LiveFire Labs' UNIX Tip, Trick, or Shell Script of the Week

Korn Shell Arrays - Array Operators - Part II

After reading last week's ksh array tip, you should have a basic understanding of how to assign values (elements) to an indexed array, and then access them individually based on their position in the array.

One example from last week demonstrated how to access and print each array element using a while loop:

$ i=0
$ while [ $i -lt 3 ]
> do
> print ${colors[$i]}
> (( i=i+1 ))
> done
RED
GREEN
BLUE
$

Hard-coding the number of elements in the array is fine for demonstrating a new concept, but obviously not very practical when it comes to using it within a shell script.  Most of the time you will not know how may elements will be stored in the array, or the number may be dynamic - changing each time the script is invoked.

Fortunately, there is an operator for determining the number of array elements:

${#arrayname[*]}

Comparing this to the syntax used for accessing individual array elements,

${arrayname[subscript]}

you should notice the addition of the # in front of the array name, and the use of an asterisk (*) in place of a subscript.  Armed with this new operator, let's tweak our while loop to print the previously defined elements:

$ i=0
$ NUM_OF_ELEMENTS=${#colors[*]}
$ while [ $i -lt $NUM_OF_ELEMENTS ]
> do
> print ${colors[$i]}
> (( i=i+1 ))
> done
RED
GREEN
BLUE
$

If we wanted to compress the code a little, the operator to detect the number of array elements could be embedded within the while statement:

$ i=0
$ while [ $i -lt ${#colors[*]} ]
> do
> print ${colors[$i]}
> (( i=i+1 ))
> done
RED
GREEN
BLUE
$

On a related note, there may be times when you need to print the entire element list at once.  This can be accomplished using the following operator:

$ print ${colors[*]}
RED GREEN BLUE
$

Notice that an asterisk is used instead of a subscript.