./pg_ms_status.sh: line 17: [: ==: unary operator expected

执行pg的shell脚本，报错

./pg_ms_status.sh: line 17: [: ==: unary operator expected

if [ $db_role == f ];then
   echo -e `date +"%F %T"` "pg master db is ok" >> $LOGFILE

报错的原因是：如果变量 db_role 的值为空,那么就if语句就变成了if [  = "f" ],这不是一个合法的条件。
为了避免出现这种情况，我们必须给变量加上引号if [  "$db_role" = "f"  ]，这样即使是空变量也提供了合法的测试条件,，if [  " " = "f"  ]