Confused about exec redirection to /dev/null, am I doing it correctly?

Question

Background

I am training myself in POSIX shell scripting, please avoid any Bashism while answering. Thank you.

I know now, thanks to Kusalananda's answer, how to determine if my script is running interactively, i.e. when it is connected to stdin.

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Question

As I rarely use exec (man page), I am unsure if I am doing the following idea correctly? Please elaborate.

If the script is running:

• interactively: Output error messages into stderr, otherwise run in the default environment settings.

• non-interactively: Redirect all output to /dev/null in this example (in the end I will likely want to redirect stdout and stderr to actual files).

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Code

# test if file descriptor 0 = standard input is connected to the terminal
running_interactively () { [ -t 0 ]; }

# if not running interactively, then redirect all output from this script to the black hole
! running_interactively && exec > /dev/null 2>&1

print_error_and_exit ()
{
    # if running interactively, then redirect all output from this function to standard error stream
    running_interactively && exec >&2

    ...
}

Answer 1

It seems I was close. Since the running_interactively function worked as expected, I was able to move on with redirecting to files as follows.

I have edited this answer to have more code for re-use.

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#!/bin/sh

is_running_interactively ()
# test if file descriptor 0 = standard input is connected to the terminal
{
    # test if stdin is connected
    [ -t 0 ]
}

redirect_output_to_files ()
# redirect stdout and stderr
# - from this script as a whole
# - to the specified files;
#   these are located in the script's directory
{
    # get the path to the script
    script_dir=$( dirname "${0}" )

    # redirect stdout and stderr to separate files
    exec >> "${script_dir}"/stdout \
        2>> "${script_dir}"/stderr
}

is_running_interactively ||
# if not running interactively, add a new line along with a date timestamp
# before any other output as we are adding the output to both logs
{
    redirect_output_to_files
    printf '\n'; printf '\n' >&2
    date; date >&2
}

print_error_and_exit ()
{
    # if running interactively redirect all output from this function to stderr
    is_running_interactively && exec >&2
    ...
}