bash while loop that breaks at a given file size

Question

So, I bought this book called Primes and Programming, and it's pretty tough going. Today I wrote this (simple) program from chapter 1:

#!/usr/bin/env python

import math

def find_gcd(a,b):
    while b > 0:
        r = a - b * math.floor(a/b)
        a = b
        b = r
    return int(a)

if __name__ == "__main__":
    import random, sys
    while True:
        print find_gcd(random.randrange(int(sys.argv[1])), random.randrange(int(sys.argv[2])))

...and just now I called it like so:

./gcd-rand.py 10000 10000 > concievablyreallyhugefile

...and now I'm dreaming of a bash one-liner that breaks when concievablyreallyhugefile has reached a certain size. I guess it would look something like:

while $(du -h f) < 32M; do ./gcd-rand.py 10000 10000 > $f; done

...but I have never written a while loop in bash before and I don't really know how the syntax works.

Answer 1

The trick is to use the test command test or the equivalent [ ... ]:

 while [ "$(du -m f|cut -f1)" -lt 32 ]
 do 
  ./gcd-rand.py 10000 10000 > "$f"
 done

See help test for more information.

Note

test or [ command is a bash builtin. The help information can be retrieved inside bash via help test or help [. man test refers to the test command that is used if a shell has no such builtin or is invoked explicitly as /usr/bin/test.

Answer 2

./gcd-rand.py 10000 10000 | head -c 32M > concievablyreallyhugefile

head will stop reading after 32MB. Soon after head stops reading, gcd-rand.py will receive a SIGPIPE signal and exit.

To avoid storing a truncated last line, as Michael Kjörling noticed:

./gcd-rand.py 10000 10000 | head -c 32M | sed '$d' > concievablyreallyhugefile

Answer 3

Your python code loops forever. Thus, you might want to run it in the background and then kill it when the file size is exceeded. As one-liner:

{ ./gcd-rand.py 10000 10000 > f & }; p=$!; while (( $(stat -c %s f) < 33554432 )); do sleep .1; done; kill $p

Note: choose sleep time as appropriate, instead of stat you can also use du, as suggested by Dirk.

Answer 4

You can use the ulimit command to restrict how large a file the shell (or its children) can create:

ulimit -f 32768