Variables and Parameters in the Korn Shell

This chapter is from the book 

Variables
Special Parameters
Variable Expansion
Array Variables
Compound Variables
Quoting

Variables and parameters are used by the Korn shell to store values. Like other high-level programming languages, the Korn shell supports data types and arrays. This is a major difference with the Bourne, C shell, and other scripting languages, which have no concept of data types.

The Korn shell supports four data types: string, integer, float, and array. If a data type is not explicitly defined, the Korn shell will assume that the variable is a string.

By default, all variables are global in scope. However, it is possible to declare a local variable within a function. This is discussed in more detail later in this chapter.

Variables

Korn shell variable names can begin with an alphabetic (a–Z) or underscore character, followed by one or more alphanumeric (a–Z, 0–9) or underscore characters. Other variable names that contain only digits (0–9) or special characters (!, @, #, %, *, ?, $) are reserved for special parameters set directly by the Korn shell.

To assign a value to a variable, you can simply name the variable and set it to a value. For example, to assign abc to variable X:

$ X=abc 

The typeset command can also be used to assign values, but unless you are setting attributes, it's a lot more work for nothing. If a value is not given, the variable is set to null. Here, X is reassigned the null value:

$ X= 

This is not the same as being undefined. As we'll see later, accessing the value of an undefined variable may return an error, while accessing the value of a null variable returns the null value.

Accessing Variable Values

To access the value of a variable, precede the name with the $ character. There can be no space between $ and the variable name. In this example, CBIN is set to /usr/ccs/bin.

$ CBIN=/usr/ccs/bin 

Table 3.1. Assigning Values to Variables

variable=	declare variable and set it to null
typeset variable=	declare variable and set it to null
variable=value	assign value to variable
typeset variable=value	assign value to variable

Now you can just type $CBIN instead of the long pathname:

$ cd $CBIN 
   $ pwd 
   /usr/ccs/bin 

Here is a new command to go along with this concept: print. It displays its arguments on your terminal, just like echo.

$ print Hello world! 
   Hello world! 

Here we use print to display the value of CBIN:

$ print $CBIN 
   /usr/ccs/bin 

Variable Attributes

Korn shell variables can have one or more attributes that specify their internal representation, access or scope, or the way they are displayed. This concept is similar to a data type in other high-level programming languages, except that in the Korn shell it is not as restrictive. Variables can be set to integer type for faster arithmetic operations, read-only so that the value cannot be changed, left/right justified for formatting purposes, and more. To assign a value and/or attribute to a Korn shell variable, use the following format with the typeset command:

typeset –attribute variable=value 

or

typeset –attribute variable 

Except for readonly, variable attributes can be set before, during, or after assignment. Functionally it makes no difference. Just remember that the attribute has precedence over the value. This means that if you change the attribute after a value has been assigned, the value may be affected.

Lowercase (–l) and Uppercase (–u) Attributes

These attributes cause the variable values to be changed to lower or uppercase. For example, the lowercase attribute and uppercase value ASPD are assigned to variable MYSYS:

$ typeset —l MYSYS=ASPD 

Despite the fact that MYSYS was assigned uppercase ASPD, when accessed, the value is displayed in lowercase:

$ print $MYSYS 
   aspd 

This is because the attribute affects the variable value, regardless of the assignment. Variable attributes can also be changed after assignment. If we wanted to display variable MYSYS in uppercase, we could just reset the attribute:

$ typeset —u MYSYS 
   $ print $MYSYS 
   ASPD 

Table 3.2. Assigning Values/Attributes to Variables

typeset – attribute variable=value	assign attribute and value to variable
typeset –attribute variable	assign attribute to variable
typeset +attribute variable	remove attribute from variable

Readonly (–r) Attribute

Once the readonly attribute is set, a variable cannot be assigned another value. Here, we use it to set up a restricted PATH:

$ typeset —r PATH=/usr/rbin 

If there is an attempt to reset PATH, an error message is generated:

$ PATH=$PATH:/usr/bin: 
   /bin/ksh: PATH: is read only 

We'll come back to this in a few pages. Unlike other variable attributes, once the readonly attribute is set, it cannot be removed.

The readonly command can also be used to specify a readonly variable.

Integer (–i) Attribute

The integer attribute (–i) is used to explicitly declare integer variables. Although it is not necessary to set this attribute when assigning integer values, there are some benefits to it. We'll cover this later in Chapter 6. In the meantime, NUM is set to an integer-type variable and assigned a value:

$ typeset —i NUM=1 
   $ print $NUM 
   1 

We could also assign NUM the number of users on the system using command substitution like this:

$ typeset —i NUM=$(who | wc —l) 
   $ print $NUM 
   3 

There is one restriction on integer variables. Once a variable is set to integer type, it can't be assigned a non-integer value:

$ typeset —i NUM=abc 
   /bin/ksh: NUM: bad number 

Float (–E, –F) Attribute

The float attributes (–E, –F) are used to declare float variables. The –E is used to specify the number of significant digits, while –F is used to specify the precision. We'll cover this later in Chapter 6. In the following example, X is set to a float variable and assigned a value using both formats:

$ typeset —E5 X=123.456 
   $ print $X 
   123.46 
   $ typeset —F5 X=123.456 
   $ print $X 
   123.45600 

The float command can also be used to declare a float variable, but does not allow for specifying the precision.

