Why are PATH variables different when running via sudo and su?

Take a look at /etc/sudoers. The default file in Fedora (as well as in RHEL, and also Ubuntu and similar) includes this line:

Defaults    secure_path = /sbin:/bin:/usr/sbin:/usr/bin

Which ensures that your path is clean when running binaries under sudo. This helps protect against some of the concerns noted in this question. It's also convenient if you don't have /sbin and /usr/sbin in your own path.

The command su - will execute the root users profile and take on that user's environment including path etc. sudo does not do that.

If you'd like sudo to behave like su - then use the option sudo -i [command which will execute the user's profile

If you'd like su - to behave like sudo then don't use the hyphen - just use su [command]

You can check why (it's different) by running sudo sudo -V.

For example on Linux run:

$ sudo sudo -V | grep PATH
Value to override user's $PATH with: /usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin

^{Note: On macOS/BSD, just run: sudo sudo -V.}

The above list is restricted due to default security policy plugin in some Linux distributions.

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This is further explained in man sudoers:

If the secure_path option is set, its value will be used for the PATH environment variable.

secure_path - Path used for every command run from sudo. If you don't trust the people running sudo to have a sane PATH environment variable you may want to use this.

Another use is if you want to have the “root path” be separate from the “user path”. Users in the group specified by the exempt_group option are not affected by secure_path. This option is not set by default.

If that's the case, you can change that by running sudo visudo and editing the configuration file and modifying your secure_path (adding extra path separated by :) or add your user into exempt_group (so you won't be affected by secure_path options).

Or in order to pass user's PATH temporary, you can run:

sudo env PATH="$PATH" my_command

and you can check that by:

sudo env PATH="$PATH" env | grep ^PATH

See also: How to make sudo preserve $PATH?

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Other reason why the environment could be different for sudo, is because you could have env_reset option enabled in your sudoers file. This causes commands to be executed with a new, minimal environment.

So you can use env_keep option (not recommended for security reasons) to preserve your user's environment variables:

Defaults        env_reset
Defaults        env_keep += "PATH PYTHONPATH"