executing command prompt command in MySql trigger
system is one of MySQL's internal commands, interpreted by the mysql command-line tool rather than by the SQL engine, as stated in the manual:

mysql sends each SQL statement that you issue to the server to be executed. There is also a set of commands that mysql itself interprets. [...]
system    (\!) Execute a system shell command.

[...]

You can't run the system command inside a stored program (and that includes a trigger), because the entire body of the stored program is sent to the server and the server won't recognise system.
However, you can get an output file with SELECT ... INTO OUTFILE FILE:
  SELECT field_1 INTO OUTFILE '/tmp/abc.txt'
  FIELDS TERMINATED BY ',' 
  LINES TERMINATED BY '\n';