Bash declare Statement: Syntax and Examples

Introduction

A variable in bash is created by assigning a value to its reference. Although the built-in declare statement does not need to be used to explicitly declare a variable in bash, the command is often employed for more advanced variable management tasks.

This tutorial will show you how to work with variables and their attributes using the bash declare command.

Prerequisites

• Access to the terminal/command line.
• Sudo user privileges.

Bash Declare Syntax

The syntax for using the bash declare command is:

declare [options] [variable-name]="[value]"

Bash declare Options

The declare command works with the following general options:

The options in the table below are used to set an attribute to a variable.

How to Declare a Variable in Bash

The following example shows how to declare a variable named testvar and assign it the value of 100.

declare testvar="100"

When successfully executed, the command produces no output. Use the -p option to check if the variable was successfully created. Since the command lists all the variables currently present on the system, limit it with the grep command.

declare -p | grep testvar

The double dash (--) sign preceding the variable name signifies that the variable has no arguments.

To set an attribute to a variable, use the syntax below. The example sets the -x attribute to the testvar variable.

declare -x testvar

Search for the variable again and confirm that the attribute has been set.

declare -p | grep testvar

declare -p testvar

To remove the attribute, precede it with a plus (+) sign.

declare +x testvar

Integers

The -i option limits the variable to integer values only.

declare -i testvar

To test this, attempt to change the variable’s value to a string of characters.

declare testvar="example"

Use the echo command to see the value of the variable.

echo $testvar

The output shows the value of zero (0):

The value of a variable can also be the result of a mathematical operation.

declare testvar="3*3"

In the example above, the testvar variable was given the value of 3*3. Checking the variable value with echo shows the result.

echo $testvar

The result of the mathematical operation does not have to be an integer.

declare testvar="10/3"

However, if the variable has the -i attribute set, the result is rounded to the closest integer.

echo $testvar

Attempting to declare a non-integer value to a variable while the -i option is set, results in an error.

declare testvar="1.5"

Cases

Use the -u attribute to convert all letters to uppercase. The example below declares the testvar variable and gives it the value of example.

declare -u testvar="example"

However, since the -u attribute was set, checking the value of the variable shows that it has the value of EXAMPLE.

echo $testvar

The -l attribute has the opposite effect.

declare -l testvar="EXAMPLE"

Setting it converts uppercase letters to lowercase.

echo $testvar

Read-Only

Set the -r attribute to the variables you want to protect against accidental change.

declare -r testvar="100"

Attempting to change the value of the read-only variable results in an error.

declare testvar="50"

Arrays

Bash variables can have more than one value. To assign multiple values to a single bash variable, convert it to an array by typing:

declare -a testvar

If the variable had a value before conversion, that value is now the first element of the array, with the index number 0. To check the value of the first element in the variable array, type:

echo ${testvar[0]}

Declare more elements of the array by specifying the element’s index number.

declare testvar[1]="200"

List all the elements of the array by typing:

echo ${testvar[*]}

Conclusion

After completing this tutorial you should be able to use the bash declare command to declare variables and set/unset their attributes.

To edit the default bash prompt, read How to Customize Bash prompt in Linux.