories a special character on the left indicates what type of file it is

d = dir
L = linked
b & c = special device files
n = named pipe
s = socket
- = other - txt, bin

Display contents of files

vi - editing a file
emas - alternative to vi
cat - concatenation - similar to "type" in DOS/Windows.
tac - reverse concatenation - types file in reverse
head - displays the top ten lines of a file
tail - displays the bottom ten lines of a file
more - displays file page by page
less - similar to more but allow line by line manipulation

Display contents of Binary files

strings = searches binary file for text
od = displays file in octal format
od -x = displays contents in hex

Key Terms

The tab completion feature is something your really going to like, it makes life much easier for you, rather than typing every single command you type the first character or two and hit the tab key and it finishes it for you.

~ metacharacter = represent user's home dir
Absolute pathname - full path to a file or starting from /
Binary data file - contains machine language (1 or 0)
cat - display file contents
cd - change directory
Command mode - vi mode for text editing
Concatenation - joining of text
egrep = grep -E
emacas = ediotr macros editor, think of as word pad
fgrep = grep -F
Gedit = GUI text editor
Head - displays first 10 lines, can specify # of lines
Insert mode - vi mode only allows text into file, no other options
less command - display file pg. by pg. cursor navigation
Linked file - file that represents another file
ll command = ls -l
more command = displays file pg. by pg. & line by line
Named pipe file - temp connection that sends info from 1 cmd or process in memory to another, *can represent a file
Nano editor - terminal text editor, uses ctrl keys for functions and navigation
od command - displays file contents in octal format
Parent dir - one dir level up
Regular expressions - special metacharacters
Relative pathname - path to a folder relative to the pwd
Socket file = named pipe between 2 computers
Special device file - files to identify hardware
Strings command - search & display text in a bin file
Tab-completion - press tab to complete path in the shell
tac command = cat backwards
Tail cmd = display last 10 lines
Text tools = program to create, modify, & search txt files
Wildcard metacharacters = used to match certain characters

Key Commands

We have some of the key commands listed alphabetically here but the cat command is much like the tac command where cat is used to display the entire contents of a text file the tac command begins with the last line of a file and ends with the the first line of the file. (Just like the word cat backwards is tac) The " global regular expression print" or grep command searches files for patterns of characters using regular expressions. The egrep & fgrep are other variants of grep that we will talk about later. The print working directory is a useful command to see where you are in a current directory in the tree. And ls command is another useful used for listing files in a given directory.

cat
cd (change directory)
egrep
grep
file
grep
head
less
ls
more
pwd (print working directory)
strings
tac
tail

Search files for regular expressions using grep

Use vi to modify text files

source: http://www.cs.colostate.edu/helpdocs/vi.html

What is vi?

The default editor that comes with the UNIX operating system is called vi (visual editor). [Alternate editors for UNIX environments include pico and emacs, a product of GNU.] The UNIX vi editor is a full screen editor and has two modes of operation: Command mode commands which cause action to be taken on the file, and Insert mode in which entered text is inserted into the file. In the command mode, every character typed is a command that does something to the text file being edited; a character typed in the command mode may even cause the vi editor to enter the insert mode. In the insert mode, every character typed is added to the text in the file; pressing the <Esc> (Escape) key turns off the Insert mode. While there are a number of vi commands, just a handful of these is usually sufficient for beginning vi users. To assist such users, this Web page contains a sampling of basic vi commands. The most basic and useful commands are marked with an asterisk (* or star) in the tables below. With practice, these commands should become automatic. NOTE: Both UNIX and vi are case-sensitive. Be sure not to use a capital letter in place of a lowercase letter; the results will not be what you expect.

To Get Into and Out Of vi

To Start vi

To use vi on a file, type in vi filename. If the file named filename exists, then the first page (or screen) of the file will be displayed; if the file does not exist, then an empty file and screen are created into which you may enter text.

vi filename edit filename starting at line 1

	vi -r filename	recover filename that was being edited when system crashed

To Exit vi

Usually the new or modified file is saved when you leave vi. However, it is also possible to quit vi without saving the file. Note: The cursor moves to bottom of screen whenever a colon (:) is typed. This type of command is completed by hitting the <Return> (or <Enter>) key.

:x<Return> quit vi, writing out modified file to file named in original invocation

	:wq<Return>	quit vi, writing out modified file to file named in original invocation
	:q<Return>	quit (or exit) vi

:q!<Return> quit vi even though latest changes have not been saved for this vi call

Moving the Cursor

Unlike many of the PC and MacIntosh editors, the mouse does not move the cursor within the vi editor screen (or window). You must use the the key commands listed below. On some UNIX platforms, the arrow keys may be used as well; however, since vi was designed with the Qwerty keyboard (containing no arrow keys) in mind, the arrow keys sometimes produce strange effects in vi and should be avoided. If you go back and forth between a PC environment and a UNIX environment, you may find that this dissimilarity in methods for cursor movement is the most frustrating difference between the two. In the table below, the symbol ^ before a letter means that the <Ctrl> key should be held down while the letter key is pressed.

j or <Return>

 [or down-arrow]	move cursor down one line

k [or up-arrow] move cursor up one line
h or <Backspace>

 [or left-arrow]	move cursor left one character

l or <Space>

 [or right-arrow]	move cursor right one character

0 (zero) move cursor to start of current line (the one with the cursor)
$ move cursor to end of current line

	w	move cursor to beginning of next word
	b	move cursor back to beginning of preceding word
	:0<Return> or 1G	move cursor to first line in file
	:n<Return> or nG	move cursor to line n
	:$<Return> or G	 move cursor to last line in file

Screen Manipulation

The following commands allow the vi editor screen (or window) to move up or down several lines and to be refreshed.

