Classic Shell Scripting

Working with Fields

For many applications, it's helpful to view your data as consisting of records and fields. A record is a single collection of related information, such as what a business might have for a customer, supplier, or employee, or what a school might have for a student. A field is a single component of a record, such as a last name, a first name, or a street address.

Text File Conventions

Because Unix encourages the use of textual data, it's common to store data in a text file, with each line representing a single record. There are two conventions for separating fields within a line from each other. The first is to just use whitespace (spaces or tabs):

$ cat myapp.data
# model     units sold     salesperson
xj11        23             jane
rj45        12             joe
cat6        65             chris
...

In this example, lines beginning with a # character represent comments, and are ignored. (This is a common convention. The ability to have comment lines is helpful, but it requires that your software be able to ignore such lines.) Each field is separated from the next by an arbitrary number of space or tab characters. The second convention is to use a particular delimiter character to separate fields, such as a colon:

$ cat myapp.data
# model:units sold:salesperson
xj11:23:jane
rj45:12:joe
cat6:65:chris
...

Each convention has advantages and disadvantages. When whitespace is the separator, it's difficult to have real whitespace within the fields' contents. (If you use a tab as the separator, you can use a space character within a ...