Objectives| Philosophy and Format of UNIX Commands| A Command for Everything| The Basic Commands| File Permissions| Symbolic and Absolute Modes| Changing File Permissions| When is a Command not a Command?| The Shell| Executing a Command| Other Special Characters| Conclusion| Review Questions

Section 2

BASIC UNIX

Objectives

At the end of this section you should

• be familiar with the command format used by UNIX
• be aware of the role of a shell
• have gained an understanding of some of the intricacies of using a shell including special characters, I/O redirection, wildcards and shell variables
• become familiar with the UNIX file and directory manipulation commands
• received an introduction to the UNIX directory hierarchy,
• become familar with some of the UNIX commands that report the status of the machine
• have used the UNIX on-line help system at least once

Philosophy and Format of UNIX Commands

There are no set rules about UNIX commands however there is a UNIX philosophy that is used by many of the commands, but not all.

• small is beautiful,
  UNIX provides the mechanisms to join commands together so commands should do one thing well.
• 10 percent of the work solves 90 percent of the problems,
  UNIX was never designed to solve all problems, it was designed to solve most requirements without too much hassle on the programmer's part.
• solve the problem, not the machine, and
  Commands should ignore any machine specific information and be portable.
• solve at the right level, and you will only have to do it once.
  The key to UNIX problem solving is only to do it once e.g. pattern matching is only implemented once, in the shell, not in every command.

A Command for Everything

Being an MS-DOS junkie from way back what command does he look for? rename of course. It doesn't work! "That's a bit silly!", he thinks, "You would think that UNIX would have a rename command."

It just so happens that this person has just completed a C programming subject in which one of the assignments was to write a rename command. So he spends the next day trying to write and compile this program. After much toil and trouble he succeeds and follows good administration policy and informs all the other users of this brand new wonderful program he has written. He goes into great detail on how to use the command and all the nice features it includes.

They all write back and tell him about the UNIX command mv that does the same thing.

The moral of this story is that if you want to do something under UNIX, then chances are that there is already a command to do it.

The Basic Commands

Reading.
UNIX is a Four Letter Word... and VI is a Two Letter Abreviation. Section 2.4.

Purpose.
Revise and introduce the basic UNIX commands.

a) echo *
b) echo *[!0-9]
c) echo m[a-df-z]*
d) echo [A-Z]*
e) echo jan*
f) echo *.*
g) echo ?????
h) echo *89
i) echo jan?? feb?? mar??
j) echo [fjm] [ae] [bnr] *

a) ls wc -1
b) rm ???
c) who wc -1
d) mv progs/* /usr/steve/backup
e) ls *.c wc -1
f) rm *.o
g) who sort
h) pwd
i) cp memo1 ..
j) plotdata 2>errors &

File Permissions

	-rw-rw-rw-  1  david  staff    227  Dec 12 19:33  note
 	    1.     2.   3.     4.	5.      6.	   7.
	
	1.	File access permissions, who can do what to the file/directory
	2.	The number of links to this file.  
	3.	The owner's user name.
	4.	The group owner's group name.
	5.	The size of the file in bytes.
	6.	The date and time the file was last modified.
	7.	The name of the file.

	Figure 2.1. Explanation of the output of the ls -l command.

Users, Groups and Others

Access to a UNIX file is grouped into three categories

• user
  The individual user who owns the file (by default the user that created the file). In figure 2.1 the owner is david.
• group
  The collection of people that belong to the group that owns the file (by default the group to which the user owner belongs). In figure 2.1 the group owner is staff.
• other
  Anybody that doesn't fall into the first two categories.

The very first character is used to indicate the type of file. Some of the possible file types are listed in table 2.1.

		tuuugggooo

	t = type of file
	u = permissions for user who is the owner of the file
	g = permissions for the group that owns the file
	o = permissions for everyone else

	Figure 2.2. Format of File Permissions.


File Type	Meaning

-		a normal file
d		a directory
l		symbolic link
b		block device file
c		character device file
p		a fifo or named pipe
s		a XENIX semaphore
m		a XENIX shared data section

	Table 2.1. Different File Types.

The three remaining fields for user, group and others all use the same format. The first entry is the read attribute, the second is the write attribute and the third is the execute attribute. If there is a - character in a particular spot then that attribute is turned off (i.e. the user can't do it). If there is a r, w or a x in the appropriate spot then the attribute is turned on and the user is allowed to perform the associated operation.

For example:

if a particular file had the permissions -rwx-w---x
Then the owner of the file has the attributes rwx which means they can read, write and execute the file.
Users in the group have the attributes -w- which means they can only write to the file.
All other users have the attributes --x which means they can only execute the file.

