Table of Contents
1.1. Defining Variables
1.2. Uppercase Variables
1.3. Lowercase Variables
2.1. Listing All Variables
2.2. Listing Specific Variables
3.1. Read-Only Variables (-r)
3.2. Integer Variables (-i)
3.3. Indexed Arrays (-a)
3.4. Associative Arrays (-A) (Bash 4 )
3.5. Exporting Variables (-x)
4.1. Setting Function Attributes
4.2. Listing All Functions (-f)
4.3. Listing Function Names (-F)
4.4. Exporting Functions
Home System Tutorial LINUX Bash declare Command Usage With Examples

Bash declare Command Usage With Examples

Mar 05, 2025 am 09:16 AM

The Bash declare command is a powerful tool for defining, manipulating, and displaying variables and functions. This tutorial provides a comprehensive guide to using declare in Bash scripting on Linux systems.

Table of Contents

    1. Basic declare Usage
    • 1.1. Defining Variables
    • 1.2. Uppercase Variables
    • 1.3. Lowercase Variables
    1. Listing Variables and Values
    • 2.1. Listing All Variables
    • 2.2. Listing Specific Variables
    1. Special declare Attributes
    • 3.1. Read-Only Variables (-r)
    • 3.2. Integer Variables (-i)
    • 3.3. Indexed Arrays (-a)
    • 3.4. Associative Arrays (-A)
    • 3.5. Exporting Variables (-x)
    1. Function Management with declare
    • 4.1. Setting Function Attributes
    • 4.2. Listing All Functions (-f)
    • 4.3. Listing Function Names (-F)
    • 4.4. Exporting Functions
    1. Checking Variable/Function Existence
    1. Removing Attributes
    1. Unsetting Variables/Functions
  • Example: Combined Variable and Function Management
  • Summary of declare Options
  • Conclusion

Bash declare Command Usage With Examples

1. Basic declare Usage

1.1. Defining Variables

declare can define variables, similar to direct assignment:

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declare my_var="Hello, World!"

echo "$my_var"

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1.2. Uppercase Variables

The -u flag converts variable values to uppercase:

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declare -u upper_var="hello world"

echo "$upper_var"  # Output: HELLO WORLD

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Copy after login

1.3. Lowercase Variables

The -l flag converts variable values to lowercase:

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declare -l lower_var="HELLO WORLD"

echo "$lower_var"  # Output: hello world

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2. Listing Variables and Values

2.1. Listing All Variables

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declare -p

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This displays all variables and their values.

2.2. Listing Specific Variables

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declare -p my_var

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Output (example):

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<code>declare -- my_var="Hello, World!"</code>

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3. Special declare Attributes

3.1. Read-Only Variables (-r)

The -r flag creates read-only variables:

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declare -r readonly_var="Fixed Value"

readonly_var="New Value"  # This will produce an error

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3.2. Integer Variables (-i)

The -i flag enforces integer type:

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declare -i num=10

num+=5

echo "$num"  # Output: 15

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Assigning non-numeric values will result in 0.

3.3. Indexed Arrays (-a)

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

echo "${my_array[@]}"  # Output: apple banana cherry

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Access individual elements using my_array[index].

3.4. Associative Arrays (-A) (Bash 4 )

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declare my_var="Hello, World!"

echo "$my_var"

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Access elements using keys.

3.5. Exporting Variables (-x)

The -x flag exports variables to subshells:

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declare -u upper_var="hello world"

echo "$upper_var"  # Output: HELLO WORLD

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This makes the variable available in child processes.

4. Function Management with declare

4.1. Setting Function Attributes

declare can set function attributes (e.g., local variables).

4.2. Listing All Functions (-f)

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declare -l lower_var="HELLO WORLD"

echo "$lower_var"  # Output: hello world

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Lists all defined functions with their definitions.

4.3. Listing Function Names (-F)

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declare -p

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Lists only the names of defined functions.

4.4. Exporting Functions

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declare -p my_var

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Exports the function to subshells.

5. Checking Variable/Function Existence

Use declare -p with error redirection to check existence.

6. Removing Attributes

Use instead of - to remove attributes (e.g., declare i num).

7. Unsetting Variables/Functions

Use the unset command (e.g., unset my_var, unset -f my_function).

Example: Combined Variable and Function Management

(Illustrative example combining variable and function management with declare.)

Summary of declare Options

(Table summarizing declare options and their purposes.)

Conclusion

The declare command is a crucial tool for advanced Bash scripting, offering fine-grained control over variables and functions. Mastering declare enhances script efficiency and readability.

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