Intro To 'tty' Command In Linux

2023-06-20 - By Robert Elder

I use the 'tty' command to identify the terminal file that's connected to standard input. If I run the 'tty' command on my desktop graphical terminal:

tty

I'll see this:

/dev/pts/0

which indicates a pseudo-terminal.

Pseudo-terminals Vs. Terminals

If I open a terminal emulator like GNU screen or tmux:

screen

and then run the 'tty' command in this terminal emulator, I'll see another pseudo-terminal reference:

tty

/dev/pts/1

If I switch out of a graphical environment, into a console (by holding CTRL + ALT + one of the F1, F2, F3, ... keys), I'll see a terminal file that looks like this:

tty

/dev/tty3

Return Codes

The return code from the tty command also provides meaningful information, as illustrated by this wrapper script:

#!/bin/bash

if [ "${1}" == "do-close-stdout" ]; then
	#  Runs tty, but with closed stdout file descriptor:
	CMD_TO_EXECUTE="tty >&-"
elif [ "${1}" == "do-bad-arguments" ]; then
	#  Runs tty, but with incorrect number of arguments:
	CMD_TO_EXECUTE="tty 1 2 3 4 5"
else
	#  Runs tty as normal
	CMD_TO_EXECUTE="tty"
fi

echo "About to run this command: '${CMD_TO_EXECUTE}'"
TTY_OUTPUT=$(bash -c "${CMD_TO_EXECUTE}")
TTY_RETURN_CODE=$?

if [ "${TTY_RETURN_CODE}" -eq 0 ]; then
     echo "Case 0: tty is '${TTY_OUTPUT}' and rtn code was ${TTY_RETURN_CODE}: Standard input is a terminal."
elif [ "${TTY_RETURN_CODE}" -eq 1 ]; then
     echo "Case 1: tty is '${TTY_OUTPUT}' and rtn code was ${TTY_RETURN_CODE}: Standard input is a non-terminal file."
elif [ "${TTY_RETURN_CODE}" -eq 2 ]; then
     echo "Case 2: tty is '${TTY_OUTPUT}' and rtn code was ${TTY_RETURN_CODE}: Given incorrect arguments."
elif [ "${TTY_RETURN_CODE}" -eq 3 ]; then
     echo "Case 3: tty is '${TTY_OUTPUT}' and rtn code was ${TTY_RETURN_CODE}: A write error occurred."
else
     echo "else Case: tty is '${TTY_OUTPUT}' and rtn code was ${TTY_RETURN_CODE}: Unknown rtn code???"
fi

The return code has a value of 0 when standard input is a terminal:

Return Code 0

./tty-command-wrapper.sh

About to run this command: 'tty'
Case 0: tty is '/dev/pts/0' and rtn code was 0: Standard input is a terminal.

Return Code 1

A value of 1 is returned when standard input is a non-terminal file:

touch file1.txt
./tty-command-wrapper.sh < file1.txt

About to run this command: 'tty'
Case 1: tty is 'not a tty' and rtn code was 1: Standard input is a non-terminal file.

Return Code 2

When the arguments to tty are incorrect, the return value is 2:

./tty-command-wrapper.sh "do-bad-arguments"

About to run this command: 'tty 1 2 3 4 5'
tty: extra operand ‘1’
Try 'tty --help' for more information.
Case 2: tty is '' and rtn code was 2: Given incorrect arguments.

Return Code 3

If a write error occurs the return value is 3:

./tty-command-wrapper.sh "do-close-stdout"

About to run this command: 'tty >&-'
tty: write error: Bad file descriptor
Case 3: tty is '' and rtn code was 3: A write error occurred.

In this case, the write error was triggered by using this special syntax to close standard out so that write operations fail:

>&-

And that's why the 'tty' command is my faviourite Linux command.

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