Bash is 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 can help 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. ..

  1. In the Bash shell, type eval to execute a command.
  2. To run multiple commands in the Bash shell, use the following syntax: eval ‘command1’ ‘command2’ …
  3. To stop the Bash shell, type exit or q to quit.

Steps to use eval in Linux Bash Scripts

Use of eval

eval Bash “echo "Hello, world!"” This will print “Hello, world!” on the screen.

In the previous step, we noticed that the environment variable ‘USER’ was expanding. This is an indication that the program is doing something it wasn’t supposed to. We need to figure out what it is doing and stop it. ..

Hello, Joe. It’s Wednesday, March 27, 01:01:06 PM EDT 2022. ..

To get more information on how to use eval, 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 up variables for the current shell with it.

Step 2: To get a list of all the variables in the file, use the command: Step 3: To get a list of all the values for a specific variable, use the command: Step 4: To change the value of a specific variable, use the command: ..

export PATH=$PATH:/usr/local/bin export TMPDIR=$TMPDIR:/tmp export CMD=/usr/local/bin/cmd Step 3: Finally, we need to set up the environment variables: export TMPDIR=/tmp export PATH=$PATH:/usr/local/bin export CMD=/usr/local/bin/cmd

Variable Expansion in String

Hello, world! Hello, Bash!

echo $foo $barFOO

  1. Is the person telling the story accurate?
  2. Are they reliable?
  3. Do their facts align with what we know?
  4. Are their stories consistent with each other?
  5. Is there a motive for the story?

Variable Indirection With eval

We’ll define a Bash variable dynamically and not know its name ahead.

Last_arg: 5

Step 2: Now, let’s test it: Step 2: Now, let’s test it:

echo “${#}”;

Step 3: Let’s catch that 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 the security risks it brings about. ..

In this example, we are considering a not safe variable indirection. This means that we are not sure which variable to use as the parameter for a function call. We can use the keyword “not” to indicate that we do not want to use a specific variable as the parameter.

Basic String Sanitization With Double Quotes

“We should use double quotes as must-be protection” Step 2: In the variable assignment cases, we should use single quotes as must-be protection: ‘We should use single quotes as must-be protection’ ..

Hello all, from evil.txt! Hello.txt variables

More Sanitization by Removing Content’s Quotes

The string from the hello_from_evil file is processed.

eval(“echo $1”) This will echo the value of $1 to the console.

The culprit is a freed command separator.

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

Final Words

Eval is a command that can be used in Linux Bash Scripts. It allows you to run commands and evaluate their results. This can be very useful when you need to combine different tools together to accomplish a task that would otherwise be difficult. ..