eval vs source: For Executing Commands Within a Shell Script

Shell scripting is a fundamental skill for anyone working with Unix-like operating systems, offering a powerful way to automate tasks and streamline workflows. Two key commands, "source" and "eval," play distinct roles in the realm of shell scripting. In this article, we delve into the core concepts of both commands, providing clear explanations, practical examples, and essential considerations for using them effectively. By understanding the nuances of "source" and "eval," you'll gain a deeper insight into how they can simplify your scripting tasks and navigate potential security concerns when handling untrusted input. Let's explore these essential tools for shell scripting and empower you to become a more proficient scripter.

What is the source Command?

The source command in Bash (also written as a single dot, .) lets you run the contents of another script inside your current script. This means any variables, functions, or settings defined in the source script become available in your current session. It’s like importing a set of tools or instructions into your workspace. You’ll often use source to load configuration files, environment variables, or reusable code, such as constants or functions.

The source command is generally safer than eval because it doesn’t execute random code it only includes the contents of a specific script file. However, you should still check that the sourced script is trustworthy to avoid running malicious code.

Example:

To begin, create a file named "constants.sh" where we will store constant values. In the following section, we will utilize the source command to incorporate this file.

Step 1: Open a Terminal

Opening a terminal using [Alt + Ctrl + T] or [Right Click -> Open in Terminal]

Step 2: Create a Configuration File

We’ll create a file called constants.sh to store some constant values. Open the terminal and type:

'nano': The 'nano' command is a text editor for Unix-like operating systems, including Linux. It is designed to be easy to use and is often preferred by those who are new to the command line or need a simple text editor for quick edits. To open a file with nano or create a new one, you can use the following command:

nano [filename]

Replace '[filename]' with the name of the file you want to open or create. If the file does not exist, 'nano' will create a new one with that name.

Once you're inside the 'nano' text editor, you can use various keyboard shortcuts to perform actions like saving, quitting, searching, and more. Common shortcuts include:

• Ctrl + O: Save the current file.
• Ctrl + X: Exit 'nano'.
• Ctrl + G: Open the help menu, which shows other available commands.
• Ctrl + W: Search for text within the file.
• Ctrl + K: Cut (delete) the current line.
• Ctrl + U: Uncut (paste) the previously cut text.

You can find more commands and options in the help menu by pressing Ctrl + G.

Keep in mind that 'nano' is a basic text editor, and if you need more advanced features, you might want to consider using other text editors like 'vim' or 'emacs'.

Step 3: Add Constants to constants.sh

In the nano editor, type the following:

# constants.sh
export PI="3.1415"
export A="Hello"

Save the file by pressing Ctrl + O, then press Enter. Exit nano with Ctrl + X.

Step 4: Create a Script to Use source

Now, create a script called source.sh to use the source source` command. Type:

#!/bin/bash
#Source the constants file
source constants.sh 
#Using echo to get values of constants.
echo "Value of PI: $PI"
echo "$A Geeks."

Note: You can use any name for the script, I used "source.sh". Also here "source.sh" and "constants.sh" both are in the the same folder hence in execution of the the source command only file name is used but the syntax of the the source command is " source /path/to/your/file".

Step 5: Run the Script

Run the script by typing:

bash source.sh

The command "bash source.sh" is used to execute a Bash script called "source.sh." When you run this command, it will execute the commands specified in the "source.sh" script.

The "source" command within the script is used to include and execute the contents of another script or file. In this case,"source.sh" is meant to include and run the "constants.sh" file.

So, running "bash source.sh" will execute the commands within "source.sh," which, in turn, sources the "constants.sh" file and displays the values of variables.

The provided Bash script sources to the file named "constants.sh" to access variables and then uses the 'echo' command to display the values of those variables. This assumes that "constants.sh" contains variable definitions for PI with the value of "3.14159" and A with the value "42."

What is the eval Command?

The eval command in Bash is used to evaluate and execute a string as a shell command. It takes a string as an argument and treats it as if it were a line of code in the script. The primary purpose of eval is to dynamically generate and execute code. For example, you can build a command as a string and then use eval to run it.

It's essential to exercise caution when using eval because it can potentially introduce security risks, especially when dealing with untrusted or user-generated input. If not properly sanitized, it can be vulnerable to code injection attacks.

Example:

Step 1: Open a Terminal

Use [Alt + Ctrl + T] or right-click and select Open in Terminal.

Step 2: Create an eval Script

Create a file called eval.sh:

nano eval.sh

Step 3: Add the Script

Type the following in the file in terminal and save (Ctrl + O, then Enter) and exit (Ctrl + X).

#!/bin/bash
# A dynamically generated command as a string
command="ls -l"
#use eval to execute the command
eval "$command"

1. "!/ bin/bash": This is known as a "shebang" line, and it specifies that the script should be executed using the Bash shell.
2. "ls -l": In this line, a variable named command is declared and assigned the value "ls -l." This value is a string, and it represents a shell command to list files and directories in long format.
3. "eval $command": The eval command is used to evaluate and execute the contents of the command variable. In this case, it takes the value of the command variable, which is "ls -l," and treats it as a shell command. This results in the execution of the "ls -l" command as if it were directly entered into the shell.

Step 4: Run the Script

Run it with:

bash eval.sh

When you run the command "bash eval.sh," it will execute the "ls -l" command using 'eval', and you'll see the output of the "ls -l" command, which will list the files and directories in the current directory in long format.

Comparison between Source and Eval:

Now that we have gained a solid understanding of both the 'source' and 'eval' commands, let's proceed with a comparative analysis of these two commands.

Conclusion

In summary, 'eval' is used for executing dynamically generated code from a string, but it poses security risks when dealing with untrusted input. On the other hand, source is used to include the content of other scripts and is generally safer, but you should be careful when sourcing external scripts

Question 2: What is the key difference in security considerations between using 'source' and 'eval' in Bash scripts, particularly when dealing with untrusted input?

Answer:

While both 'source' and 'eval' can be considered safe when employed with trusted input, it's important to note that 'eval' necessitates an additional layer of meticulous input validation to safeguard against potential code injections. Consequently, it tends to be riskier when handling untrusted input.

Question 2: Can I use 'source' to execute a single command?

Answer:

Indeed, it is possible to employ the 'source' command to execute a single command by encapsulating it within a file and then sourcing that file. Notably, there is no imposed limit on the file size that the 'source' command can handle and execute.

Question 3: What happens if there's an error in the sourced script or evaluated command?

Answer:

In either scenario, errors have the potential to impact the ongoing shell session. It is essential to exercise caution and perform thorough error handling and testing prior to executing scripts or files that incorporate these commands.

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