you're trying to run it as a shell command, it's not possible .... when you run mplayer for the first time it will create the file "/home/user/.mplayer/config" ... then you have to edit this file. The file is empty by default, so it should be completely safe to run command "echo 'subcp = windows-1250' >> /home/user/.mplayer/config" - this will add the needed line at the end of the file. You can check this afterwards by "cat /home/user/.mplayer/config" - it will print the content of that file. Another option is runnig mplayer as "mplayer -subcp windows-1250 some_file", but this is not permanent ...