Linux less behavior and stderr

Question

I'm watching the output of my complicated command with less, problem is the stderr gets lost. stderr lines normally get listed inbetween stdout lines inside less. I would like them to be printed to the console, and when I exit less, to see them there together.

I realize there might be no solution to this, I read about tee and multitee but no luck so far.

Answer 1

Maybe

command 2> command.err | less; cat command.err; rm command.err

Answer 2

You have to redirect stderr to stdout:

$ ./somecommad 2>&1 | less

Check the manual for you shell (e.g. man bash.)

Answer 3

just tell the shell to redirect fd 2 to fd 1 (stderr to stdout)

 make 2>&1 | less

Answer 4

You need to understand the concept of "file descriptors". Usually, a unix application will start with three special file descriptors:

• Standard input
• Standard output
• Standard error

The "pipe" | in the shell connects stdout from one process with stdin of the next.

Errors are - by design - not fed to stdin of the next process. They will often not make sense to the next application, and should not be hidden from the user.

If you want to mix the errors into stdout, you can use e.g. 2>&1, which says essentially "append stderr to stdout". For example

find /etc 2>&1 | less

should also include error output from inaccessible files.

find /etc 2>&1 >/dev/null | less

will give you the errors only.

Answer 5

I'm confused about your question, as far as I ca tell your desired behavior is the default.

When I use

#include <stdio.h>

int main(int argc, char**argv){
  for (int j=0; j<10; ++j){
    fprintf( (j%2 ? stdout : stderr) , "%d\n" , j);
  }
  return 0;
}

to get a simple test,

$ ./testredirection | less

does just what you ask. That is I see

1
3
5
7
9
(END) 

in less and

$ ./testredirection | less
0
2
4
6
8
$ 

when I quit less

Answer 6

I happen to encounter this problem in one of my Debian 5.0's recently. for example, ls abc | less I find that the error message go into less, which against my knowledge.

After some attempts, I found that it is just something related to screen buffers. stderr does NOT go into less actually. You can use Up or Down arrow keys (or j/k) to demonstrate.