There are two other possible letters which may appear in the permissions field for a file.

• s - the set user/group id bit (set uid/ set gid bit)
  When someone runs a program the resulting process runs with the privileges of the user who ran it (both user and group). If the set user/group bit are set the resulting process will run with the privileges of the user/group owner of the file.
  

  e.g. The passwd command has the set uid bit set,
  and is owned by root. This means that whoever runs
  can write their new password to the passwd file.
  

  YOU MUST BE CAREFUL WHEN SETTING THESE BITS.
  They are a common source of security problems.
• t - the sticky bit
  When you run a program UNIX reads the program code from disk into memory and creates a process. This takes time. If you have a large program which you run frequently it might make more sense to keep it in memory all the time. To do this you set the sticky bit.

	Figure 2.3. Special Permission Bits.

Attribute Type	Meaning for a file		Meaning for a directory

	r	the contents of the		ability to obtain
		 file can be viewed		 a directory listing
	w	the contents of the		ability to create and remove
		 file can be changed		 files from directory
	x	the file can be			ability to change into that
		 executed as a command		 directory, and access its
						 contents

		Table 2.2. The Meanings of Permissions.

	          jonesd   rwxr-w--x
	             |
	             |  all owned by jonesd
	             |  group owner admin
	   __________|_______________________
	  |                                  |
	assign   r-x------                 docs   rwxr-xrwx

• astudent belongs to the group users
• astaff belongs to the group admin

a) Can astudent obtain a directory listing of the jonesd directory?
b) Can astaff obtain a directory listing of the docs directory?
c) Can astudent obtain a directory listing of the docs directory?
d) Can astudent create a file in the docs directory?

Symbolic and Absolute Modes

In absolute mode each access class (user, group and other) are represented by a number no bigger than seven. This number is arrived at by converting the symbolic permissions into a binary digit as shown in figure 2.4

	Converting Symbolic to Numeric.

	r w x     r - -     r - x	symbolic
	1 1 1     1 0 0     1 0 1
	  7         4         5
		   754			numeric

	Figure 2.4. Converting Symbolic Permissons to Numeric.

a) rw----r-x
b) rwxrwxrwx
c) ---rwx-wx

a) 111
b) 550
c) 750

Changing File Permissions

Command	Purpose

chmod	change the file permissions for a file
umask	set the default file permissions for any files to be created.
	  Usually run as the user logs in.
chgrp	change the group owner of a file.
chown	change the user owner of a file.

	Table 2.3. File Permission Commands.

The chmod Command

For example:

chmod u+rwx temp.dat
turn on all permissions for the owner of the file
chmod gw-rwx temp.dat
turn off all permissions for all the users except the owner of the file
chmod -R a-rwx /
turn off all permissions for everyone for all files
chmod -R a= /
turn off all permissions for everyone for all files
chmod 770 temp.dat
allow the user and group read, write and execute and others no access

	chmod Command Format.

chmod [-R]  operation files

-R	recursively descend each directory

operation can be either symbolic or absolute permissions.
When using absolute permissions operation is simply the
numeric permissions e.g. 770 200

When using symbolic permissions operation takes the form of 
	whooppermission	where:

who			u 	for owner of file
			g	for group
			o	for others
			a	for all categories
op			+	add permission
			-	remove permission
			=	set permission
permission		r	read
			w	write
			x	execute		
			s	set uid/gid
			t	set sticky bit

Figure 2.5. chmod Command Format.

The chown and chgrp Commands

There are some limitations on how you can use chown and chgrp. Only the root user or the current owner of the file can use chown to change the ownership of a file. Only the root user can arbitrarily change groups. The owner of a file can only change the group to another group to which the owner belongs.

On some systems you cannot give away ownership of files at all, only root can. Two reasons with this are

• in a filesystem with quotas (quotas place an upper limit of how many files and how much disk space a user can use) a person could avoid the quota system by giving away the ownership to another person
• if anyone can give ownership of a file to root they could create a program that is setuid to the owner of the file and then change the owner of the file to root.

	chown/chgrp Command Format.

chown [-R] owner files
chgrp [-R] group files

-R	change the ownership on all sub-directories and the files
	 within them

owner	is either a numeric user identifier or a user name listed
	 in /etc/passwd .*
group	is either a numeric group identifier or a group name listed
	 in /etc/group.
files	is a list of files of which you wish to change the ownership

Figure 2.6. chown/chgrp Command Format.

