首先重定向到文件 STDOUT 然后是 STDERR

Question

I have a gawk code like this.

#!/usr/bin/gawk -f

1 {
    for (i=0;i<5;i++){
        print $0
        print $0, i > "/dev/stderr"
    }
}

I want to redirect to a file tmp, first the stdout and then the stderr. I tried this:

gawk -f Documents/gawk_script.awk ./file &> tmp

But this call append to the file first stderr. I don't want to divide them into two files, so I'm asking if there's a way to do that.

In ./file there's such a line:

hello
hello
howareyou
well
well

while in tmp file

hello
hello
hello
hello
hello
hello
hello
hello
hello
hello
howareyou
howareyou
howareyou
howareyou
howareyou
well
well
well
well
well
well
well
well
well
well
well
hello 0
hello 1
hello 2
hello 3
hello 4 
hello 0
hello 1
hello 2
hello 3
hello 4
howareyou 0
howareyou 1
howareyou 2
howareyou 3
howareyou 4
well 0
well 1
well 2
well 3
well 4
well 0
well 1
well 2
well 3
well 4

Answer 1

The problem you are encountering is due to the buffering of the streams stdout and stderr . Both streams have different default buffer-settings. While stdout is line-buffered when writing to a terminal, it is very well buffered when it writes to a stream/pipe/file. The stream stderr on the other hand is always unbuffered. That is why you first encounter the output of stderr and only later the output of stdout in your file tmp . Be aware, however, that the output will be interleaved when you output more lines as all of a sudden the buffer of stdout will be full and written to the file, following again with some output of stderr until the next buffer of stdout is full. The problem is nicely explained in the following page:

• http://www.pixelbeat.org/programming/stdio_buffering/

One of the ugly hacks you can apply is the usage of stdbuf to change the buffering of the datastreams of awk :

$ stdbuf -oL -eL awk '{...}' file.txt &> tmp.txt

Here we use stdbuf to set buffering mode of the streams stdout and stderr to line-buffering and thus the output of your code would look like:

hello
hello 1
hello
hello 2
...

If you really want first all output of stdout followed by all output of stderr , you should follow the approach mentioned by Ed Morton .

Answer 2

This is a small example:

I am not sure what you mean, if I understand it...

If you want to redirect stdout to file_out , redirect stderr to file_err , you can do this...

command > file_out 2> file_err

Answer 3

There is no good way* to tell awk or the shell that it must buffer stderr until the tool finishes executing. Keep it simple and just do this:

awk -f script.awk file > out 2>tmp; cat tmp >> out && rm -f tmp

Otherwise you could buffer stderr yourself and print at the end (but this will only work for stderr messages you are manually printing, not messages gawk is generating itself):

{
    for (i=0;i<5;i++){
        print $0
        errs = errs $0 OFS i ORS
    }
}
END {
    printf "%s", errs > "/dev/stderr"
}

and then call as:

awk -f script.awk file > out 2>&1

Of course you don't actually need to use stderr at all if that's all you're doing with it, just print to stdout.

*There may be some arcane incantation you can use to make this happen if the planets align a certain way and/or you have certain tools or a certain shell but just keep it simple as shown above.

Answer 4

Lit bit old, but if someone still needs it:

echo "some stuff" | awk '{
    for (i=0;i<5;i++){
        print $0;
        # print i on stderr
        system(">&2 echo " i);
    }
}'