How could I remember how to use redirection?

Question

I know what

  program > /dev/null 2>&1 

does. It redirects the output to /dev/null and 2>&1 means to redirect the error output in the same place where the output is sent.

My problem is I always have to google it because I never remember it.

So, I try &2>1, 1>2&, 1>&2... I try every combinaison until I google it...

What's the trick to remember it easily ?

Answer 1

Output is better than error so it comes first (1 vs 2).

> is shorthand for 'goes to'. On the left is what I want to send and on the right is where I want to send it. Since 'where' is (almost) always a file, something like

program > /dev/null 2>1

would redirect to a file named 1. Thus, the ampersand modifies the file to file descriptor.

Unfortunately, I haven't come across nor developed my own mnemonic, but when I was first learning *nix, I found this logical way to work well. After a few run-throughs, it becomes second nature.

Answer 2

Seeing the & as a knot might help : think about what you want to do as taking the output of 2, so 2>, and tying it together with 1, so 2>&1

Answer 3

Actually, it depends on what shell you're using. Bash is usually very forgiving and you can just do:

program &> file

Answer 4

One trick is just to remember that 1 = standard output, 2 = standard error. So:

2>&1 = standard error stream goes into standard output stream.
1>&2 = vice versa.

If you have ever programmed in a C-like language, it's easy to remember the ampersand (&). I choose to think of it as referring to the "address of" existing file descriptor, so that you don't change the file itself or create a new one.

Answer 5

Draw it in your wallpaper :)

Now, seriously, this and other basic stuff I kept forgetting, so I added a quick tips menu to an app I developed and that I use daily. You might want to give it a try or use something like gnote to keep a note.