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

The ’eval’ command is used to execute arguments as a single command in the current shell and return its result. It can be used to perform various operations, including calling a command with arguments and setting variables in the current shell.

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

Code:

eval "echo foo"

Motivation: The motivation behind using this example is to illustrate how the ’eval’ command can be used to execute a command with arguments in the current shell. In this case, we want to call the ’echo’ command with the argument “foo”.

Explanation: The ’eval’ command is followed by a string that represents the command and its arguments to be executed. In this example, the string “echo foo” is passed as an argument to the ’eval’ command.

Example output:

foo

Explanation: When the ’eval’ command is executed, it evaluates the string passed as its argument and treats it as a command. In this case, the command “echo” is executed with the argument “foo”, resulting in the output “foo” being displayed in the shell.

Use case 2: Set a variable in the current shell

Code:

eval "foo=bar"

Motivation: The motivation behind using this example is to demonstrate how the ’eval’ command can be used to set a variable in the current shell. In this case, we want to set a variable named ‘foo’ with the value ‘bar’.

Explanation: The ’eval’ command is used to execute the string passed as its argument in the current shell. In this example, the string “foo=bar” is evaluated and treated as a command to set a variable in the shell.

Example output: (no output)

Explanation: When the ’eval’ command is executed, it evaluates the string passed as its argument, treating it as a command. In this case, the command “foo=bar” is executed, setting the variable ‘foo’ to the value ‘bar’ in the current shell. Since variable assignment commands do not produce any output, there will be no output displayed in the shell.

Conclusion:

The ’eval’ command is a powerful tool for executing commands and evaluating strings as commands in the current shell. It can be used to call commands with arguments and set variables, among other use cases. However, it is important to exercise caution when using the ’eval’ command, as it can execute arbitrary code and potentially lead to security vulnerabilities if not used properly.