How to use eval in Linux Bash Scripts

Today, In this article we will show you how to use eval in Linux Bash Scripts. The `eval` command is used in Bash to execute arguments like a shell command. Arguments are concatenated into a string and taken as input to the shell command to execute the command. The `eval` command executes the command in the current shell. This command is useful when executing a command that contains a special operator or reserved keywords. This command can be used in any script, even if the variable name is unknown until the script is executed.

Bash is just a shell that allows you to communicate with the computer and give it instructions. A script is basically just a set of instructions given to the computer to perform various useful tasks. A script helps you automate various tasks with the luxury of getting results faster than the normal process. Normally, you write a simple or advanced Bash command in the terminal, which is executed immediately.

In Bash scripts, you can issue multiple statements or commands at once, and the computer executes them all only when you run the script. In short, a single Bash command can be executed in the terminal, but to execute a combination of multiple commands at once, you must create a Bash script. We mentioned below are the steps to use eval in Linux Bash Scripts.

Steps to use eval in Linux Bash Scripts

Use of eval

Step 1: Let’s run Bash commands through eval:

$ eval “pwd; echo Hello $USER; date”

/home/joeHello joeSun Mar 27 01:01:06 PM EDT 2022

Step 2: Let’s notice the expansion of the environment variable ‘USER’.

Step 3: For more information, we should type help eval in the terminal.

Setting Variables in Current Shell

The eval built-in doesn’t spawn a child process. Therefore, we’re going to set variables for the current shell with it.

Step 1: Let’s assume that the file ‘vars’ contains variable definitions:

echo “Setting variables”foo=FOObar=BAR

Step 2: Now, let’s put these variables into the current shell:

$ eval “$(cat vars)”Setting variables

$ echo $foo $barFOO BAR

Variable Expansion in String

Step 1: Now, let’s substitute Bash variables embedded in a greeting template hello.txt:

Hello $USER! Welcome to $HOSTNAME.

Step 2: So let’s check:

$ eval echo ”$(cat hello.txt)”

Variable Indirection With eval

We’re going to define a Bash variable dynamically without knowing its name ahead.

Step 1: So, let’s print the value of the last argument passed to the script last_arg:

#!/bin/bash

echo echo ”${$#}” # just for testeval echo ”${$#}”

Step 2: Now, let’s test it:

$ ./last_arg foo bar foobar

echo “${3}”foobar

Step 3: Let’s catch that firstly we substitute the variable ‘#’. Its value is the number of arguments, 3 in our case.

Security Issues

When using eval, we should be aware of security issues it brings about.

Step 1: Let’s consider a not safe variable indirection:

$ foo=bar bar=”Hello all *”$ eval echo $$foo

Hello all hello_from_evil.txt hello.txt vars

Basic String Sanitization With Double Quotes

Step 1: In the variable indirection cases, we should use double quotes as must-be protection:

$ foo=bar; bar=”Hello all *”$ eval echo ”$$foo”Hello all *

More Sanitization by Removing Content’s Quotes

Now let’s safely process the string from the hello_from_evil file.

Step 1: First, let’s examine the eval‘s argument using echo instead:

$ content=”$(echo ”$(cat hello_from_evil.txt)”)”$ echo “$content”“Hello $USER! Welcome to $HOSTNAME. The system is up to “;date””

Step 2: So the culprit is a freed command separator.

Step 3: Therefore, we’re going to remove all quotes using the replacement pattern ${content//”}:

$ eval echo ””${content//”}””Hello joe! Welcome to fedora. The system is up to ;date

Final Words

We hope you like our article on how to use eval in Linux Bash Scripts. By bash is meant not only the scripting language, but also the tools that come with the Linux operating system. Every single tool in Linux has its task and each one performs a different task individually. Bash is very useful whenever you need to combine all these tools and chain them together so that they all work in harmony to accomplish a task that is otherwise difficult to accomplish.