Update installation on debian

- Redirect stderr messages, to not get extra lines of output like No LSB modules are available.
- Separate the codename variable to be more efficient and readable 

Signed-off-by: Mohammed Abdallah <[email protected]>

Author: mohamedabdallah20

installation/linux/debian.md

@@ -52,13 +52,16 @@ Refer to the [supported platform documentation](../supported-platforms.md) to se
 ## Update your sources lists
 
 For Debian, you must add the Fluent Bit APT server entry to your sources lists.
+Ensure codename is set to your specific [Debian release name](https://wiki.debian.org/DebianReleases#Production\_Releases). (for example: `bookworm` for Debian 12)
 
+```bash
+codename=$(grep -oP '(?<=VERSION_CODENAME=).*' /etc/os-release 2>/dev/null || lsb_release -cs 2>/dev/null)
+```
+Now Update your sources listes
 ```bash copy
-echo "deb [signed-by=/usr/share/keyrings/fluentbit-keyring.gpg] https://packages.fluentbit.io/ubuntu/$(grep -oP '(?<=VERSION_CODENAME=).*' /etc/os-release || lsb_release -cs) $(grep -oP '(?<=VERSION_CODENAME=).*' /etc/os-release || lsb_release -cs) main" | sudo tee /etc/apt/sources.list.d/fluent-bit.list
+echo "deb [signed-by=/usr/share/keyrings/fluentbit-keyring.gpg] https://packages.fluentbit.io/ubuntu/$codename $codename main" | sudo tee /etc/apt/sources.list.d/fluent-bit.list
 ```
 
-Replace _`CODENAME`_ with your specific [Debian release name](https://wiki.debian.org/DebianReleases#Production\_Releases) (for example: `bookworm` for Debian 12)
-
 ## Update your repositories database
 
 Update your system's `apt` database: