Redirect stdout and stderr after redirection command - bash

Question

In a bash script I am enabling IP forwarding using the following command:
echo 1 > /proc/sys/net/ipv4/ip_forward
However I also want to include my own error messages so I have to redirect the stdout and stderr to /dev/null, which looks like the following:
echo 1 > /proc/sys/net/ipv4/ip_forward >/dev/null 2>&1
This works for commands that do not have the redirection symbol in it, for example:
route add default gw 10.8.0.1 > /dev/null 2>&1
Is there any way I can make this work for commands that do have a redirection in them? Is there a workaround for this? Any other way I can do this better?

Answer 1

Your redirections are nonsensical (no offense).
This:
echo 1 > /proc/sys/net/ipv4/ip_forward >/dev/null 2>&1
will:
redirect echo's standard output to /proc/sys/net/ipv4/ip_forward;
redirect echo's standard output to /dev/null
redirect echo's standard error to where file descriptor 1 points, i.e., to /dev/null.
Hence, globally, redirection 2 cancels redirection 1: nothing is going to go to /proc/sys/net/ipv4/ip_forward!
I guess you want to redirect echo's standard output to /proc/sys/net/ipv4/ip_forward and echo's standard error to /dev/null. This is achieved by:
echo 1 > /proc/sys/net/ipv4/ip_forward 2>/dev/null
But read on, I believe that's not the answer you're looking for!
Why do you want to redirect echo's standard error to /dev/null?
echo very rarely writes to standard error. In fact, the only time echo will write to standard error is when there's a write error. There are a couple of ways this could happen:
if the disk is full (that can be simulated with /dev/full):
$ echo hello >/dev/full
bash: echo: write error: No space left on device
$ echo hello >/dev/full 2>/dev/null
$
(no error messages shown with the redirection 2>/dev/null).
if echo's standard out is closed:
$ ( exec >&-; echo hello )
bash: echo: write error: Bad file descriptor
$ ( exec >&-; echo hello 2> /dev/null )
$
(no error messages shown with the redirection 2>/dev/null).
There might be other cases where echo outputs to standard error. But the following are certainly not among them:
Redirecting echo's standard output to a non-existent file descriptor:
$ echo hello >&42
bash: 42: Bad file descriptor
$ echo hello >&42 2>/dev/null
bash: 42: Bad file descriptor
$
The redirection 2>/dev/null doesn't fix anything; you can actually see that the error comes from bash and not from echo (the latter would have bash: echo: as a prefix).
Redirecting echo's standard output to a file without write permission:
$ touch testfile
$ chmod -w testfile
$ echo hello > testfile
bash: testfile: Permission denied
$ echo hello > testfile 2>/dev/null
bash: testfile: Permission denied
$
Same as above, the redirection 2>/dev/null doesn't fix anything.
The previous cases are not fixed by 2>/dev/null, since the error occurs at Bash's level, before the command is even executed and the redirections performed, because it's at the moment of the redirection that Bash encounters an error: it can't open the stream for writing and outputs the error message to standard output.†
Now I guess you're trying to fix the following scenario: when the user doesn't have enough rights to write to /proc/sys/net/ipv4/ip_forward:
$ echo 1 >/proc/sys/net/ipv4/ip_forward
bash: /proc/sys/net/ipv4/ip_forward: Permission denied
$
The error message on standard error can't be redirected with a simple redirection‡:
$ echo 1 >/proc/sys/net/ipv4/ip_forward 2>/dev/null
bash: /proc/sys/net/ipv4/ip_forward: Permission denied
$
A standard way to redirect the error that occurs at the redirection level (i.e., before the command is even executed) is to use groupings:
$ { echo 1 >/proc/sys/net/ipv4/ip_forward; } 2>/dev/null
Now that explains why the solution you posted as an answer works: let's go through it and we'll see that there's something that shows you didn't completely understand redirections (and hopefully this post will help you understanding a few things); your code is:
function ip_forward
{
echo 1 > /proc/sys/net/ipv4/ip_forward
}
ip_forward >/dev/null 2>&1
This will run the function ip_forward, and redirect:
its standard output to /dev/null;
and then its standard error to where its standard output points (i.e., to /dev/null).
But the function ip_forward doesn't output anything to standard output! so the redirection >/dev/null is only useful for the 2>&1 part of the redirection. In fact, your code is completely equivalent to:
function ip_forward
{
echo 1 > /proc/sys/net/ipv4/ip_forward
}
ip_forward 2>/dev/null
But then (since you only use a function construct as a way to achieve what you wanted—not because you want a function), it's much better to write your code as either:
echo 1 2>/dev/null >/proc/sys/net/ipv4/ip_forward
or
{ echo 1 > /proc/sys/net/ipv4/ip_forward; } 2>/dev/null
(the latter being preferred).
Sorry for this long post!
†
There's something we should be aware of: the order of redirection. They are performed from left to right, as Bash reads them. How about we first redirect standard error, and then standard output to a non-existent/non-writable stream?
$ echo hello 2>/dev/null >&42
$
That's right, it works.
‡
Well, can, if you understood the previous footnote:
$ echo 1 2>/dev/null >/proc/sys/net/ipv4/ip_forward
$ echo $?
1
$
No error on standard error! that's because of the order of the redirections.

Answer 2

Solved
echo 1 > /proc/sys/net/ipv4/ip_forward ip_forward 2>/dev/null
echo 1 is the stdout part so this does not have to be redirected anymore. I only had to redirect stderr by adding 2>/dev/null.