	^f	move forward one screen
	^b	move backward one screen
	^d	move down (forward) one half screen
	^u	move up (back) one half screen
	^l	redraws the screen
	^r	redraws the screen, removing deleted lines

Adding, Changing, and Deleting Text

Unlike PC editors, you cannot replace or delete text by highlighting it with the mouse. Instead use the commands in the following tables. Perhaps the most important command is the one that allows you to back up and undo your last action. Unfortunately, this command acts like a toggle, undoing and redoing your most recent action. You cannot go back more than one step.

u UNDO WHATEVER YOU JUST DID; a simple toggle

The main purpose of an editor is to create, add, or modify text for a file. Inserting or Adding Text

The following commands allow you to insert and add text. Each of these commands puts the vi editor into insert mode; thus, the <Esc> key must be pressed to terminate the entry of text and to put the vi editor back into command mode.

i insert text before cursor, until <Esc> hit

	I	insert text at beginning of current line, until <Esc> hit

a append text after cursor, until <Esc> hit

	A	append text to end of current line, until <Esc> hit

o open and put text in a new line below current line, until <Esc> hit
O open and put text in a new line above current line, until <Esc> hit

Changing Text

The following commands allow you to modify text.

r replace single character under cursor (no <Esc> needed)

	R	replace characters, starting with current cursor position, until <Esc> hit
	cw	change the current word with new text,

starting with the character under cursor, until <Esc> hit

	cNw	change N words beginning with character under cursor, until <Esc> hit; 
 e.g., c5w changes 5 words
	C	change (replace) the characters in the current line, until <Esc> hit
	cc	change (replace) the entire current line, stopping when <Esc> is hit
	Ncc or cNc	change (replace) the next N lines, starting with the current line,

stopping when <Esc> is hit

Deleting Text

The following commands allow you to delete text.

x delete single character under cursor

	Nx	delete N characters, starting with character under cursor
	dw	delete the single word beginning with character under cursor
	dNw	delete N words beginning with character under cursor; 
 e.g., d5w deletes 5 words
	D	delete the remainder of the line, starting with current cursor position

dd delete entire current line

	Ndd or dNd	delete N lines, beginning with the current line; 
 e.g., 5dd deletes 5 lines

Cutting and Pasting Text

The following commands allow you to copy and paste text.

	yy	copy (yank, cut) the current line into the buffer
	Nyy or yNy	copy (yank, cut) the next N lines, including the current line, into the buffer
	p	put (paste) the line(s) in the buffer into the text after the current line

Other Commands

Searching Text

A common occurrence in text editing is to replace one word or phase by another. To locate instances of particular sets of characters (or strings), use the following commands.

	/string	 search forward for occurrence of string in text
	?string	 search backward for occurrence of string in text
	n	 move to next occurrence of search string
	N	 move to next occurrence of search string in opposite direction

Determining Line Numbers

Being able to determine the line number of the current line or the total number of lines in the file being edited is sometimes useful.

	:.=	 returns line number of current line at bottom of screen
	:=	 returns the total number of lines at bottom of screen
	^g	provides the current line number, along with the total number of lines,

in the file at the bottom of the screen Saving and Reading Files

These commands permit you to input and output files other than the named file with which you are currently working.

	:r filename<Return>	 read file named filename and insert after current line

(the line with cursor)

	:w<Return>	write current contents to file named in original vi call
	:w newfile<Return>	write current contents to a new file named newfile
	:12,35w smallfile<Return>	write the contents of the lines numbered 12 through 35 to a new file named smallfile
	:w! prevfile<Return>	write current contents over a pre-existing file named prevfile

Regular Expressions

Search files for regular expressions using grep

grep allows you to search through a file for text argument.

*grep "localhost" /etc/hosts return lines that include localhost in the file /etc/hosts

grep -v "localhost" /etc/hosts

return lines that DONT include localhost in the file /etc/hosts

grep -l "LOCALHOST" /etc/hosts

return lines that include localhost in the file /etc/hosts that is not case-sensitive

Commands to Display Contents of Binary Files

strings = searching binary file for text
od = displays file in octal format
od -x = displays contents in hex

Searching for Text within Files

Tools: grep, awk, sed, vi, emacs, ex, ed, C++, PERL, tcl

Regular expressions

Wildcard metacharacters are interpreted by the shell

Regular expressions are interpreted by text tool program

Wildcard metacharacters match characters in file & dir names

Regular expressions match charters within file

Wildcard metacharacters typically have different definitions than regular expressions

metacharacters

There are more regular expression metacharacters than wildcard metacharacters

===Emacs===
Not installed by default in Fedora 13.

Can run in a GUI environment.

More GUI than vi.

Written in C and Emacs Lisp.

Licensed GNU GPL

Users can combine commands into macros to automate task.

Summary

Linux file system is a hierarchy, series of directories

paths are absolute or relative

Many types of files - text, scripts, executable, dirs, linked, special device
ls - view file names and many options to modify view
Wildcard metacharacters help to select multiple files
Regular expression metacharacters are used in many ways.
Text files can be viewed differently with commands such as, head, tail, cat, tac, more and less.
vi is the most common text editor however GUI options exits