The umask Command

For example imagine what would happen if all the files and directories that were created were given the default access permissions 777. Anyone could read, write or execute those files.

For example:

When files are created with the following umask values
umask 027
the user will have all permissions, group will not have write and others will have none
umask 022
the user will have all permissions, group and others will not have write.

	The umask Command.

umask [ ooo ]

With no parameter the current umask value is displayed.

ooo	are octal digits (numbers ranging from 0-7).  The specified
	 digits are subtracted from the default access permissions.
	e.g.	027	will subtract write permission for the
		 group and all permissions for others

Figure 2.7. umask Command.

When is a Command not a Command?

For example, the command which ls on my machine aldur returns /usr/bin/ls. This means that the program for ls is in the directory /usr/bin. If which can't find the command it reports no command in path. This implies one of two things

• there is no such program, or
• your search path does not include the directory in which the program is located.

The Shell

The shell you will more than likely use under Linux is called bash (Bourne Again Shell). The basic syntax used by bash is identical to the Bourne shell but it also provides additional abilities including command line editing using cursor keys.

Different people have their favourite shells. As a Systems Administrator you will have to know the Bourne shell syntax. This is because almost all of the shell programs that are used to maintain a UNIX operating system are written using the Bourne shell syntax.

All of the commands talked about in the previous readings are actually executable programs stored somewhere in the directory hierarchy. When you ask the shell to cd /home it runs the executable program cd to perform the task.

The shell itself is just an executable program. Table 2.4 lists some of the program names for the various shells. If you enter one of these program names, for example csh, it will execute the program csh and start a version of the C shell. (You exit the shell program by typing logout, exit or using the key combination CTRL-D.)

Shell	Program Name	Description

Bourne shell	sh	the original shell from AT&T,
			 available on all UNIX machines
C shell		csh	shell developed as part of BSD
Korn shell	ksh	AT&T improvement of the Bourne shell
Bourne again	bash	Shell distributed with Linux, version
 shell			 of Bourne shell that includes command
			 line editing and other nice things

	Table 2.4. Examples of UNIX Command Shells.

Exercise 2-7. Execute one of the other shells on your system? Does it change the variable shell?

Among the shell variables displayed by entering the set command you should see PATH or path. This is the shell variable that holds your current search path, the list of directories UNIX looks in to find executable programs.

Shell Responsibilities

• executing programs or commands as requested by the user,
• performing variable and file name substitution,
  It is the shell that performs the translation of let* into the list of filenames that start with let.
• performing I/O redirection,
  Making sure that input/output ends up in the correct place as specified by the user's use of the > >> << and other characters.
• environment control, and
  The shell also controls and stores settings about the type of terminal being used amongst other information.
• providing an interpreted programming language.
  Shells provide the ability to write programs using UNIX commands. The shell provides the looping and conditional commands necessary for a programming language.

Executing a Command

• takes a line of text from the user,
• parses the text and performs the necessary I/O redirection,
• replaces any shell variables in the text with the actual value,
• replaces any wild card characters with the appropriate file names, and then executes the command.

Read the command line

Perform I/O Redirection

Character		Meaning
command < file		Take standard input from file.
command > file		Put the output of command into file.
			  Overwrite file if it already exists.
command >> file		Put the output of command into file.  Append the
			  output onto the end of file if it already exists. 
command << label	Take standard input for command from the following
			  lines until a line that contains only label
`command`		Execute command and replace `command` with the
			  standard output of the command.
command1 | command2	Use the standard output of command1 as the standard
			 input of command2

	Table 2.5. Types of I/O Redirection.

	ls | cat > hello.dat

Replace Shell Variables

For example:

echo $SHELL
will display on my system /bin/bash

Replace Wildcard Characters

Execute the Command

Order is Important

1. I/O redirection
2. Shell variables, and then
3. Wildcard characters

pipe=\|
creates a shell variable called pipe. The \ character is explained in the following section
echo cat $pipe more
remember I/O redirection is checked for first. What would happen if the shell variable substitution was done first?

Doing it Twice

For example:

name="david"
variable=name
echo $variable
these three commands will produce the output
name
replace the last command with
eval echo \$$variable
will produce the output
david

Other Special Characters

Character(s)		Meaning

white space		Any white space characters (tabs, spaces) are used
			  to separate arguments (multiple white space
			  characters are ignored)
newline character	used to indicate the end of the command-line

' " \			special quote characters which change the way the
			  shell interprets special characters
&			Used after a command, tells the shell to run the
			  command in the background e.g. ls &
< >> << ` |		redirection characters change where I/O is sent

$			used to indicate a shell variable name
			  (more on these later)
;			used to execute more than one command in
			  one command line

	Table 2.6. Special Shell Characters.

echo hello there my friend
displays
hello there my friend
the shell ignores multiple spaces
ls ; cd /etc ; ls
the ; is used to separate the three commands which are executed one after the other

Quotes

Exercise 2-8. What's the output from this command?

echo Multiply is signified by the * symbol

string=hello there how are you echo $string

  Character	   Name		Purpose

	'	single quote	causes the shell to ignore all special
				  characters contained within a pair of
				  single quotes
	"	double quote	causes the shell to ignore all special
				  characters EXCEPT $ ` \ contained within
				  a pair of double quotes
	\	backslash	causes the shell to ignore any special
				  character immediately following a backslash

	Table 2.7. The Quote Characters.

Examples of Using Quote Characters

hello_string='hello there'
echo $hello_string		echo '$hello_string'
echo "$hello_string"		echo \$hello_string
echo "\$hello_string"

echo *				echo I\'m David.
echo '*'			echo \*

echo one      two  three      four
echo 'one      two  three      four'
echo "one      two  three      four"

echo hello there \
my name is david

echo > temp.dat			echo \> temp.dat

a) stars*
b) hello my friend
c) "goodbye"
d) Now delete them from the file system.

b) echo "** hello **"
b) echo this is a star *
b) echo ain\\\\'t you my friend
b) echo "the output of the ls command is `ls`"
b) echo 'the output of the pwd command is `pwd`'

Getting Help

		UNIX People have a Sense of Humour?

	Some people say that UNIX warps the mind of its users.  The following
	 is a list of comments taken from actual UNIX man pages.  

		"Acts oddly on nights with full moon."
		"This manual page is confusing."
		"This manual page is still confusing."
		 (next release of same command)
	"It can be used if a disk or the processor is on fire."

	Figure 2.8. UNIX People have a sense of humour.

If you are not sure of the exact word to give the man command you can use one of man -k or apropos (same command different name). This command searches for occurrences of a given keyword in the manual pages and displays the proper word for use with man.

The manual pages are divided into sections with each section dealing with a specific area. Table 2.8 provides a summary of the major sections. These can differ slightly from machine to machine. In particular, manufacturers will add sections specific to commands they add to the operating system.

	Section Number	Purpose

		1	User Commands
		2	System Calls
		3	Library routines
		4	Device drivers
		5	File formats
		6	Games
		7	Miscellaneous: ASCII, macro packages etc.
		8	Commands for system administration

	Table 2.8. Section Numbers for Manual Pages (BSD).

Under a Linux box the following command can be used to display all the contents of a specific manual section.
ls /usr/man/preform/catn | -F \. '{ print $1 }'
Where the n is replaced with the number of the section.

Manual pages are stored in a special format called troff or nroff. It is difficult to read a manual page on the screen and use that information at the same time. It is often preferable to print the manual page or produce a file with the manual page in it. The formatting language used by manual pages can make the result difficult to read.

The following sequence of commands should produce a file containing a man page that can be read. (It is up to you to use the man command to figure out what is going on here)

		man manpage | ul -tlp > filename

manpage represents the manual page you require
filename represents the file in which to place the output

Figure 2.9. Printing a man page.

The man Command

1. man [ section ] title
2. man -k keyword
3. man -f filename

1. Displays an entire manual page (page at a time) with the title title. section refers to the section of the manual the manual page will come from. Refer to Table 2.8 for more information on manual sections.
2. Display a one-line summary of a manual page which discusses the keyword. Exactly the same as the apropos command.
3. Display a one-line summary of a manual page which refers to the file with name filename.

Conclusion

• the UNIX commands to manipulate files and directories,
• the concepts of filename substitution and I/O redirection,
• the concept of a shell and its responsibilities,
• commands to list users and processes on a UNIX machine,
• commands to display the status of a machine,

Review Questions

2.1. Write commands to carry out the following tasks:

a) change into the directory /usr/local
b) produce a file called listing that contains the names of all the files in the /etc directory c) count the number of words in the file /etc/passwd

2.2. What is a link?

2.3. What are the major responsibilities of a UNIX shell?

2.4. What do the following commands do?

a) echo 'the *'s are out tonight\'
b) ls | cat | wc -l > count
c) ls `cat file.list`

2.5. Explain what filenames the following file specifications will match

a) ?*
b) [!a-z]??[a-z]
c) *\*?
d) '*?*?'abc*