Using the typeset command (with examples)

The typeset command in Bash is used to declare variables and assign attributes to them. This command provides flexibility in defining the type and properties of variables in a shell script.

1: Declare a string variable with the specified value

typeset variable="value"

Motivation: By declaring a string variable, we can store and manipulate textual data within our script. This is useful when dealing with user input, file operations, or text processing.

Explanation:

• typeset: The command used to declare variables and assign attributes.
• variable: The name of the variable to be declared.
• "value": The value assigned to the variable. It can be any string enclosed in double-quotes.

Example output:

typeset greeting="Hello, World!"
echo $greeting
# Output: Hello, World!

2: Declare an integer variable with the specified value

typeset -i variable="value"

Motivation: When we need to perform mathematical operations or comparisons, declaring an integer variable ensures that the values are treated as numbers rather than strings.

Explanation:

• typeset -i: The -i option is used to declare an integer variable.
• variable: The name of the variable to be declared.
• "value": The value assigned to the variable. It should be a valid integer.

Example output:

typeset -i count=5
echo $count
# Output: 5

3: Declare an array variable with the specified value

typeset variable=(item_a item_b item_c)

Motivation: Arrays allow us to store multiple values under a single variable name. This is useful for operations that involve a collection of related items.

Explanation:

• typeset: The command used to declare variables and assign attributes.
• variable: The name of the variable to be declared.
• (item_a item_b item_c): The values assigned to the variable, enclosed in parentheses and separated by spaces.

Example output:

typeset fruits=(apple banana cherry)
echo ${fruits[0]}
# Output: apple

4: Declare an associative array variable with the specified value

typeset -A variable=([key_a]=item_a [key_b]=item_b [key_c]=item_c)

Motivation: Associative arrays are similar to regular arrays but allow us to use custom keys instead of numeric indices. This is useful for creating lookup tables or mapping values to specific keys.

Explanation:

• typeset -A: The -A option is used to declare an associative array variable.
• variable: The name of the variable to be declared.
• ([key_a]=item_a [key_b]=item_b [key_c]=item_c): The key-value pairs assigned to the variable, enclosed in parentheses and separated by spaces.

Example output:

typeset -A ages=([John]=25 [Emily]=30 [Michael]=35)
echo ${ages[Emily]}
# Output: 30

5: Declare a readonly variable with the specified value

typeset -r variable="value"

Motivation: Read-only variables cannot be modified or reassigned once they are declared. This is useful when we want to enforce immutability for certain values in our script.

Explanation:

• typeset -r: The -r option is used to declare a readonly variable.
• variable: The name of the variable to be declared.
• "value": The value assigned to the variable. It can be any string enclosed in double-quotes.

Example output:

typeset -r pi="3.14"
echo $pi
# Output: 3.14

# Trying to modify the readonly variable
pi="3.14159"
# Output: -bash: pi: readonly variable

6: Declare a global variable within a function with the specified value

typeset -g variable="value"

Motivation: By default, variables declared within a function are local to that function and cannot be accessed outside. Using the -g option allows us to declare a global variable that can be accessed and modified from any part of the script.

Explanation:

• typeset -g: The -g option is used to declare a global variable within a function.
• variable: The name of the variable to be declared.
• "value": The value assigned to the variable. It can be any string enclosed in double-quotes.

Example output:

function setGlobalVariable {
  typeset -g globalVar="I'm a global variable"
}

setGlobalVariable
echo $globalVar
# Output: I'm a global variable

In conclusion, the typeset command provides a way to declare variables with specific attributes in Bash. By using different options and values when using typeset, we can ensure data integrity and provide clearer intentions in our scripts. Whether it’s declaring string variables, integer variables, arrays, associative arrays, readonly variables, or global variables within functions, the typeset command offers the flexibility needed to handle various scenarios.