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August 4, 2003 - Using set to Process Unknown/Varying Numbers of Fields

It's relatively simple to write a shell script that will handle records or variables when the number of fields is known or fixed, but how would you write code to process records and variables when this number is unknown or varies from record to record?  This week's tip will provide you with one option for accomplishing this task.

Consider the following excerpt of shell script code:

<previous code>

set $RECORD

while [ $1 ]
do
     <field processing code>
     shift
done

<subsequent code>
The first line of actual code ("set $RECORD") will assign each field of the record stored in the variable RECORD to a positional parameter (e.g. $1, $2, $3, etc.).  These fields are typically delimited with a blank space, but not always.

Next, the while statement checks the value of $1 to decide whether or not the commands within the loop should be executed.  If the value of $1 is null, processing will continue at the statement immediately following the loop's closing statement ("done").

Located within the while loop are field processing statements, and the shift command.  shift will move (or "shift") the values in the positional parameters to the left, or down, one position.  This means that the value in $1 will be discarded, the value in $2 will be assigned to $1, the value to $3 will be assigned to $2, and so on.

After shift is executed, the value in $1 is checked again for a null value.  The loop will continue until all of the record's fields have been processed.  Since the code is only concerned about the value stored in $1, the number of total fields in each record is really irrelevant.

If multiple records are being processed (e.g. read individually from an external file), it's safe to assume that this snippet of code will be enclosed within another loop that stores each record in the RECORD variable.


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