8 Different Use Cases of the `source` Command (with examples)
Use Case 1: Evaluate contents of a given file
source path/to/file
Motivation: This use case is helpful when you want to execute a series of commands or set environment variables defined in a file, without creating a new subshell. Using source allows the changes made by the commands in the file to persist in the current shell session.
Explanation: The source command takes a file path as an argument and evaluates the contents of that file in the current shell context. It reads and executes each line of the file as if it were entered directly into the shell prompt.
Example Output: Suppose the file path/to/file contains the following commands:
echo "Hello, world!"
export MY_VAR="example"
When executed with source path/to/file, the output will be:
Hello, world!
And the environment variable MY_VAR will be set to “example” in the current shell session.
Use Case 2: Evaluate contents of a file using the . operator
. path/to/file
Motivation: This use case is an alternative way to achieve the same result as the previous use case. The . operator, when used as a command, is functionally equivalent to the source command.
Explanation: The . operator (dot operator) can be used instead of source to evaluate the contents of a file in the current shell context. It behaves the same way as the source command, reading and executing each line of the file as if it were entered directly into the shell prompt.
Example Output: Following the same example as in Use Case 1, executing . path/to/file will produce the same output:
Hello, world!
And the environment variable MY_VAR will be set to “example” in the current shell session.
Use Case 3: Sourcing multiple files sequentially
source file1 file2 file3
Motivation: Sometimes, you may need to execute multiple files in a specific order, each containing different sets of commands or environment variable definitions. By providing multiple file paths to the source command, you can easily execute them sequentially within the same shell session.
Explanation: When multiple file paths are provided as arguments to the source command, each file is evaluated in the order they are specified. The commands and environment variable definitions in each file are executed in the current shell context.
Example Output: Suppose we have three files: file1, file2, and file3, with the following contents, respectively:
echo "Executing file1"
export VAR1="value1"
echo "Executing file2"
export VAR2="value2"
echo "Executing file3"
export VAR3="value3"
When executed with source file1 file2 file3, the output will be:
Executing file1
Executing file2
Executing file3
And the environment variables VAR1, VAR2, and VAR3 will be set accordingly in the current shell session.
Use Case 4: Execute a configuration file for an application
source config.cfg
Motivation: Many applications use configuration files to manage their settings. By sourcing the configuration file using the source command, you can load the configuration into the current shell session, making the settings immediately available for the application to use.
Explanation: The source command is commonly used to execute configuration files for applications. The configuration file typically contains settings in the form of environment variable assignments, which are evaluated in the current shell context. These settings can be used by the application to customize its behavior.
Example Output: Suppose the config.cfg file contains the following settings:
export DB_HOST="localhost"
export DB_PORT=5432
export LOG_LEVEL="debug"
When executed with source config.cfg, the configuration settings will be loaded into the current shell session, making the values of DB_HOST, DB_PORT, and LOG_LEVEL available for the application to use.
Use Case 5: Source a script to extend or modify shell behavior
source custom-script.sh
Motivation: You may have a custom shell script that enhances or modifies the behavior of your shell session. By sourcing the script, you can leverage its functionality within the current shell environment.
Explanation: The source command can be used to execute a custom script that extends the behavior of your shell session. The script can contain functions, aliases, or other modifications that alter how commands are executed or provide additional functionality.
Example Output: Suppose custom-script.sh contains the following lines:
alias ll='ls -l'
welcome_message() {
echo "Welcome to the custom shell!"
}
When executed with source custom-script.sh, the ll alias will be available, allowing you to use ll as a shortcut for ls -l. Additionally, the welcome_message function will be defined, and you can invoke it by executing welcome_message in the shell.
Use Case 6: Source a script to set up environment variables for a specific task
source setup-env.sh
Motivation: In certain cases, you may require specific environment variables to be set before performing a task. By sourcing a script that sets up those environment variables, you can ensure they are properly configured for the task at hand.
Explanation: The source command is commonly used to set up environment variables for a specific task. By creating a script that contains the necessary environment variable assignments, you can easily source it using source to ensure the environment is correctly configured. This is particularly useful when dealing with complex tasks that require a specific environment setup.
Example Output: Suppose setup-env.sh contains the following environment variable assignments:
export AWS_ACCESS_KEY="your_access_key"
export AWS_SECRET_KEY="your_secret_key"
export AWS_REGION="us-west-2"
When executed with source setup-env.sh, the environment variables AWS_ACCESS_KEY, AWS_SECRET_KEY, and AWS_REGION will be set with the provided values, allowing subsequent commands or scripts to access them as needed.
Use Case 7: Source a shell script to define functions for reuse
source utils.sh
Motivation: If you have a collection of useful shell functions that you want to reuse across different projects or scripts, you can define them in a separate script and source it whenever you need to use those functions.
Explanation: The source command is well-suited for sourcing a shell script that defines reusable functions. By sourcing the script, you make those functions available in the current shell session, enabling you to call them from other scripts or directly from the shell prompt.
Example Output: Suppose utils.sh contains the following shell function:
greet() {
echo "Hello, $1!"
}
When executed with source utils.sh, the greet function will be defined and can be used in subsequent commands or scripts. For example, executing greet "Alice" will output:
Hello, Alice!
Thus, allowing you to easily reuse the defined functions without needing to redefine them every time.
Use Case 8: Source a script to override or extend existing shell commands
source overrides.sh
Motivation: If you want to modify the behavior of certain existing shell commands or extend their functionality, you can define overrides in a script and source it into the current shell session.
Explanation: The source command is useful for sourcing a script that includes overrides or extensions for existing shell commands. By doing so, you can redefine the behavior of a command or add additional functionality to it.
Example Output: Suppose overrides.sh contains the following lines:
alias ls='ls --color=auto'
alias rm='rm -i'
When executed with source overrides.sh, the ls command will be aliased to include the --color=auto option, enabling colored output. Additionally, the rm command will be aliased to include the -i option, prompting for confirmation before deleting files.
In conclusion, the source command is a powerful tool for executing commands or setting up the environment within the current shell context. It allows you to evaluate the contents of files, execute configuration scripts, modify shell behavior, set up environment variables, and much more. By understanding and utilizing the different use cases of the source command, you can enhance your shell experience and streamline your workflow.
