How to use the command 'eval' (with examples)

The eval command is a powerful built-in shell utility that interprets and executes the arguments passed to it as a single command line in the current shell environment. By leveraging eval, you can dynamically construct and execute commands, which can be especially useful in scripting and when you want to manipulate or evaluate variables or commands on the fly. It’s important, however, to use eval judiciously as it can pose security risks if used with untrusted input.

Use case 1: Call echo with the “foo” argument

Code:

eval "echo foo"

Motivation:

The first use case demonstrates a simple execution of the echo command using eval. The primary motivation for using eval in conjunction with echo might appear trivial in this case, but it serves as an introductory illustration to show how eval can process its argument as a full command. This is particularly useful in scripting scenarios where commands need to be constructed dynamically. eval allows the command to be executed in the current shell context, ensuring that any environment variables or command outputs generated are accessible within the same execution shell.

Explanation:

• eval: This is the command that takes the following string and executes it as if it were a command typed directly into the shell. It allows for the evaluation of complex dynamic command setups.
• "echo foo": This is the argument to eval. The echo command is used for outputting text to the terminal, and here it is instructed to print foo. Enclosing echo foo in quotes ensures that it is treated as a single string argument passed to eval.

Example output:

foo

This output comes from the echo command inside the eval execution, printing the string “foo” to the terminal.

Use case 2: Set a variable in the current shell

Code:

eval "foo=bar"

Motivation:

The second use case demonstrates how eval can be used to define variables within the current shell environment. One of the main motivations for using eval in this context is its ability to manipulate shell variables dynamically. When you are writing scripts that require variable names or values determined at runtime, eval can be used to construct the variable declaration strings and execute them. Using eval, you ensure that the variable modification takes place in the current shell environment, making it accessible immediately without needing subprocesses.

Explanation:

• eval: This command will interpret the provided string as a shell command. The capability of eval to interpret strings allows it to execute dynamic constructs, which can be essential for advanced shell scripts.
• "foo=bar": This string is evaluated by eval, which treats it as a shell command to assign the string bar to the variable foo. The quotes are crucial in ensuring the string is handled correctly as a single command input by eval.

Example output:

There is no visible output from this command directly, as it performs a variable assignment. However, you can verify the assignment by subsequently using the echo command:

echo $foo

This would result in:

bar

Indicating that the variable foo has been correctly assigned the value bar within the current shell session.

Conclusion:

The eval command is a versatile tool within the shell that can be leveraged for the dynamic execution of commands in the current shell environment. Through the examples, we explored how to use eval for executing basic commands and setting variables dynamically. These use cases exemplify the utility of eval in scripting and command-line operations. However, due to its capability of executing arbitrary code, care should be taken to ensure its arguments are sanitized, especially when dealing with input from untrusted sources.