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Asked: 2024-09-25T05:39:50+05:30 In:

What is the method to substitute a specific string in a file with the contents of another file using the sed command in a Linux environment?

Hey everyone, I’m stuck on something and could really use your help. So, I’ve been working on this project where I need to edit a bunch of configuration files for a server setup. The issue is that there’s a specific string in these files that needs to be replaced with the contents from another file. I’ve heard that the `sed` command in Linux is super powerful for this kind of task, but I’m a bit overwhelmed with all the options and syntax.

Here’s what I’m trying to do: Imagine I have a file called `config.txt`, and inside that file, there’s a line that says `{{PLACEHOLDER}}`. I want to replace that entire line with the contents from another file named `replacement.txt`. The problem is, I don’t just want to replace the text literally; I want to take everything inside `replacement.txt` and insert it in place of `{{PLACEHOLDER}}` in `config.txt`.

I’ve tried a few things, like basic `sed` commands, but I keep either messing up the syntax or ending up with a bunch of error messages. I heard something about using `sed` with the `-i` option to edit files in place, which sounds like it might be what I need, but I’m not sure how to use it properly in this context.

Could someone explain how to do this? Like, what would be the exact command I need to run in the terminal? Also, are there any potential risks I should be aware of, like accidentally overwriting important data or files? I’d like to avoid any mishaps since these are important configuration files.

Any insights or solutions would be greatly appreciated! If you could also break down the command a bit, so it’s easy to follow, that would be awesome. Thanks in advance for your help!

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    2 Answers


    1. Sounds like you’re on a tricky mission, but don’t worry, `sed` can definitely help you with this! Here’s how you can do it:

      To replace `{{PLACEHOLDER}}` in `config.txt` with the contents of `replacement.txt`, you can use the following command in your terminal:

      sed -i "/{{PLACEHOLDER}}/r replacement.txt" config.txt

      Let me break this down a bit:

      • sed: This is the command you’re using.
      • -i: This option tells `sed` to edit in place, meaning it will change the original file directly instead of just showing you the output in the terminal.
      • “/{{PLACEHOLDER}}/r replacement.txt”: This part does two things:
        • The /{{PLACEHOLDER}}/ is a search pattern that tells `sed` which line to look for.
        • The r replacement.txt part tells `sed` to read the content from the replacement.txt file and insert it right after the line containing your placeholder.
      • config.txt: This is the file you’re working on.

      So when you run this command, it looks for the line with `{{PLACEHOLDER}}` and replaces that line’s content with everything from `replacement.txt`!

      Now, about potential risks: always make sure to back up your files before doing any in-place editing, especially since these are important configuration files. You can create a backup like this:

      cp config.txt config_backup.txt

      This way, if something goes wrong, you still have your original file to fall back on!

      Hope this helps you out! Just give it a try, and you should be good to go!



    2. The `sed` command is a powerful tool for text manipulation in Linux, and for your need to replace a specific placeholder in `config.txt` with the contents of `replacement.txt`, you can use the following command:
      “`bash
      sed -i “/{{PLACEHOLDER}}/r replacement.txt” config.txt
      sed -i “/{{PLACEHOLDER}}/d” config.txt
      “`
      This command works in two parts. The first `sed` command uses the `-i` option to edit the file in place, searching for the line containing `{{PLACEHOLDER}}` and reading (`r`) the contents of `replacement.txt` to insert right after the matched line. The second command deletes the line containing `{{PLACEHOLDER}}` to ensure that it is removed from `config.txt`. This sequence effectively replaces the placeholder with the text from `replacement.txt`. Make sure to back up your original `config.txt` file beforehand to avoid accidental data loss.

      When using the `-i` option, there is a potential risk of overwriting the original file without warnings. To mitigate this risk, it is good practice to create a backup by using `sed -i.bak` which creates a copy of the original as `config.txt.bak`. Here’s the updated command:
      “`bash
      sed -i.bak “/{{PLACEHOLDER}}/r replacement.txt” config.txt
      sed -i “/{{PLACEHOLDER}}/d” config.txt
      “`
      This way, if something goes wrong, you can always revert to the backup file. Be cautious with your file paths and ensure that `replacement.txt` contains the desired contents before running the command. With these steps, you should be able to execute your task confidently!


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