/var/adm/wtmp file handling in AIX...

Examples
       1    To convert a binary record in wtmp format to an ASCII record called dummy.file, enter:

            /usr/sbin/acct/fwtmp  <  /var/adm/wtmp  >  dummy.file
            The content of a binary wtmp file is redirected to a dummy ASCII file.
       2    To convert an ASCII dummy.file to a binary file in wtmp format called /var/adm/wtmp, enter the fwtmp command with the
            -ic switch:

            /usr/sbin/acct/fwtmp -ic < dummy.file > /var/adm/wtmp 
            The dummy ASCII file is redirected to a binary wtmp file.