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Full Discussion: Bash script unable to disable expected output
Top Forums Shell Programming and Scripting Bash script unable to disable expected output Post 303016771 by Don Cragun on Thursday 3rd of May 2018 01:09:30 PM
Old 05-03-2018
Expanding a little on what Corona688 has already said, the who utility uses its standard input file descriptor to determine the terminal being used as your controlling terminal. If your script is not the first process in a pipeline or if standard input is redirected from a non-terminal device file, it might not know how to determine who you are.

On a macOS or BSD system, these conditions would lead to output similar to:
Code:
dwc      tty??    May  3 09:59

instead of the normal output:
Code:
dwc      ttys007  Apr 28 08:53

that I get when standard input is connected to my terminal. I'm surprised that Linux (at least Ubuntu Linux) doesn't give you any output at all.
This User Gave Thanks to Don Cragun For This Post:
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LEARN ABOUT OPENSOLARIS

tty

tty(1)								   User Commands							    tty(1)

NAME

       tty - return user's terminal name

SYNOPSIS

       tty [-l] [-s]

DESCRIPTION

       The  tty utility writes to the standard output the name of the terminal that is open as standard input. The name that is used is equivalent
       to the string that would be returned by the ttyname(3C) function.

OPTIONS

       The following options are supported:

       -l    Prints the synchronous line number to which the user's terminal is connected, if it is on an active synchronous line.

       -s    Inhibits printing of the terminal path name, allowing one to test just the exit status.

ENVIRONMENT VARIABLES

       See environ(5) for descriptions of the following environment variables that affect the execution of  tty: LANG, LC_ALL, LC_CTYPE,   LC_MES-
       SAGES,  and NLSPATH.

EXIT STATUS

       The following exit values are returned:

       0     Standard input is a terminal.

       1     Standard input is not a terminal.

       >1    An error occurred.

ATTRIBUTES

       See attributes(5) for descriptions of the following attributes:

       +-----------------------------+-----------------------------+
       |      ATTRIBUTE TYPE	     |	    ATTRIBUTE VALUE	   |
       +-----------------------------+-----------------------------+
       |Availability		     |SUNWcsu			   |
       +-----------------------------+-----------------------------+
       |CSI			     |Enabled			   |
       +-----------------------------+-----------------------------+
       |Interface Stability	     |Committed 		   |
       +-----------------------------+-----------------------------+
       |Standard		     |standards(5).		   |
       +-----------------------------+-----------------------------+

SEE ALSO

       isatty(3C), ttyname(3C), attributes(5), environ(5), standards(5)

DIAGNOSTICS

       not on an active synchronous line

	   The standard input is not a synchronous terminal and -l is specified.

       not a tty

	   The standard input is not a terminal and -s is not specified.

NOTES

       The -s option is useful only if the exit status is wanted. It does not rely on the ability to form a valid path name. Portable applications
       should use test -t.

SunOS 5.11							    1 Feb 1995								    tty(1)
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