I have tried below command to append some path to system path variable by batch-file :
setx PATH \"%PATH%;C:\\Program Files\\MySQL\\MySQL Server 5.5\\bin\"
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To update and expand on Endoro's answer for Windows 10, manually add the path to your Path system variable as a new variable. I wasn't able to get setx to work even changing the flags around. Doing it manually was simple.
To get to your system environmental variables -> Windows Key -> Edit the system environmental variables -> Click Environmental Variables -> Select the Path variable in the System variables frame -> Click Edit -> Click New -> Add the path -> Click Okay
Make sure you close all your CLI windows and open a new one if you're trying to verify by checking the version.
Windows showing where to edit the Path environmental variable