How to resize a datafile and free up the space at OS level

execute below sql query to find out how much space can be reclaimed at dataffile level as below
SQL> select file_name,
  2  ceil( (nvl(hwm,1)*&&blksize)/1024/1024 ) smallest,
ceil( blocks*&&blksize/1024/1024) currsize,
  3    4  ceil( blocks*&&blksize/1024/1024) -
  5  ceil( (nvl(hwm,1)*&&blksize)/1024/1024 ) savings
  6  from dba_data_files a,
  7  ( select file_id, max(block_id+blocks-1) hwm
  8  from dba_extents
  9  group by file_id ) b
 10  where a.file_id = b.file_id(+)
 11  /

                                                   Smallest
                                                       Size  Current    Poss.
FILE_NAME                                             Poss.     Size  Savings
-------------------------------------------------- -------- -------- --------
/Ora10g/prod/system01.dbf                   421      544      123
/Ora10gdb10/oradata/prod/collectord01.dbf             1,223    1,374      151
/sv101/db14/oradata/prod/prdsselx00p_06.dbf           19,991   20,000        9
/Ora10gdb01/oradata/prod/disceuld_01.dbf              5,502    5,502        0
/Ora10gdb03/oradata/prod/prodprds2d01.dbf               1,585    1,585        0
/Ora10gdb03/oradata/prod/prodprds2x01.dbf               1,350    1,350        0
/Ora10gdb07/oradata/prod/prdsotlhx00p_01.dbf          19,991   19,991        0
/Ora10gdb08/oradata/prod/prdssrd00p_01.dbf            24,991   25,000        9
/Ora10gdb09/oradata/prod/prdssrx00p_01.dbf            17,251   17,401      150
/Ora10gdb08/oradata/prod/prdssrd00p_02.dbf            24,991   25,000        9
/Ora10gdb04/oradata/prod/tools01.dbf                     20       20        0
/Ora10gdb01/oradata/prod/users_01.dbf                     4       10        6
/Ora10gdb05/oradata/prod/ctsreportsd02.dbf           11,511   11,550       39
/Ora10gdb01/oradata/prod/prdsmatrix_04.dbf            20,000   20,000        0
/Ora10gdb01/oradata/prod/prdsmatrix_05.dbf            20,000   20,000        0
/Ora10gdb01/oradata/prod/datamartd_05.dbf            19,997   19,997        0
/Ora10gdb01/oradata/prod/datamartd_06.dbf            19,997   20,000        3
/Ora10gdb01/oradata/prod/datamartd_07.dbf            19,997   20,000        3
/Ora10gdb05/oradata/prod/prdssachd00p_08.dbf           9,421   19,991   10,570

below query will provide the statement that can executed to shrink the datfiles and reclaim space at OS level
SQL> SQL> SQL> select 'alter database datafile ''' || file_name || ''' resize ' ||
  2  ceil( (nvl(hwm,1)*&&blksize)/1024/1024 ) || 'm;' cmd
  3  from dba_data_files a,
  4  ( select file_id, max(block_id+blocks-1) hwm
  5  from dba_extents
  6  group by file_id ) b
where a.file_id = b.file_id(+)
  7    8  and ceil( blocks*&&blksize/1024/1024) -
  9  ceil( (nvl(hwm,1)*&&blksize)/1024/1024 ) > 0
 10  /

alter database datafile '/Ora10gdb10/oradata/prod/collectord01.dbf' resize
1223 m;