$ZIO contains the translated name of the current device, in contrast to $IO, which contains the name as specified by the USE command.

GT.M does not permit the SET or NEW command to modify $ZIO.

An example where $ZIO contains a value different from $IO is if the logical GTM$PRINCIPAL is defined.

Example:

    $ define gtm$principal foo
    GTM>WRITE $IO
    FOO
    GTM>WRITE $ZIO
    _TNA275:
    

Notice that $ZIO contains the actual terminal device name while $IO contains the string pointed to by the logical GTM$PRINCIPAL.

    $ gtm_principal="foo"
    $ export gtm_principal
    GTM>WRITE $IO
    foo
    GTM>WRITE $ZIO
    /dev/pts/8