Associating attributes with variables

The typeset command lets you associate attributes with shell variables. This process is analogous to declaring the type of a variable in a conventional programming language. For example:

typeset –i8 y

says that y is an octal integer. In this way, you can make sure that arithmetic with y is always performed in base 8 rather than the usual base 10.

Other attributes may specify how the variable's value is displayed when the variable is expanded. Attributes of this kind are:

–Ln: The value should always be displayed with n characters, left-justified within that space.
–Rn: The value should always be displayed with n characters, right-justified within that space.
–RZn: The value should always be displayed with n characters, right-justified and with enough leading zeros to fill out the rest of the space.
–Zn: The same as -RZn.
–LZn: The value should always be displayed with n characters, left-justified and with leading zeros stripped off.

All of these options may lead to truncation of a value that is longer than the specified length.

You can use the –u attribute of typeset for variables with string values. Then whenever such a variable is assigned a new value, all lowercase letters in the value are automatically converted to uppercase. Similarly, the –l attribute specifies that whenever a variable is assigned a new value, all uppercase letters in the value are automatically converted to lowercase.

The read-only attribute –r is useful when a variable is marked for export. The command:

typeset –r name

says that the variable name cannot be changed from its present value. Then subsequent commands cannot change this value. You can also use the format:

typeset –r name=value

which sets the variable to the given value and marks it read-only so that the value cannot be changed.