Bash Declare Statement: Syntax and Examples

As a Linux user, you've probably heard of Bash shell. Bash is a powerful tool that allows you to perform a variety of tasks on your system. One of most useful features of Bash is declare statement. In this article, we'll explore what declare statement is, how it works, and provide examples of how you can use it in your scripts.

What is Declare Statement?

The declare statement is a built-in Bash command that allows you to set attributes for variables. When you declare a variable, you are telling Bash to treat that variable in a certain way. For example, you can declare a variable to be read-only or to be an array.

Syntax of Declare Statement

The syntax of declare statement is simple. Here's basic format ?

declare [options] variable=value

The declare keyword tells Bash that you want to declare a variable. options allow you to set attributes for variable. Finally, you assign a value to variable.

Options for Declare Statement

There are several options you can use with declare statement. Here are some of most common ones ?

• -a ? This option declares variable to be an array.

• -i ? This option declares variable to be an integer.

• -r ? This option declares variable to be read-only.

• -x ? This option exports variable to environment.

Examples of Declare Statement

Let's take a look at some examples of how you can use declare statement in your scripts.

Declare a Variable as an Array

declare -a my_array=("apple" "banana" "cherry")

In this example, we are declaring a variable called my_array as an array. We are also assigning three values to array: "apple", "banana", and "cherry".

Declare a Variable as an Integer

declare -i my_num=5

In this example, we are declaring a variable called my_num as an integer. We are also assigning a value of 5 to variable.

Declare a read-only Variable

declare -r my_name="John"

In this example, we are declaring a variable called my_name and making it read-only. This means that value of my_name cannot be changed once it has been set.

Export a Variable to Environment

declare -x my_path="/usr/local/bin"

In this example, we are declaring a variable called my_path and exporting it to environment. This means that other programs that are run from same shell session will be able to access value of my_path.

Additional Examples of Declare Statement

Let's dive into some more examples of declare statement to gain a deeper understanding of its functionality ?

Declare a Variable with a Default Value

declare my_var="Hello World!"

In this example, we are declaring a variable called my_var and assigning it a default value of "Hello World!".

Declare an Integer Variable and Perform Arithmetic Operations

declare -i num1=5
declare -i num2=10
declare -i result=num1+num2
echo "The result is: $result"

In this example, we are declaring two integer variables called num1 and num2, and then declaring a third integer variable called result, which is sum of first two variables. We then print out result using echo command.

Declare an Associative Array

declare -A my_array=([apple]="red" [banana]="yellow" [cherry]="red")
echo "${my_array[banana]}"

In this example, we are declaring an associative array called my_array with three key-value pairs. We then print out value of banana key using ${my_array[banana]} syntax.

Declare a Function

declare -f my_func
my_func() {
   echo "Hello World!"
}
my_func

In this example, we are declaring a function called my_func using -f option. We then define function, which simply prints out "Hello World!". Finally, we call function using my_func command.

Declare a Variable Using a Reference

declare -n my_ref=my_var
my_var="Hello Universe!"
echo "$my_ref"

In this example, we are declaring a variable called my_var with a value of "Hello Universe!". We then declare a reference variable called my_ref, which is assigned value of my_var. We then print out value of my_ref, which is same as my_var.

Additionally, you can use the "declare" statement to set attributes for variables that affect their behavior in different ways. For example, you can use the "-t" option to set a trace variable, which allows you to debug Bash scripts by tracing the execution of commands.

Example

declare -t var1="This is a trace variable"
echo $var1

Output

+ echo 'This is a trace variable'
This is a trace variable

Here, we set the "var1" variable as a trace variable using the "-t" option. When we ran the script, Bash displayed a "+" sign before executing the "echo" command, indicating that the trace variable is enabled.

Another useful attribute you can set with the "declare" statement is the "-u" option, which converts the value of a variable to uppercase.

Example 

declare -u var2="this is a lowercase string"
echo $var2

Output

THIS IS A LOWERCASE STRING

Here, we declared a variable "var2" with the "-u" option, which converted the lowercase string to uppercase.

One important thing to note is that the "declare" statement can also be used to declare and assign multiple variables in a single command. 

Example 

declare var3=10 var4="This is another variable" var5=("apple" "banana" "orange")
echo $var3
echo $var4
echo ${var5[1]}

Output

10
This is another variable
banana

Here, we used a single "declare" statement to declare and assign three variables: "var3", "var4", and "var5". Then, we accessed the values of these variables using their names and indices.

Conclusion

The declare statement is a powerful tool that allows you to set attributes for variables in Bash. By using options available with declare statement, you can make your scripts more efficient and easier to read. Whether you are declaring an array, an integer, a read-only variable, or exporting a variable to environment, declare statement is an essential command for any Bash programmer.