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shellawkio-redirection

Linux output redirection to file

This is in RHEL 7.

Simple shell script:

while [ 1 ] ;
do
  echo "Test Message 1"
  echo "Test Message 2"
  echo ""
  sleep 10
done

Following works fine:

./loop-test.sh | gawk '{ print strftime("[%Y-%m-%d %H:%M:%S]"), $0 }'
[2023-12-21 10:04:47] Test Message 1
[2023-12-21 10:04:47] Test Message 2

I want to redirect output to a file. So, I did: ./loop-test.sh | gawk '{ print strftime("[%Y-%m-%d %H:%M:%S]"), $0 }' > t.txt

It creates output file t.txt, but it is empty. I tried several redirections... including "tee" with no luck.

Is there a way I can capture the output in a file?

Solution

  • This is happening due to output buffering. When the output is redirected to a file, both gawk and the shell buffer the output before writing it to the file. To disable buffering in gawk, you can use the fflush() function. Something like:

    ./loop-test.sh | gawk '{ print strftime("[%Y-%m-%d %H:%M:%S]"), $0; fflush() }' > t.txt

    SAMPLE DEMO