How to rate-limit a pipe under linux?

Is there a filter which I could use to rate-limit a pipe on linux? If this exists, let call it rate-limit, I want to be able to type in a terminal something like

cat /dev/urandom | rate-limit 3 -k | foo

in order to send a a stream of random bytes to foo's standard input at a rate (lower than) 3 kbytes/s.

Solution 1:

Pipe Viewer has this feature.

cat /dev/urandom | pv -L 3k | foo

From the pv man page:

-L RATE, --rate-limit RATE

Limit the transfer to a maximum of RATE bytes per second. A suffix of "k", "m", "g", or "t" can be added to denote kilobytes (*1024), megabytes, and so on.

Solution 2:

I'd say that Juliano has got the right answer if you have that tool, but I'd also suggest that this is a neat little K&R style exercise: just write a specialized version of cat that reads one character at a time from stdin, outputs each to stdout and then usleeps before moving on. Be sure to unbuffer the standard output, or this will run rather jerkily.

I called this slowcat.c:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int main(int argc, char**argv){
  int c;
  useconds_t stime=10000; // defaults to 100 Hz

  if (argc>1) { // Argument is interperted as Hz
    stime=1000000/atoi(argv[1]);
  }

  setvbuf(stdout,NULL,_IONBF,0);

  while ((c=fgetc(stdin)) != EOF){
    fputc(c,stdout);
    usleep(stime);
  }

  return 0;
}

Compile it and try with

$ ./slowcat 10 < slowcat.c

Solution 3:

throttle seems designed specifically for this. e.g.

cat /dev/urandom | throttle -k 3 | foo

Solution 4:

Here's an all-shell solution that won't lose input (cf. the head -1 idea from Mike S):

hexdump -c /dev/urandom | (lines=0; while read line; do echo $line; if [ $((++lines % 10)) -eq 0 ]; then sleep 1; fi; done) | cat -n

Solution 5:

Quick and dirty, all shell all the time (I have CentOS and pv doesn't come with it by default):

hexdump -c /dev/urandom | while true; do head -1; sleep 1; done | \
your_program_that_reads_stdin  -your_argument_list

...dirty because although it's rate limited, it's bursty at a rate that I can't tell you :-) . But it's handy when you just need to send data from one place to the other (I was doing some socat tests) and you don't want to swamp your screen with garbage.