An urgent assistance

Shell Programming and Scripting
1

Hi.
Does any one know when the below will print hi ?

[ -t 0 ] || echo hi

Thanks in advance

2

It belongs to which shell and which script..

3

Very simple way to test: enter it in your terminal emulation / console.

4 
man test :

  -t [fildes]    True if the open file whose file descriptor number
                          is fildes (1 by default) is associated with a
                          terminal device.
5

If standard output is NOT connected to a terminal echo hi.

6

It will print "hi" when there is no terminal (e.g. from "cron" or "at").

Quick test if you have permissions to use "at":

at now
[ -t 0 ] || echo hi
ctrl/D

Result is in unix mail for the user submitting the "at" job.