Bash declare Command Usage With Examples
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
-
- Basic
declare
Usage
- 1.1. Defining Variables
- 1.2. Uppercase Variables
- 1.3. Lowercase Variables
- Basic
-
- Listing Variables and Values
- 2.1. Listing All Variables
- 2.2. Listing Specific Variables
-
- 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
)
- Special
-
- 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
- Function Management with
-
- Checking Variable/Function Existence
-
- Removing Attributes
-
- Unsetting Variables/Functions
- Example: Combined Variable and Function Management
- Summary of
declare
Options - Conclusion
1. Basic declare
Usage
1.1. Defining Variables
declare
can define variables, similar to direct assignment:
1 2 |
|
1.2. Uppercase Variables
The -u
flag converts variable values to uppercase:
1 2 |
|
1.3. Lowercase Variables
The -l
flag converts variable values to lowercase:
1 2 |
|
2. Listing Variables and Values
2.1. Listing All Variables
1 |
|
This displays all variables and their values.
2.2. Listing Specific Variables
1 |
|
Output (example):
1 |
|
3. Special declare
Attributes
3.1. Read-Only Variables (-r
)
The -r
flag creates read-only variables:
1 2 |
|
3.2. Integer Variables (-i
)
The -i
flag enforces integer type:
1 2 3 |
|
Assigning non-numeric values will result in 0.
3.3. Indexed Arrays (-a
)
1 2 |
|
Access individual elements using my_array[index]
.
3.4. Associative Arrays (-A
) (Bash 4 )
1 2 |
|
Access elements using keys.
3.5. Exporting Variables (-x
)
The -x
flag exports variables to subshells:
1 2 |
|
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
)
1 2 |
|
Lists all defined functions with their definitions.
4.3. Listing Function Names (-F
)
1 |
|
Lists only the names of defined functions.
4.4. Exporting Functions
1 |
|
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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