This is obviously some kind of Linux, so look for a file in / etc that ends in -release or _version:
ls /etc/*-release /etc/*_version
This will help you determine which Linux distribution you have. lsb-release is typically Ubuntu, while Fedora, redhat-release shows Fedora, RedHat, or CentOS. You can find a more complete list here .
Alternatively, you can see if rpm or dpkg is installed, trying to execute these commands:
rpm --version dpkg --version
If dpkg is installed, it may be based on Ubuntu / Debian. If rpm is installed, it may be RedHat / CentOS or openSUSE.
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