Predefined Shell Variable	Purpose

$0		the name of the shell procedure being executed
$1 thru $9	the first through ninth parameter passed to the shell
		  program (see shift)
$#		the number of parameters passed
$*		used to represent all the parameters passed 
$$		contains the process id number of the current process,
		  often used to create unique temporary files
$?		used to hold the exit status of the last command, a value
		  of 0 means the last command succeeded, 1 means it failed
"$@"		used to represent all the parameters passed, especially if
		  the parameters contain spaces, must use the surrounding ""

	Table 3.2. Predefined Shell Variables (Bourne Shell)

For example:
arguments hello there
arguments accepted 2 command line arguments
they were hello there

Your shell program should use shell variables wherever possible.

Using More than 9 Parameters

Instead you must use the shell command shift. shift moves all the parameters one parameter to the left i.e. the first parameter disappears, the second parameter becomes the first, the third the second and so on. So after executing the shift command the tenth parameter has become the ninth parameter.

For example:

The following shell program called args2 demonstrates the use of shift
echo parameters are $*
shift
echo parameters are $*
shift
Example use of the script
$ args2 hello there friend
parameters are hello there friend
parameters are there friend

The exit Command

An exit status of 0 indicates success.
An exit status of anything else indicates failure.

			exit Command Format.

		exit number

	number should be 0 if the shell procedure succeeded.
	  Any other value indicates failure.

	Figure 3.1. The exit Command.

For example:

The following is code for a shell program called search
!#/bin/sh
# use the -s (silent) option of grep
# first parameter should be a username
# see if the username exists
grep -s $1 /etc/passwd
echo $?
Generally speaking this program will display 0 if the grep found the username in the password file and 1 if it didn't.

The Conditional Commands

The if Command

			if Command Format.

	if commandt
	then				
		command-list
	fi

	if commandt
	then
		command-list1
	elif command1
		command-list2
	fi

	command-list is a list of 1 or more UNIX commands.
	commandt is any UNIX command.

	Figure 3.2. The if Command.

For example:

	if grep -s $1 /etc/passwd
	then
		echo user $1 was found in password file
	else
		echo user $1 NOT FOUND
	fi

The case Command

			case Command Format.

		case value in
			pat1)	command
				command;;
			pat2)	command-list;;
			*)	command-list;;
		esac

		Figure 3.3. case Command Format.

Once the end of the appropriate command list is reached, execution is restarted at the command directly after the esac. Only the first pattern matched will ever be executed.

The * pattern is optional and is used to match any value and so it acts as the default. (The * should never be the first choice. * matches everything and so the other choices will never get a look in.)

The * is an example of using wildcard characters in the patterns. You can also use the ? and [] wildcard symbols in the patterns. The | symbol can be used as an OR operator for patterns. i.e. pat1 | pat2 will be matched if at least one pattern is a match

For example:

	case $1 in
		hello ) echo hello there;;
		goodbye ) echo why goodbye
			    echo you haven't said hello;;
		T* | *T) echo that word started or ended with a T;;
		[a-z]?) echo that word started with a
			  echo lowercase letter and was
			  echo two letters long;;
		*) echo sorry I don't know that word;;
	esac

	case $# in
		0) echo no parameters;;
		1) echo one parameter;;
		2) echo two parameters;;
		3) echo three parameters;;
	esac

The test Command

Almost every person who has ever written a shell procedure has written one called test and then wondered why it wouldn't work. The reason is that there is already a UNIX program called test. When you enter test at the command line it will typical find the UNIX command before it finds your shell procedure.

		test Command Format.

	test expr
	[ expr ]

	The [ is actually the name of an executable program (Try the command
	 which [ or whereis [).  When it is executed you are actually running
	 the program called test.

	Figure 3.4. test Command Format.

	Expression		Result

	-z string		true if the length of string is 0
	-n string		true if the length of string is not zero
	string1 = string2	true if the two strings are identical
	string1 != string2	true if the two strings are NOT identical
	string			true if string is NOT the null string

		Table 3.4. Expressions for testing String Conditions

	Expression		Result

	int1 -eq int2		true if the two integers are equal
	int1 -ne int2		true if the two integers are not equal
	int1 -gt int2		true if int1 is greater than int2
	int1 -ge int2		true if int1 is greater than or equal to int2
	int1 -lt int2		true if int1 is less than int2
	int1 -le int2		true if int1 is less than or equal to int2

		Table 3.5. Expressions for testing Integer Conditions.

