T0 vs S0 in inittab for serial ttyS0 access

xiconfjs asked:

I can’t find any documentation on what the difference between “S0” and “T0” in the inittab file for granting serial access to console is. People seem to use it interchangeably:

T0:12345:respawn:/sbin/getty -L ttyS0 115200 vt100


S0:12345:respawn:/sbin/getty -L ttyS0 115200 vt100

Thank you in advance

My answer:

The first field was an arbitrary identifier of 1-4 characters. It can be pretty much anything. Its value is treated as an opaque blob by the machine; it’s only relevant to human readers.

In ancient versions of Linux and older Unixes, this field was limited to 2 characters.

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