History

Invented by David Korn of AT&T Bell Labs in the mid-1980s.

• Almost entirely upward compatible with Bourne shell.

Variable Expression and Modifier

Variable expressions can be tested and modified by using special modifiers.

• ${variable:–word}:
  • If variable is set and is nonnull, substitute its value;
  • Otherwise, substitute word.
• ${variable:=word}:
  • If variable is not set or is null, set it to word; the value of variable is substituted permanently.
  • Positional parameters may not be assigned in this way.
• ${variable:+word}:
  • If variable is set and is nonnull, substitute word;
  • Otherwise substitute nothing.
• ${variable:?word}:
  • If variable is set and is nonnull, substitute its value;
  • Otherwise, print word and exit from the shell.

Built-In Commands

• set: Print all variables, local and global.
• env: Print only global variables.
• typeset: Print all variables, integers, functions, and exported variables.
• unset: Undefines a non-readonly variable.

print

Used instead of echo.

• Has more options.

More on -un:

• If file descriptor is greater than 2, it must first be opened with exec.
• If n is not specified, default file descriptor is 1 (standard output).

Functions

Besides the Bash way of declaring functions, Korn also supports another way to declare functions:

# Bash syntax for function declaration
my_function() { commands; }
# Korn alternative syntax
function my_function { commands; }

Positional Parameters

Like Bourne, but with some C shell features.

• $0: References the name of the current shell script.
• $1–$9: Positional parameters 1–9.
• ${10}: Positional parameter 10.
• $#: Evaluates to the number of positional parameters.
• $*: Evaluates to all the positional parameters.
• $@: Same as $*, except when double quoted.
• "$*”: Evaluates to “$1 $2 $3”,etc.
• "$@”: Evaluates to “$1” “$2” “$3”, etc.

Test Operator ([[ ]])

The Korn test command has additional operators and features over Bash.

• Wildcards can be used in string-matching tests

String Operator Options:

let command and expression evaluation

let is a built-in command used to perform integer arithmetic.

• Replaces Bourne shell integer testing.
• Alternative way to use let is (())
  • (()) doesn’t require $ before variable names.

Example: Using let

$ i=5
$ let i=i+1
$ print $i
6

typeset

typeset: Lets you assign variable types.

# Floating point number assignment to variable.
typeset –F my_var

Note: On floating-points Korn supports integer and floating-point arithmetic.

• Floating-point arithmetic available only on versions newer than 1988.