Questions[redirect](ubuntu)

The simplest way to display file contents is using the cat command:

cat file.txt

I can get the same result using input redirection:

cat < file.txt

Then, what is the difference between them?

I am doing my homework and I have a question about one line. I don't understand what this >&2 means:

if test -d $2 
  then echo "$2: Is directory" >&2 ; exit 21
fi

I have this code that works:

# Hide irrelevant errors so chrome doesn't email us in cron
if [[ $fCron == true ]] ; then
    google-chrome --headless --disable-gpu --dump-dom \
        "$RobWebAddress" > "$DownloadName" 2>/dev/null
else
    # Get silly error messages when running from terminal
    google-chrome --headless --disable-gpu --dump-dom \
        "$RobWebAddress" > "$DownloadName"
fi

If I try to shorten it like this:

# Hide irrelevant errors so chrome doesn't email us in cron
local HideErrors
[[ $fCron == true ]] && HideErrors="2>/dev/null"

google-chrome --headless --disable-gpu --dump-dom \
    "$RobWebAddress" > "$DownloadName" "$HideErrors"

I get error messages:

[0826/043058.634775:ERROR:headless_shell.cc(597)] Open multiple tabs is only supported when remote debugging is enabled.
[0826/043058.672587:ERROR:headless_shell.cc(597)] Open multiple tabs is only supported when remote debugging is enabled.
[0826/043058.711640:ERROR:headless_shell.cc(597)] Open multiple tabs is only supported when remote debugging is enabled.
(... SNIP ...)

Why does a hard-coded argument work but not an argument as a variable?

Edit 2:

Currently I found success with second answer's alternate suggestion:

# Redirect errors when cron is used to /dev/null to reduce emails
ErrorPipe=/dev/stderr
[[ $fCron == true ]] && ErrorPipe=/dev/null

google-chrome --headless --disable-gpu --dump-dom \
                "$RobWebAddress" > "$DownloadName" 2>"$ErrorPipe"

Edit 1:

Based on the first answer, I should point out program header already contains:

[[ $fCron != true ]] &&
    exec 2> >(grep -v 'GtkDialog mapped without a transient parent' >&2)

Every time starting the machine, I run the following program:

$ cat start.sh
#! /bin/bash
google-chrome &> /dev/null &
lantern &> /dev/null &
xdg-open . &> /dev/null &
emacs &> /dev/null  &
code ~/Programs/ &> /dev/null &
xdg-open ~/Reference/topic_regex.md &> /dev/null &

Cumbersome &> /dev/null &... How could I shorten the logic?

Oftentimes I run into small bash scripts that use this sort of syntax in if statements:

some command > /dev/null 2>&1

What is the purpose of outputting to /dev/null like that, and what does the 2>&1 mean?

It always seems to work but I'd like to know what it's doing.