consider a fixed length file

12345abcd8901
12345abcd7777
12345njdu8888
12345hdku8388

i would like to dump it in to oracle DB using sql loader
12345 in to first coloumn
abcd in to second coloumn
8901 in to 3rd coloumn

Two steps

1.Make a csv file

awk 'BEGIN{FIELDWIDTHS = "5 4 4"}; {print $1","$2","$3;}' filename

load data
          infile 'filename.csv'
		  into table tablename
          fields terminated by ","

You can edit the control file of the sqlloader by the following:

LOAD DATA
INFILE <data_file_path_and_name>
INTO TABLE <table_name> (
<column_name> POSITION(<integer>:<integer>) <data_type>,
<column_name> POSITION(<integer>:<integer>) <data_type>,
<column_name> POSITION(<integer>:<integer>) <data_type>)

I.e:
LOAD DATA
   INFILE /tmp/load_data.dat
   INTO TABLE t1
     (col1 POSITION(1:5) ,
      col2 POSITION(6:9) , 
      col3 POSITION(10:13))