Confusion with the redirection of output to 2>&1?

Question

I have checked couple of relevant posts regarding this in stackoverflow and other sources regarding the usage of 2>&1.

Unfortunately so far have not get my head around it completely.

I understand that 2 is the stderr and 1 is the stdout and we are combining with the 2>&1.

But my question is what is difference between:

1. mycommand > /dev/null       
2. mycommand 2> /dev/null      
3. mycommand > /dev/null 2>&1  

I was thinking:

1. will redirect stdout and stderr to /dev/null
2. will redirect stderr to /dev/null
3. will redirect stdout and stderr to /dev/null

Relevant posts:

• What does "/dev/null" mean at the end of shell commands)

• i/o stream redirection on linux shell. how does the shell process a command with redirection?

• What does “> /dev/null 2>&1″ mean? (http://www.xaprb.com/blog/2006/06/06/what-does-devnull-21-mean/)

Answer 1

See this:

mycommand > /dev/null

it will redirect channel 1 (which is stdout) of mycommand to /dev/null

mycommand 2> /dev/null

it will redirect channel 2 (which is stderr) to /dev/null

mycommand > /dev/null 2>&1

it will redirect channel 1 to /dev/null and then bind channel 2 (stderr) to channel 1 (stdout). Both will go into /dev/null

There is another one (just to complete)

mycommand 2>&1 > /dev/null

In this second case, I bind (the child's) stderr to stdout (of the parent) and then I find the child's stdout to /dev/null. The result is that you now get the child's stderr output on stdout and the stdout goes to the file. This is useful for processing stderr in a pipe, for example. (see this answer)

Answer 2

(errfile doesn't exist)

$ cat errfile
cat: 0652-050 Cannot open errfile.

$ cat errfile > /tmp/stream.out
cat: 0652-050 Cannot open errfile.


$ cat errfile > /tmp/stream.out 2>&1

$ cat /tmp/stream.out
cat: 0652-050 Cannot open errfile.

($ rm /tmp/stream.out)

$ cat errfile 2>&1 > /tmp/stream.out
cat: 0652-050 Cannot open errfile.

$ cat /tmp/stream.out

$

Order is thus important and 2>&1 1>out is different than 1>out 2>&1 due to stream redirection at shell interpretation. You shoud redirect in "reverse" order. stdout > final than source > stdout

Answer 3

Try these to get the differences:

echo "stderr" > /dev/fd/2 | >/dev/null
stderr
echo "stdout" > /dev/fd/1 | >/dev/null

both commands redirected to /dev/null but in first one we're writing to stderr which prints stderr but in second one it prints nothing