How to do a continous 'wc -l' with gnu texttools?

Question

I know of course that

cat logfile.txt | wc -l
120

will tell me the number of lines in a file.

Whereas

tail -f logfile.txt

will show me the new lines that another program writes to logfile.txt.

Is it possible to combine both so that I get a continuous updating line count of logfile.txt with standard text utilities?

I do know about

watch wc -l logfile.txt

but I do not want to re-count the whole file each time, that seems to be a waste. One would need an appended-only count every second or so and probably an \r instead of an \n at the end of line.

Answer 1

Maybe:

tail -n +1 -f file | awk '{printf "\r%lu", NR}'

Beware that it would output a number for every line of input (though overriding the previous value if sent to a terminal).

Or you can implement the tail -f by hand in shell:

n=0
while :; do 
  n=$(($n + $(wc -l)))
  printf '\r%s' "$n"
  sleep 1
done < file

(note that it runs up to one wc and one sleep command per second which not all shells have built in. With ksh93 while sleep is builtin, to get a built in wc (at least on Debian), you need to add /opt/ast/bin at the front of $PATH (regardless of whether that directory exists or not) or use command /opt/ast/bin/wc (don't ask...)).

You could use pv, as in:

tail -n +1 -f file | pv -bl > /dev/null

But beware that it adds k, M... suffixes when the number is over 1000 (and there doesn't seem to be a way around that).

Answer 2

Try to count it with pure bash without wc:

a=0 ; tail -f file | while read -r line ; do ((a++)) ; echo $a ; done

or even like this to rewrite previous value:

a=0 ; tail -f file | while read -r line ; do ((a++)) ; echo -ne "\r$a" ; done

Answer 3

I don' believe there is anything like that. But it should be easy to whip up something along the lines of:

#!/usr/bin/perl

$for_a_while = 1;

$oldcount = -1;
$count = 0;
open($fh, "<", $ARGV[0]);

for (;;) {
  for ($curpos = tell($fh); <$fh>; $curpos = tell($fh)) {
    $count++;
  }
  if($count != $oldcount) {
    print "$count\n";
    $oldcount = $count;
  }
  sleep($for_a_while);
  seek($fh, $curpos, 0);
}

(General idea cribbed from perlfunc(1))

Answer 4

Continuing the awk-based solution: you might not need to see the counter ticking for each line in your log; it that's the case, you can have it like this (number would change for each 10 lines):

tail -n +0 logfile.txt | \
    awk 'a+=1{}a%10==0{printf "\r%lu", a}END{printf "\r%lu", a}'