	Expression	Result

	-r file		true if file exists and is readable
	-w file		true if file exists and is writable
	-x file		true if file exists and is executable
	-f file		true if file exists and is a regular file,
			 or true if file exists and is not a directory
	-d file		true if file exists and is a directory
	-h file		true if file exists and is a symbolic link
	-c file		true if file exists and is a character special file
	-b file		true if file exists and is a block special file
	-p file		true if file exists and is a named pipe
	-u file		true if file exists and its set user ID bit is set
	-g file		true if file exists and its set group ID bit is set
	-k file		true if file exists and its sticky bit is set
	-s file		true if file exists and has a size greater than 0

		Table 3.6. Expressions for testing File Conditions.

	Expression	Result

	!		the unary negation operator, reverses the result
			 of an expression
	-a		binary AND operator
	-o		binary OR operator
	( expr )	parentheses used for grouping, parentheses have
			 special meaning to the shell so to use them in the
			 test command you must quote them

		Table 3.7. Logical Operators.

Example uses of the test Command

	if test $1 = hello
	then
		echo hello there
	fi

	if [ $1 -gt $2 -o $1 -eq $2 ]
	then
		echo $1 is greater than or equal to $2
	fi

	if [ $# -eq 0 ]
	then
		echo You didn\'t enter any parameters
	fi

Exercise 3-7. Write a shell program exists that takes a filename as a command line parameters. If there are no command line parameters it should display a message explaining how to use it. If there are parameters it should test to see if a file with that name exists and display an appropriate message.
For example:
exists /etc/passwd
The file /etc/passwd exists.

Repeated Action Commands

• for
• while
• until

The for command

		for Command Format.

	for variable in word1 word2 word3 ... wordn
	do
		command-list
	done

	Figure 3.5. for Command Format.

For example:

	count=0
	for param in $*	# for every parameter
	do
	  count=`expr $count + 1`	# expr talked about later
	  echo parameter $count is $param
	  # display the number parameter and its value
	done

	echo days of the week are
	for today in sun mon tue wed thu fri sat
	do
	  echo $today
	done

The while Command

		while Command Format.

	while command1
	do
		command-list
	done

	Figure 3.6. while Command Format.

For example:

	display a countdown from 10 to 0
	count=10
	while [ $count -ge 0 ]
	do
		echo  $count
		count=`expr $count - 1`
	done

The until Command

		until Command Format.

	until command1
	do
		command-list
	done

	Figure 3.7. until Command Format.

For example:

	display a countdown from 10 to 0
	count=10
	until [ $count -lt 0 ]
	do
		echo $count
		count=`expr $count - 1`
	done

Some Other Useful Commands

	Command	Purpose

	expr	evaluates logical and arithmetic expressions
	cut	used to extract fields of data from the lines of a file
		 or the output of a command
	sort	sorts its standard input into order
	head	display the first X number of lines of standard input
	tail	display the last X number of lines of standard output
	file	attempts to describe the type of a file
	grep	search for lines containing a character string
	uniq	remove duplicated lines from standard input
	strings	displays any ASCII characters contained in a binary file
	tr	translates specified character(s) into other character(s)
	paste	the opposite to cut, puts lines back together

		Table 3.8. Some other useful commands.

Examples

expr 5 + 6
expr 5 \* 10 + 5
echo "105 - 5 = " `expr 105 \- 5`

cut -d: -f1,6 /etc/passwd
cut -c1-8 /etc/passwd
cut -d: -f1 /etc/passwd	t -d: -f1 /etc/passwd | sort

cat /etc/group		
sort /etc/group	
sort -r /etc/group

cut -d: -f1,6 /etc/passwd | tr : '    '
cut -d: -f1 /etc/passwd | tr [a-z] [A-Z]

Conclusion

Review Questions

	bash$ my_sum 1 2 3 4 5
	Total = 15

	csh> my_sum 1 2 3 4 5
	1 + 2 + 3 + 4 + 5 = 15