The command below generates output in different I/O streams: error messages should go tostderr stream and "normal" messages tostdout stream. But if you run it in a terminal both streams are written to the screen:

$ ls -d /tmp nothingls: cannot access 'nothing': No such file or directory/tmp

You can redirect the output to a file. Note that the error message is written in the screen, not in in the file:

$ ls -d /tmp nothing > stdout.txtls: cannot access 'nothing': No such file or directory$ cat stdout.txt/tmp

Or you can redirect only the error message:

$ ls -d /tmp nothing 2> stderr.txt/tmp$ cat stderr.txtls: cannot access 'nothing': No such file or directory

Also, you can redirect both streams at the same time todistinct files:

$ ls -d /tmp nothing 2> stderr.txt > stdout.txt$ cat stderr.txt ls: cannot access 'nothing': No such file or directory$ cat stdout.txt /tmp

Or you can redirect everything to thesame file:

$ ls -d /tmp nothing &> all.txt$ cat all.txt ls: cannot access 'nothing': No such file or directory/tmp

Another possibility is to redirect stderr to stdout. It can be useful when you are passing data to another command. In the example below, both lines are sent to stdout andwc will count 2 lines:

$ ls -d /tmp nothing 2>&1 | wc -l2

Redirecting stdout to stderr is also possible. In this case both lines are sent to stderr that is displayed at the terminal screen, and nothing is sent to stdout, sowc will count 0 lines:

$ ls -d /tmp nothing 1>&2 | wc -l0ls: cannot access 'nothing': No such file or directory/tmp