Problem

When I tried to install Flutter 2 on Linux, running flutter doctor was required.

Running it, however, resulted in the "Permission denied" error below.

$ flutter doctor
touch: cannot touch '/opt/flutter/bin/cache/.dartignore': Permission denied

Should I do it as root, superuser? No. It is not recommended:

$ sudo flutter doctor
   Woah! You appear to be trying to run flutter as root.
   We strongly recommend running the flutter tool without superuser privileges.

Solution

Invite the user you use to "flutterusers" group.
Here is an Arch Linux example. Run this command:

$ sudo usermod -a -G flutterusers <your-user>
$ # alternatively:
$ #sudo gpasswd -a <your-user> flutterusers

Log out and login again.
Then, try flutter doctor again.

$ flutter doctor
   Building flutter tool...

Solved 🙂

Optional: Another way (less recommended)

Alternatively, it may be solved by changing /opt/flutter permission. But it is perhaps less recommended.

$ sudo chown -R <your-user>: /opt/flutter