A `-' on the command line indicates placement of the standard input in the list of files, allowing the use of union in pipes.
$ cat journal Date Amount Account Ref Description ---- ------ ------- --- ----------- 920107 14.00 meal v meal with jones 920119 81.72 vitamin c sundown vitaminsSEE ALSO$ cat carexpense Date Amount Account Ref Description ---- ------ ------- --- ----------- 920114 81.80 insur c car insurance allstate
$ union journal carexpense Date Amount Account Ref Description ---- ------ ------- --- ----------- 920107 14.00 meal v meal with jones 920119 81.72 vitamin c sundown vitamins 920114 81.80 insur c car insurance allstate