Question

I have a program which will be running continuously writing output to stdout, and I would like to continuously take the last set of output from it and write this to a file. Each time a new set of output comes along, I’d like to overwrite the same file.

An example would be the following command:

iostat -xkd 5

Every 5 seconds this prints some output, with a blank line after each one. I’d like the last “set” to be written to a file, and I think this should be achievable with something simple.

So far I’ve tried using xargs, which I could only get to do this with a single line, rather than a group of lines delimited by something.

I think it might be possible with awk, but I can’t figure out how to get it to either buffer the data as it goes along so it can write it out using system, or get it to close and reopen the same output file.

EDIT: To clarify, this is a single command which will run continuously. I do NOT want to start a new command and read the output of that.

SOLUTION

I adapted the answer from @Bittrance to get the following:

iostat -xkd 5 | (while read r; do
        if [ -z "$r" ]; then
                mv -f /tmp/foo.out.tmp /tmp/foo.out
        else
                echo "$r" >> /tmp/foo.out.tmp
        fi
done)

This was basically the same, apart from detecting the end of a “section” and writing to a temp file so that whenever an external process tried to read the output file it would be complete.

Answer 1

Partial answer:

./your_command | (while read r ; do 
    if ... ; then
        rm -f foo.out
    fi
    echo $r >> foo.out
done)

If there is a condition (the …) such that you know that you are receiving the first line of the “set” this would work.