This chapter is from the book
This chapter is from the book
This chapter is from the book
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
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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 |
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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 |
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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
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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
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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
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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
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Extracting values from variables |
To extract the value from variables, a dollar sign is used. EXAMPLE
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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, print -n "What is your name?" read name read name1 name2 ... |
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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 |
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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 ... Array
set -A fruit apples pears plums
print ${fruit[0]} Prints apple
${fruit[1]} = oranges Assign a new value |
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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 is declared as an integer (( n=5 + 5 )) The let command |
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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
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Operators |
The Korn shell uses the built-in test command operators to test numbers and strings, similar to C language operators. EXAMPLE
Logical: && and |
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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
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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
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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 |
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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
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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
-
This line lets the kernel know that you are running a Korn shell script.
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This is a comment. It is ignored by the shell, but important for anyone trying to understand what the script is doing.
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The variable guestfile is set to the full pathname of a file called guests.
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This line reads: If the file guests does not exist, then print to the screen "guests nonexistent" and exit from the script.
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An environment variable is assigned a value and exported (made available to subshells).
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The output of the UNIX/Linux command, the hour of the day, is assigned to the variable called Time. Variables are assigned the values for the place and time.
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The list of foods to bring is assigned to an array called foods with the set –A command. Each item on the list can be accessed with an index starting at 0.
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The typeset –i command is used to create an integer value.
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For each person on the guest list a mail message will be created inviting the person to a party at a given place and time, and assigning a food from the list to bring.
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The condition tests for the user root. If the user is root, control will go back to the top of the loop and assign the next user in the guest list to the variable, person.
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The mail message is sent. The message body is contained in a here document.
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The variable n is incremented by 1.
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If the number of elements in the array is equal to the value of the variable, then the end of the array has been reached.
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This marks the end of the looping statements.