redirecting both stdout and stderr

sigint-ninja · 08-04-2017, 08:38 AM

hi guys,

why doesnt

ls /etc /root 2>&1 redirectfile1

not work

is this the correct way,it is working:
ls /etc /root > redirectfile1 2>&1

and to append its
ls /etc /root >> redirectfile1 2>&1

but i always thought 2>&1 always worked on its own
is the above the right way to do it? or is there a better way?

also how would you write stdout to a file and stderr to a different file using one command?

thanks

hydrurga · 08-04-2017, 08:47 AM

Redirect both to same file:

Code:

ls /etc /root &> redirectfile1

Redirect both to different files:

Code:

ls /etc /root 2> errorfile 1> outputfile

To append to one or more of the files above rather than overwrite them, replace the relevant > with >>.

rknichols · 08-04-2017, 09:08 AM

The order is important. The redirections are processed left-to-right, and "2>&1" means "send stderr to the place stdout is now going". Subsequent redirections of stdout will not affect stderr.

Code:

ls /etc /root 2>&1 >redirectfile1

First, redirect stderr to the place stdout is now going, which is probably the terminal. Since stderr was probably already going to the terminal, this is pretty much a no-op.
Then, redirect stdout to file "redirectfile1". This has no effect on stderr.

I have to use words like "probably" and "pretty much" because this command might be in a script invoked with stderr and/or stdout already redirected.

Code:

ls /etc /root >redirectfile1 2>&1

First, redirect stdout to file "redirectfile1".
Then, redirect stderr to the place stdout is now going, i.e., the file.

Note that I did not have to use the word "probably" in the second example. Nothing there is dependent on how stderr or stdout might have been previously redirected.

sigint-ninja · 08-04-2017, 09:10 AM

very efficient and easy to remember some other forums showed nasty ways of accomplishing this

thanks very much kudos!!!