2.5. The Korn Shell Constructs

The Korn and Bash shells are very similar. The following constructs will work for both shells. To see all the subtle variations, see the individual chapters for these shells.

Table 2.3. Korn Shell Syntax and Constructs

The shbang line	The "shbang" line is the very first line of the script and lets the kernel know what shell will be interpreting the lines in the script. The shbang line consists of a #! followed by the full pathname to the shell, and can be followed by options to control the behavior of the shell. EXAMPLE #!/bin/ksh
Comments	Comments are descriptive material preceded by a # sign. They are in effect until the end of a line and can be started anywhere on the line. EXAMPLE # This program will test some files
Wildcards	There are some characters that are evaluated by the shell in a special way. They are called shell metacharacters or "wildcards." These characters are neither numbers nor letters. For example, the *, ?, and [ ] are used for filename expansion. The <, >, 2>, >>, and | symbols are used for standard I/O redirection and pipes. To prevent these characters from being interpreted by the shell they must be quoted. EXAMPLE rm *; ls ??; cat file[1-3]; echo "How are you?"
Displaying output	To print output to the screen, the echo command can be used. Wildcards must be escaped with either a backslash or matching quotes. Korn shell also provides a built-in print function to replace the echo command. EXAMPLE echo "Who are you?" print "How are you?"
Local variables	Local variables are in scope for the current shell. When a script ends or the shell exits, they are no longer available; i.e., they go out of scope. The typeset built-in command can also be used to declare variables. Local variables are set and assigned values. EXAMPLE variable_name=value typeset variable_name=value name="John Doe" x=5
Global variables	Global variables are called environment variables. They are set for the currently running shell and any process spawned from that shell. They go out of scope when the script ends or the shell where they were defined exits. EXAMPLE export VARIABLE_NAME =value export PATH=/bin:/usr/bin:.
Extracting values from variables	To extract the value from variables, a dollar sign is used. EXAMPLE echo $variable_name echo $name echo $PATH
Reading user input	The user will be asked to enter input. The read command is used to accept a line of input. Multiple arguments to read will cause a line to be broken into words, and each word will be assigned to the named variable. The Korn shell allows the prompt and read command to be combined. EXAMPLE
	read name?"What is your name?"				The prompt is in quotes. After it is displayed, the read command waits for user input
	print -n "What is your name?" read name read name1 name2 ...
Arguments	Arguments can be passed to a script from the command line. Positional parameters are used to receive their values from within the script. EXAMPLE At the command line: $ scriptname arg1 arg2 arg3 ... In a script:
	echo $1 $2 $			Positional parameters, $1 is assigned arg1, $2 is assigned arg2, ...
	echo $*			All the positional paramters
	echo $#			The number of positional parameters
Arrays	The Bourne shell utilizes positional parameters to create a word list. In addition to positional parameters, the Korn shell also supports an array syntax whereby the elements are accessed with a subscript, starting at 0. Korn shell arrays are created with the set -A command. EXAMPLE
	set apples pears peaches					Positional parameters
	print $1 $2 $3					$1 is apples, $2 is pears, $3 is peaches
	set -A array_name word1 word2 word3 ... set -A fruit apples pears plums					Array
	print ${fruit[0]}					Prints apple
	${fruit[1]} = oranges					Assign a new value
Arithmetic	The Korn shell supports integer arithmetic.The typeset i command will declare an integer type variable. Integer arithmetic can be performed on variables declared this way. Otherwise, the (( )) syntax (let command) is used for arithmetic operations. EXAMPLE
	typeset -i variable_name			Declare integer
	typeset -i num num=5+4			num is declared as an integer
	print $num			Prints 9
	(( n=5 + 5 ))			The let command
	print $n			Prints 10
Command substitution	Like the C/TC shells and the Bourne shell, the output of a UNIX/Linux command can be assigned to a variable, or used as the output of a command in a string, by enclosing the command in backquotes. The Korn shell also provides a new syntax. Instead of placing the command between backquotes, it is enclosed in a set of parentheses, preceded by a dollar sign. EXAMPLE variable_name=`command` variable_name=$( command ) echo $variable_name echo "Today is `date`" echo "Today is $(date)"
Operators	The Korn shell uses the built-in test command operators to test numbers and strings, similar to C language operators. EXAMPLE
	Equality:					Relational:
	=	string, equal to				>	greater than
	!=	string, not equal to				>=	greater than, equal to
	==	number, equal to				<	less than
	!=	number, not equal to				<=	less than, equal to
	Logical:
	&&	and
	||	Or
	!	Not
Conditional statements	The if construct is followed by an expression enclosed in parentheses. The operators are similar to C operators. The then keyword is placed after the closing parenthesis. An if must end with a fi. The new test command [[ ]] is now used to allow pattern matching in conditional expressions. The old test command [ ] is still available for backward compatibility with the Bourne shell. The case command is an alternative to if/else. EXAMPLE
