The New KornShell—ksh93

ksh93 provides two methods for function definitions. The formats are given as:

function name
{
    body
}
name()
{
    body
}

The second function format is provided for compatibility with POSIX standards. The primary distinction is that of variable name scope. In a POSIX function, a variable definition has global scope. In the following POSIX function bar, variable foo is redefined to a value of 6.

typeset foo=5
bar()
{
    typeset foo=6
    echo $foo
}
bar
6
echo $foo
6

Variable definitions in ksh93 functions have local scope. In the following ksh93 function bar, a local variable foo is defined and has precedence over the global variable foo.

typeset foo=5
function bar
{
    typeset foo=6
    echo $foo
}
bar
6
echo $foo
5

ksh93 provides active variables through a series of discipline functions. From the shell level, you can write get, set, and unset disciplines. Through the KornShell Development Kit, you can also add disciplines unique to your environment.

When a variable is referenced, as in $foo, ksh will invoke the get discipline associated with foo. The default discipline is to simply return the current value associated with foo. From the shell level, you can define a foo.get discipline function.

The set discipline is called when a value is assigned to a variable. Within the set discipline, the special variable .sh.name is the name of the variable whose value is being set.

On line 31 of lsc, we define a max_stringSize.get discipline function. Every reference to ${max_stringSize} will result in this function being executed. The value of the special .sh.value variable is the value returned from the discipline.

In ksh93, a printf statement is available following the ANSI C printf definition. This permits formatting specifications to be applied to each argument. To appreciate the differences between the standard print and printf statements, consider how you would output the contents of the entries array (from the lsc example), one per line. The standard print statement would display the file names as space-separated tokens on a single line. Using the printf statement with a "%s\n" format, however, would produce the desired results.

ksh93 statements of the form (( expression )) are called arithmetic commands. Arithmetic commands return True when the value of the enclosed expression is non-zero, and False when the expression evaluates to zero. The construct $((expression)) can be used as a word or part of a word. It is replaced by the value of expression.

In the lsc example, line 38, we evaluate the value of the discipline function using:

(( .sh.value = $(strlenList ${entries[@]}) + 3 ))

ksh93 will evaluate the expression, which includes an assignment to the .sh.value variable. Note that the:

$(strlenList ${entries[@]})

will invoke the strlenList built-in function and return the maximum width of the strings (given as element values) in the entries[] array. We add 3 to this value for formatting purposes.

An ANSI C string is defined by preceding the single-quoted string with a $. For example, $'*' is the literal asterisk, *. With ANSI C strings, all characters between the single quotes retain their literal meaning, except for escape sequences. An escape sequence is introduced by the escape character \.

ANSI C string support provides an essential feature for shell programmers. Consider, for example, having to process variables with embedded tabs in their values. Without ANSI C string support, we would not be able to effectively test the value of the variable for embedded tabs. As an example, consider the following script:

print "foo\tbar" > /tmp/foobar
read aline < /tmp/foobar
if [[ "${aline}" == "foo\tbar" ]]
then print TRUE
fi

The comparison (see Conditional Commands, below) will fail. We can replace the conditional with ANSI C strings and ensure proper functionality. The example above should be rewritten as:

print "foo\tbar" > /tmp/foobar
read aline < /tmp/foobar
if [[ "${aline}" == $'foo       bar' ]]
then    print TRUE
fi

On line 45 of Listing 1, we must test to see if the directory is empty. The preceding entries=(*) in an empty directory will set the entries variable to the literal asterisk if no files are found.