There are other differences. By default, Ubuntu locks down the root user, instead requiring that a local user be created during installation; that user is then granted sudo privileges in order to perform tasks as the root user. CentOS/RHEL have no such restriction and allow for general access by the root user.
Some distributions also have customized configuration tools. Suse Linux (and OpenSuse) sport YaST (Yet another Setup Tool), a system setup and configuration manager that isn't used in other Linux distributions.
The devil is in the daemon
In other cases, there may be broad similarities among current distributions that aren't shared by older versions. For example, systemd has been introduced as a replacement for sysvinit in recent releases of the major Linux distributions, fundamentally changing the way servers running these distributions boot, load, and manage services (and not without controversy). Older releases of these distributions will not have systemd, and thus will not benefit from the greater efficiency and performance improvements that it brings. That may be a factor worth considering if you're presented with a choice of release versions.
Another key element that differentiates Linux distributions is the package update mechanism and the frequency of package updates. On Ubuntu and Debian, you use Apt (Advanced Package Tool) to handle distribution updates. This tool allows you to update individual packages or the system as a whole. On RHEL, CentOS, Fedora, and others, Yum (Yellowdog Updater, Modified) is the package manager. Apt and Yum operate in similar ways, letting admins configure custom package repositories, and easily upgrade packages and maintain the system.
Generally speaking, the major Linux distributions align as described in the table below.
Making the right choice
The choice of which distribution to use typically reduces to a few key elements:
Familiarity. The advantages of sticking with a Linux distribution you're extremely familiar with will generally outweigh the advantages of adopting a different distribution. If CentOS and RHEL are what you know, sticking with these distributions will likely be easiest in the long run. Unless server requirements dictate otherwise, go with what you know.
Simplicity. There is a lot to be said for homogeneity in application stacks. Maintaining the same distribution across your stack is almost always the way to proceed. If your application servers run CentOS, then your database servers should too, unless there is an overriding reason to go a different direction for those servers.
Server requirements. Ultimately, your choice of distro will be limited by the applications and services you need to deploy. For example, if the server must run the very latest releases of a common package like MySQL or PHP, choosing a stabler distribution like CentOS or RHEL with a longer release schedule will complicate matters. You will likely need to resort to additional, third-party package repositories and use packages built by a variety of contributors rather than the official releases supported by the distribution. In some cases, you may have to create your own package RPMs or custom package compilations for very recent package releases.
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