Standard Output, Standard Error and Standard Input

ls exists 1> stdout.txt    # Redirect the standard output to a file
ls noexist 2> stderror.txt # Redirect the standard error output to a file
ls > out.txt 2>&1          # Redirect standard output and error to a file

# similar examples 
cat exists.txt > output.txt 2>&1
cat output.txt
#foo
#bar
#baz

cat noexist.txt > output.txt 2>&1
cat output.txt
#cat: nop.txt: No such file or directory