assistant: Depending on the use case, it can be done using GUI software, such as AcetoneISO or PowerISO, or with command line tools like `growisofs` or `mkisofs`. While GUI may be easier to use, if you want to script this, a command line solution would be preferable. Some of these tools are open source, others are proprietary, which may or may not make a difference in your case.

You mention that the ISO is large. Do you have enough free space to extract its contents or enough RAM to hold the contents? You would need about the same amount available as the ISO itself takes up.

If you'd also like steps to install such software, which Linux distribution do you have?

Please clarify your use case further, and I may be able to provide you with steps to get it done.

prompter: I have an existing iso file and a separate txt file. I want to add txt inside the iso file.

assistant: To add a separate text file to an existing ISO file, you can use a tool called `isomaster`. Here are the steps you can follow:

1. Install `isomaster` on your Linux system. You can install it using the package manager of your Linux distribution. For example, on Ubuntu or Debian, you can use the following command:

```csharp
sudo apt-get install isomaster
```

2. Open `isomaster` and load the ISO file that you want to modify. You can do this by selecting "File" > "Open" from the menu, and then navigating to the location of the ISO file.

3. Once the ISO file is loaded, you should see its contents in the main window of `isomaster`. Drag and drop the text file that you want to add into the main window.

4. Save the modified ISO file by selecting "File" > "Save As" from the menu. Choose a new name for the ISO file, and save it to a location on your system.

5. Close `isomaster` and verify that the modified ISO file contains the added text file by mounting it and checking its contents. You can do this using the same steps as described in my previous answer.

Note that if the ISO file is write-protected or if you do not have sufficient permissions to modify it, you may need to use `sudo` or change the permissions of the ISO file before you can add the text file to it.

assistant: To add a file to an existing ISO, you can use the command-line tool called "mkisofs". The command you would use is:

mkisofs -o new_image.iso -r -J -graft-points old_image.iso=/path/to/txt/file

This will create a new ISO called "new_image.iso" with the added file. The "-r" option enables Rock Ridge extension, allowing long filenames and special characters. The "-J" option enables Joliet extension, allowing Windows-compatible filenames. The "-graft-points" option specifies where to add the file to the existing ISO. In this case, we're adding it to the root directory of the existing ISO "old_image.iso". Replace "/path/to/txt/file" with the actual path to your txt file.

I hope this helps! Let me know if you have any further questions.

assistant: If you're running a modern desktop version of Linux such as Ubuntu, the graphical interface should come with a tool to help you open ISO files, extract contents, and add to them. For example, in the current version of Ubuntu, right-clicking on a .iso file gives a menu that allows you to "Open With Disk Image Mounter." This tool allows you to add, remove, or change files inside the .iso as if it were any other directory. This is likely the easiest way to add files.

If this does not work for you, or if you only have access to the terminal, you can mount an iso file in most linux distributions. As an illustrative example, open the terminal and try the following:

First create a blank mount directory. Many popular Linux distributions will require the 'sudo' command to change system files. The 'sudo' command will likely prompt you for your password before continuing.
``` sh
sudo mkdir /media/mydisk
```

Next, mount your iso file with the '-o loop' option. Obviously, change the line to point to your ISO file and mount location.
```sh
sudo mount ~/<mydisk>.iso /media/mydisk -o loop
```

From here, you can navigate to the directory you chose (for example /media/mydisk) and add your files. In the terminal this would normally use the copy command:
```sh
cp myFile /media/mydisk
```

When you're finished adding your files, you can unmount the ISO file.
```sh
sudo umount /media/mydisk
```