Using tail -f together with sed and xargs

Question

Suppose I have the following process to write a number + single quote to the file /tmp/test every second:

for i in {1..1000} ; do echo $i\' ; sleep 1 ; done > /tmp/test

I then want to use tail -f and run this through another function. For testing purposes, I'm using echo:

tail -f /tmp/test | xargs echo

This give me the following error (running this without a quote in the input works fine):

xargs: unmatched single quote; by default quotes are special to xargs unless you use the -0 option

If I add sed to the equation, I don't get any output at all:

tail -f /tmp/test | sed "s/['\"]//g" | xargs echo

Any ideas why I get no output after adding sed, or another way how I can strip the quotes from the input before it gets to xargs?

Answer 1

There's a number of things.

First: "tail -f" will never finish, so "xargs" will likewise remain running indefinitely. You can open a second window to "kill -HUP" the "tail -f" process to make both the "tail -f" and "xargs" finish, and thus produce output.

Second: "xargs" is buffering the input, and not getting what it considers to be a "full buffer", so by the time the 1000 second marks have been output, it hasn't yet run the echo command even once. If you'd like "xargs" to run the echo separately for each input row, you'd need to add "-n 1" option to xargs.

As for "xargs" not liking the single quote marks, you could use option "-d '\n'" to disable the quote processing and only separate the input arguments by a newline.

Answer 2

Maybe

tail -f /tmp/test | sed "s/['\"]//g" | while read line; do
  do_something "$line"
done

And check this: https://stackoverflow.com/questions/199266/make-xargs-execute-the-command-once-for-each-line-of-input