I am running this:

cat /dev/urandom|hexdump| grep -i "ffff f" > random

and I get nothing in file random, it remains zero length after the command is interrupted.

How to make it writing output to a file?

I need to write a result to a file, which should contains output data like this:

021bc40 7724 d4f5 59ec bcbb ffff fd26 ab3c 9b7c

03a9100 b3a5 8601 fa33 ffff f23c 4326 2e7f 0c8a

0449810 e459 d5af 4e11 ffff fc55 8660 9efb 3c9b

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Use the --line-buffered option for grep (and also get rid of the useless cat):

hexdump /dev/urandom | grep --line-buffered -i "ffff f" > random

This way the output is not buffered but every line put into random immediately. I would also recommend to use tee in your pipe to see how many lines have been produced:

hexdump /dev/urandom | grep --line-buffered -i "ffff f" | tee random

Your file is empty because the process is interrupted before the file is written to disk. That is how redirection works. As a workaround, try this:

script -c 'cat /dev/urandom|hexdump|grep -i "ffff f"' -f random

This will basically write all screen output to the file.

  • It involved one more command, but delivered. Thank you. – Ruslan Gerasimov Jun 26 '14 at 9:05

cat /dev/urandom|hexdump or hexdump /dev/urandom writes continuously to the stdout and if you press Ctrl+C nothing will be executed after this. But you can limit the output with head

hexdump /dev/urandom | head -n1000000 | grep "ffff f" > random  

this will grep in the first 1000000 lines of the output and writes the result to a file.

  • Involved one more command, but gave a good result. Thank you. – Ruslan Gerasimov Jun 26 '14 at 9:07

You will need to write the output of cat /dev/urandom | hexdump to a file before you execute it the next time. The script below should accomplish what you are trying:

cat /dev/urandom | hexdump |
  while IFS= read -r line; do
    printf '%s\n' "$line" >> random;

IFS is used to split the output into lines here.



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