In the context of the Bash shell scripting language, the “declare” command is used to declare variables and assign attributes to them. It provides a way to define variables with specific characteristics that affect their behavior and usage within a script. Here’s an elaboration on the “declare” command:
1. Variable Declaration: The primary purpose of the “declare” command is to declare variables. By using “declare,” you can create new variables and assign values to them. For example:
# declare variable="value"
2. Variable Attributes: The “declare” command allows you to assign attributes to variables, which modify their behavior or provide additional information about them. Some commonly used attributes include:
- “readonly“: Makes the variable read-only, preventing its value from being changed.
- “export“: Marks the variable for export, making it available to child processes.
- “integer“: Specifies that the variable should be treated as an integer, enabling arithmetic operations.
- “array“: Indicates that the variable is an array, allowing multiple values to be stored in it.
- “local“: Restricts the scope of the variable to the current function or code block.
- “declare -a array_name“: Declares an array explicitly.
You can combine multiple attributes together, such as “declare -r variable” to declare a read-only variable.
3. Displaying Variables: The “declare” command can be used to display the attributes and values of variables. By executing “declare” without any arguments, it will show a list of all currently declared variables along with their attributes.
4. Local Variables: By using “declare” with the “local” attribute, you can create local variables within functions or code blocks. These variables are only accessible within the scope where they are defined and do not overwrite global variables with the same name.
5. Variable Scoping: Variables declared with “declare” without any attributes are considered local by default. They are visible within the current scope, including subshells and functions. However, they are not automatically exported to child processes.
6. Alternative Syntax: The “declare” command can also be used with the “typeset” command, which is essentially a synonym for “declare.” Both commands serve the same purpose of declaring variables and assigning attributes.
declare Command Examples
1. Declare a string variable with the specified value:
# declare variable="value"
2. Declare an integer variable with the specified value:
# declare -i variable="value"
3. Declare an array variable with the specified value:
# declare -a variable=(item_a item_b item_c)
4. Declare an associative array variable with the specified value:
# declare -A variable=([key_a]=item_a [key_b]=item_b [key_c]=item_c)
5. Declare a readonly string variable with the specified value:
# declare -r variable="value"
6. Declare a global variable within a function with the specified value:
# declare -g variable="value"
Summary
Overall, the “declare” command in Bash provides a convenient way to define variables with specific attributes that affect their behavior and scope. By utilizing attributes like “readonly,” “export,” “integer,” and others, you can customize the variables according to your script’s requirements.