How To Manage Bash Functions Using declare Command In Linux

In Fish shell, there is a dedicated command namely functions to manage and interact with Functions. But there isn’t a built-in command exactly like Fish shell’s functions command for managing functions in Bash. However, Bash provides other tools and techniques to achieve similar functionality. In this tutorial, we will learn how to manage Bash functions declare command effectively.

How to Manage Functions in Bash

1. Create or Define a New Function

We can create or define a function either directly in the ~/.bashrc file or save the functions in a separate files under a dedicated directory, for example ~/.bash_functions.

Storing all functions under a single directory allows you to keep your functions organized and easy to manage.

Create a dedicated directory for storing all functions:

mkdir ~/.bash_fucntions

Store Functions in Separate Files:

Save each function in a separate file under ~/.bash_functions/.

Let us create an example function named cdls.

nano ~/.bash_functions/cdls.sh

Add the following lines in it:

cdls () 
{ 
    cd "$@" && ls --color=auto
}

Source Functions from ~/.bashrc:

Add the following lines to your ~/.bashrc to automatically load all function files:

for func_file in ~/.bash_functions/*.sh; do
    source "$func_file"
done

You can now use the cdls function to list the contents of a directory automatically whenever you cd into it.

cdls Downloads/

Now, you can use the declare command to list, view, or delete functions as needed.

2. List All Functions

To list all currently defined functions in Bash, use the declare command with the -F option:

declare -F

This will output the names of all functions. If you want to see both the names and definitions, use:

declare -f

3. View a Specific Function

To view the definition of a specific function, use declare -f followed by the function name:

declare -f function_name

For example, to view the definition of a function called cdls, run:

declare -f cdls

Sample Output:

cdls () 
{ 
    cd "$@" && ls --color=auto
}

4. Delete a Function

To delete (unset) a function, use the unset command:

unset -f function_name

For example, to delete the cdls function:

unset -f cdls

5. Edit a Function

To edit a function, you can either:

• Manually edit the file where the function is defined (e.g., ~/.bashrc or ~/.bash_functions/).
• Use a combination of declare -f and a text editor to edit the function directly in the terminal.

For example, to edit the cdls function:

declare -f cdls > /tmp/cdls.sh
nano /tmp/cdls.sh
source /tmp/cdls.sh

6. Save Functions to a File

To save a function’s definition to a file, use declare -f and redirect the output:

declare -f function_name > filename.sh

For example, to save the cdls function to a file:

declare -f cdls > ~/.bash_functions/cdls.sh

7. Load Functions from a File

To load a function from a file, use the source command:

source filename.sh

For example, to load the cdls function:

source ~/.bash_functions/cdls.sh

8. Copy a Function

To copy a function to a new name, use declare -f and eval:

declare -f original_function > /tmp/temp_function
sed 's/original_function/new_function/g' /tmp/temp_function | source /dev/stdin

For example, to copy cdls to mycdls:

declare -f cdls > /tmp/temp_function
sed 's/cdls/mycdls/g' /tmp/temp_function | source /dev/stdin

9. Check if a Function Exists

To check if a function exists, use declare -F and grep:

if declare -F function_name > /dev/null; then
    echo "Function exists"
else
    echo "Function does not exist"
fi

Cheatsheet: Managing Functions in Bash

List All Functions:

declare -F

View a Specific Function:

declare -f cdls

Delete a Function:

unset -f cdls

Save a Function to a File:

declare -f cdls > ~/.bash_functions/cdls.sh

Load a Function from a File:

source ~/.bash_functions/cdls.sh

Summary

While Bash doesn’t have a dedicated functions command like Fish shell, you can achieve similar functionality using the declare command. By storing functions in separate files and sourcing them from ~/.bashrc, you can keep your setup clean and organized.