file descriptor test command

Shell Programming and Scripting
1

Can someone delve more into what below test command do?

if [ "$1" = "-t" ]; then

I know -t is for file descriptor

-t [FD]
file descriptor FD (stdout by default) is opened on a terminal

But not sure how to relate to it..

$1 is supposedly first argument, so let's say if

scriptname: doit
firstargument: file1

and if ./doit file1

then,

above test command is checking to see ...? (My guess would be if file1 file is already open on terminal?

can someone advise?

thanks

2

You are confusing two different things. First,
if [ "$1" = "-t" ]; then
is asking if the first argument to this script is "-t" or not. If it is "-t", that would mean whatever the author of the script intended it to mean. It might mean "output a total line" or it might mean something else entirely. It may or may not have anything to do with the built-in file descriptor test. But to discuss that test, it would go something like:
if [ -t ] ; then
This test requires the single bracket form of test. The newer [[ does not handle this test. We are asking if stdout is connected to a terminal. Or to put it another way, are we being run interactively? If we are interactive, we can pose a question and request an answer. A cron job cannot interact with a user like that.

An example of a command that behaves differently when interactive is "ls".
Just plain:
ls
will, on many systems, produce multi-column output while something like:
ls | cat
will produce single column output. Behavior like this is a little bit controversial, but some program do it anyway.

3

Looks like I did confuse myself ..

when running with -t option, script does run with TEST in the front.. I guess author wanted to run it in test mode if -t argument was given..
thanks.