The Tcl console that runs under the Vivado Integrated Design Environment
(IDE) or Tcl mode (vivado -mode tcl or vivado -mode
gui) is slightly different from the one that runs under the batch mode
(vivado -mode batch).
This behavior is inherent to Tcl and is not related to the Vivado tool. Literature can be found on the web that explains the underlying reasons.
But the consequence is that during an interactive session (Tcl or GUI modes), Vivado runs under tclsh mode. However, in batch mode,
Vivado runs in Tcl script mode.
Although in all three modes (Tcl, GUI and batch) the Tcl console behaves mostly the same, the Tcl
environment is slightly different. One difference is that in batch mode, the unknown
commands are not sent to the OS for execution. To make sure that a script will work
under all the three Vivado modes, it is recommended to explicitly
use the exec command to execute external programs.