	The if construct is: if command then block of statements fi ---------------------------- if [[ string expression ]] then block of statements fi					---------------------------- if (( numeric expression )) then block of statements fi
	The if/else construct is: if command then block of statements else block of statements fi -------------------------- if [[ expression ]] then block of statements else block of statements fi --------------------------- if (( numeric expression )) then block of statements else block of statements fi The case construct is: case variable_name in pattern1) statements ;; pattern2) statements ;; pattern3) ;; esac ------------------------- case "$color" in blue) echo $color is blue ;; green) echo $color is green ;; red|orange) echo $color is red or orange ;; esac					The if/else/else if construct is: if command then block of statements elif command then block of statements elif command then block of statements else block of statements fi --------------------------- if [[ string expression ]] then block of statements elif [[ string expression ]] then block of statements elif [[ string expression ]] then block of statements else block of statements fi ---------------------------- if (( numeric expression )) then block of statements elif (( numeric expression )) then block of statements elif (( numeric expression )) then block of statements else block of statements fi
Loops	There are four types of loops: while, until, for, and select. The while loop is followed by an expression enclosed in square brackets, a do keyword, a block of statements, and terminated with the done keyword. As long as the expression is true, the body of statements between do and done will be executed. The until loop is just like the while loop, except the body of the loop will be executed as long as the expression is false. The for loop is used to iterate through a list of words, processing a word and then shifting it off, to process the next word. When all words have been shifted from the list, it ends. The select loop is used to provide a prompt (PS3 variable) and a menu of numbered items from which the user inputs a selection The input will be stored in the special built-in REPLY variable. The select loop is normally used with the case command. The loop control commands are break and continue. The break command allows control to exit the loop before reaching the end of it; the continue command allows control to return to the looping expression before reaching the end. EXAMPLE
	while command do block of statements done ----------------------------- while [[ string expression ]] do block of statements done ----------------------------- while (( numeric expression )) do block of statements done until command do block of statements done ----------------------------- until [[ string expression ]] do block of statements done ----------------------------- until (( numeric expression )) do block of statements done					for variable in word_list do block of statements done ----------------------------- for name in Tom Dick Harry do print "Hi $name" done select variable in word_list do block of statements done ------------------------------- PS3="Select an item from the menu" for item in blue red green echo $item done Shows menu: blue red green
File testing	The Korn shell uses the test command to evaluate conditional expressions and has a built-in set of options for testing attributes of files, such as whether it is a directory, a plain file (not a directory), a readable file, and so forth. See Example 2.5. EXAMPLE
	-d	File is a directory
	-a	File exists and is not a directory
	–r	Current user can read the file
	–s	File is of nonzero size
	–w	Current user can write to the file
	–x	Current user can execute the file
	Example 2.5. #!/bin/sh 1 if [ –a file ] then echo file exists fi 2 if [ –d file ] then echo file is a directory fi 3 if [ -s file ] then echo file is not of zero length fi 4 if [ -r file -a -w file ] then echo file is readable and writable fi
Functions	Functions allow you to define a section of shell code and give it a name. There are two formats: one from the Bourne shell, and the Korn shell version that uses the function keyword. EXAMPLE function_name() { block of code } function function_name { block of code } ------------------------- function lister { echo Your present working directory is `pwd` echo Your files are: ls }

2.5.1 The Korn Shell Script

Example 2.6.

1   #!/bin/ksh

2   # The Party Program––Invitations to friends from the "guest" file

3   guestfile=~/shell/guests

4   if [[ ! –a "$guestfile" ]]

    then

        print "${guestfile##*/} non–existent"

        exit 1

    fi

5   export PLACE="Sarotini's"

6   (( Time=$(date +%H) + 1 ))

7   set -A foods cheese crackers shrimp drinks "hot dogs" sandwiches

8   typeset -i n=0

9   for person in $(< $guestfile)

    do

10      if  [[ $person = root ]]

        then

              continue

        else

              # Start of here document

11           mail –v –s "Party" $person <<- FINIS

             Hi ${person}! Please join me at $PLACE for a party!

             Meet me at $Time o'clock.

             I'll bring the ice cream. Would you please bring

             ${foods[$n]} and anything else you would like to eat? Let

             me know if you can make it.

                   Hope to see you soon.

                            Your pal,

                            ellie@`hostname`

             FINIS

12           n=n+1



13           if (( ${#foods[*]} == $n ))

             then

14             set -A foods cheese crackers shrimp drinks "hot dogs"

               sandwiches

             fi

          fi

15  done

    print "Bye..."

EXPLANATION