Understanding Shell Script's idiom: 2>&1
When we are working with a programming or scripting language we are constantly using some idioms, some things
that are done in this certain way, the common solution to a problem. With Shell Script this is not different,
and a quite common idiom, but not so well understood, is the
2>&1, like in
ls foo > /dev/null 2>&1.
Let me explain what is going on here and why this works the way it does.
A quick introduction to I/O redirection
Simply put, redirection is the mechanism used to send the output of a command to another place. For instance, if we just
cat a file,
its output will be printed in the screen, by default:
$ cat foo.txt foo bar baz
But we can redirect this output to another place. Here, for example, we are redirecting it to a file called
$ cat foo.txt > output.txt $ cat output.txt foo bar baz
Note that in the first
cat we don’t see any output in the screen. We changed the standard output (
stdout) location to a file, so it doesn’t use
the screen anymore.
It’s also important to know that there are this other place, called standard error (
stderr), to where programs can send their error messages. So if we
cat a file that doesn’t exist, like this:
$ cat nop.txt > output.txt cat: nop.txt: No such file or directory
Even if we redirect the
stdout to a file, we still see the error output in the screen, because we are redirecting just the standard output, not the standard error.
And a quick introduction to file descriptors
A file descriptor is nothing more that a positive integer that represents an open file. If you have 100 open files, you will have 100 file descriptors for them.
The only caveat is that, in Unix systems, everything is a file. But that’s not really important now, we just need to know
that there are file descriptors for the Standard Output (
stdout) and Standard Error (
In plain English, it means that there are “ids” that identify these two locations, and it will always be
Putting the pieces together
Going back to our first example, when we redirected the output of
cat foo.txt to
output.txt, we could rewrite the command like this:
$ cat foo.txt 1> output.txt
1 is just the file descriptor for
stdout. The syntax for redirecting is
[FILE_DESCRIPTOR]>, leaving the file descriptor out is just a shortcut to
So, to redirect
stderr, it should be just a matter of adding the right file descriptor in place:
# Using stderr file descriptor (2) to redirect the errors to a file $ cat nop.txt 2> error.txt $ cat error.txt cat: nop.txt: No such file or directory
At this point you probably already know what the
2>&1 idiom is doing, but let’s make it official.
&1 to reference the value of the file descriptor 1 (
stdout). So when you use
2>&1 you are basically saying “Redirect the
stderr to the same place we are redirecting the
And that’s why we can do something like this to redirect both
stderr to the same place:
$ cat foo.txt > output.txt 2>&1 $ cat output.txt foo bar baz $ cat nop.txt > output.txt 2>&1 $ cat output.txt cat: nop.txt: No such file or directory
- There are two places programs send output to: Standard output (
stdout) and Standard Error (
- You can redirect these outputs to a different place (like a file);
- File descriptors are used to identify
command > outputis just a shortcut for
command 1> output;
- You can use
&[FILE_DESCRIPTOR]to reference a file descriptor value;
stderrto whatever value is set to
1>&2will do the opposite).
And if you want to learn more about Shell Script, I highly recommend the Classic Shell Scripting book.
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