Right (–R) and Left (–L) Justify Attributes

The right and left justify attributes cause variable values to be justified within their width and are be used to format data. Here, variables A and B are set to right-justify with a field width of 7 characters. Notice that integer values are used, even though the integer attribute is not set.

$ typeset —R7 A=10 B=10000 
   $ print :$A: 
   :     10: 
   $ print :$B: 
   :     10000: 

If the field width is not large enough for the variable assignment, the value gets truncated. Variable X is assigned a seven-character wide value, but the field width is set to 3, so the first four characters are lost:

$ typeset —R3 X=ABCDEFG 
   $ print $X 
   EFG 

If a field width is not given, then it is set with the first variable assignment. Variable Y is assigned a three-character wide value, so the field width is set to 3.

$ typeset —L Y=ABC 
   $ print $Y 
   ABC 

Without explicitly resetting the field width, a subsequent assignment would be restricted to a three-character wide value:

$ Y=ZYXWVUT 
   $ print $Y 
   ZYX 

Autoexport (–x) Attribute

This is another useful attribute. It allows you to set and export a variable in one command. Instead of

$ typeset X=abc 
   $ export X 

you can do this:

$ typeset —x X=abc 

We could use this attribute to add the /lbin directory to the PATH variable and export it all in one command:

$ typeset —x PATH=$PATH:/lbin 

Table 3.3. Some Variable Attributes

typeset –i var	Set the type of var to be integer
typeset –l var	Set var to lower case
typeset –L var	Left justify var; the field width is specified by the first assignment
typeset –Ln var	Left justify var; set field width to n
typeset –LZn var	Left justify var; set field width to n and strip leading zeros
typeset –r var	Set var to be readonly (same as the readonly command)
typeset –R var	Right justify var; the field width is specified by the first assignment
typeset –Rn var	Right justify var; set field width to n
typeset –RZn var	Right justify var; set field width to n and fill with leading zeros
typeset –t var	Set the user-defined attribute for var. This has no meaning to the Korn shell.
typeset –u var	Set var to upper case
typeset –x var	Automatically export var to the environment (same as the export command)
typeset –Z var	Same as typeset –RZ

Removing Variable Attributes

Except for readonly, variable attributes are removed with the typeset +attribute command. Assuming that the integer attribute was set on the NUM variable, we could remove it like this:

$ typeset +i NUM 

and then reassign it a non-integer value:

$ NUM=abc 

Once the readonly attribute is set, it cannot be removed. When we try to do this with the PATH variable that was previously set, we get an error message:

$ typeset +r PATH 
   /bin/ksh: PATH:  is read only 

The only way to reassign a readonly variable is to unset it first, then assign a value from scratch.

Multiple Attributes

Multiple attributes can also be assigned to variables. This command sets the integer and autoexport attributes for TMOUT:

$ typeset —ix TMOUT=3000 

To set and automatically export ORACLE_SID to uppercase prod:

$ typeset —ux ORACLE_SID=prod 
   $ print $ORACLE_SID 
   PROD 

Obviously, some attributes like left and right justify are mutually exclusive, so they shouldn't be set together.

Checking Variable Attributes

Attributes of Korn shell variables are listed using the typeset – attribute command. For example, to list all the integer type variables and their values:

$ typeset —i 
   ERRNO=0 
   MAILCHECK=600 
   PPID=177 
   RANDOM=22272 
   SECONDS=4558 
   TMOUT=0 

To list only the names of variables with a specific attribute, use the typeset + attribute command.

More with Variables

You can do other things with variables, such as assign them the value of another variable, the output of a command, or even the contents of a file. Here Y is assigned the value of variable X:

$ X=$HOME 
   $ Y=$X 
   $ print $Y 
   /home/anatole 

Variables can be assigned command output using this format:

variable=$(command) 

or

variable=`command` 

The second format is provided for compatibility with the Bourne shell. Here, UCSC is set to its internet ID by assigning the output of the grep and cut commands:

$ UCSC=$(grep UCSC /etc/hosts | cut —f1 —d" ") 
   $ print $UCSC 
   128.114.129.1 

Variables can also be assigned the contents of files like this:

variable=$(<file) 

or

variable=`cat file` 

The first format is equivalent to variable=$(cat file). The second format is much slower, but is provided for compatibility with the Bourne shell. Here, the FSYS variable is set to the contents of the /etc/fstab file:

$ FSYS=$(</etc/fstab) 
   $ print $FSYS 
   /dev/roota / /dev/rootg /usr 

Notice that the entries were displayed all on one line, instead of each on separate lines as in the file. We'll talk about this in the Quoting section later in this chapter.

A nameref variable is a synonym for another variable and will always have the same value as its associated variable They are created using the following formats:

nameref nameref_variable=variable 

or

typeset –n nameref_variable=variable 

For example:

$ X=abc 
   $ nameref Y=X 
   $ print $X 
   abc 
   $ print $Y 
   abc 

Unsetting Variables

Variable definitions are removed using the unset command. The TMOUT variable is not being used, so let's unset it:

$ unset TMOUT 

Now to check and see:

$ print $TMOUT 
   
   $ 

This is not the same as being set to null. As we'll see later in this chapter, variable expansion may be performed differently, depending on whether the variable value is set to null.

Unsetting either the base or nameref variable will unset both variables.

$ unset Y 
   $ print $X 
   
   $ print $Y 
   
   $