Ryfujk

The strings command behaves weirdly, apparently it doesn't stop writing to a file even if drive run out of space. Or perhaps I'm missing something?

I run the following:

# strings /dev/urandom > random.txt

this was keep running and didn't stop even after filling the disk (a regular usb flash).

then to be quicker I created a ramdisk and tried again the same command. it also didn't stop.

I understand that urandom isn't a regular file and also strings's output is redirected, however in both cases above, the cat command reported the error when there was no more space.

# cat /dev/urandom > random.txt

cat: write error: No space left on device

1. Is this normal behavior of strings? If so, why?


2. Where is the data written after there's no more space left?

If GNU cat can't write out what it read, it will exit with an error:

/* Write this block out.  */



{

  /* The following is ok, since we know that 0 < n_read.  */

  size_t n = n_read;

  if (full_write (STDOUT_FILENO, buf, n) != n)

    die (EXIT_FAILURE, errno, _("write error"));

}

GNU strings, on the other hand, doesn't care whether it managed to write successfully:

while (1)

  {

    c = get_char (stream, &address, &magiccount, &magic);

    if (c == EOF)

      break;

    if (! STRING_ISGRAPHIC (c))

      {

        unget_part_char (c, &address, &magiccount, &magic);

        break;

      }

    putchar (c);

  }

So all those writes fail, but strings continues merrily along, until it reaches end of input, which will be never.

$ strace -e write strings /dev/urandom > foo/bar

write(1, "7[\Zn]juKwnl [1nTc9gn0&}x(xn/y^7"..., 4096) = 4096

write(1, "nXaki%ndHB0n?5:Qn6bX-np!E[n'&=7n"..., 4096) = 4096

write(1, "%M6sn=4C.%n&7)nnQ_%JncT+";nK*<%n"..., 4096) = 4096

write(1, "&d<nj~g0nm]=ona=^0n%s]2WnM7C%nUK"..., 4096) = -1 ENOSPC (No space left on device)

write(1, "~nd3qQn^^u1#na#5\n^=t"bn*91_n ]o"..., 4096) = -1 ENOSPC (No space left on device)

write(1, "Ln6QO1xna,yEnk>",@ZnyM.urn~ztFnr"..., 4096) = -1 ENOSPC (No space left on device)

write(1, "n61]Rnyg9CnfLVun<Ez:n.tV-cnw_'>e"..., 4096) = -1 ENOSPC (No space left on device)

write(1, "nCj)anT]X:uAn_KH"BnRfQ4Gn3retn&s"..., 4096) = -1 ENOSPC (No space left on device)

write(1, "jnk7@%n9E?^NnJ#8Vn*]i,nXDxh?nr_1"..., 4096) = -1 ENOSPC (No space left on device)

write(1, "iatInQ)ZwnnV0JnE3-W n@0-N2vnK{15"..., 4096) = -1 ENOSPC (No space left on device)

write(1, "nZ~*gn)FQnnUY:GndRbNnn..FnvF{,n+"..., 4096) = -1 ENOSPC (No space left on device)

...

Yes this is the proper behavior

The data is not written anywhere when the disk is full, a write error is generated and the data is "dropped on the floor".

 Anything that starts with /dev is considered to be a device.

  So this is a device that constantly generates random bytes...



dd if=/dev/urandom of=~/urandom_test count=4 bs=1024

Creates a file containing 4K of random bytes.



cat /dev/urandom > ~/urandom_test2 

Will continue to write random bytes to that file until you hit Ctrl-C.



head -30 /dev/urandom > ~/urandom_test3

Will write 30 lines of random bytes