Ttyjfyk

Just hit this problem, and learned a lot from the chosen answer: Create random data with dd and get "partial read warning". Is the data after the warning now really random?

Unfortunately the suggested solution head -c is not portable.

For folks who insist that dd is the answer, please carefully read the linked answer which explains in great detail why dd can not be the answer. Also, please observe this:

$ dd bs=1000000 count=10 if=/dev/random of=random
dd: warning: partial read (89 bytes); suggest iflag=fullblock
0+10 records in
0+10 records out
143 bytes (143 B) copied, 99.3918 s, 0.0 kB/s
$ ls -l random ; du -kP random
-rw-rw-r-- 1 me me 143 Apr 22 19:19 random
4 random
$ pwd
/tmp

Unfortunately, to manipulate the content of a binary file, dd is pretty much the only tool in POSIX. Although most modern implementations of text processing tools (cat, sed, awk, …) can manipulate binary files, this is not required by POSIX: some older implementations do choke on null bytes, input not terminated by a newline, or invalid byte sequences in the ambient character encoding.

It is possible, but difficult, to use dd safely. The reason I spend a lot of energy steering people away from it is that there's a lot of advice out there that promotes dd in situations where it is neither useful nor safe.

The problem with dd is its notion of blocks: it assumes that a call to read returns one block; if read returns less data, you get a partial block, which throws things like skip and count off. Here's an example that illustrates the problem, where dd is reading from a pipe that delivers data relatively slowly:

yes hello | while read line; do echo $line; done | dd ibs=4 count=1000 | wc -c

On a bog-standard Linux (Debian jessie, Linux kernel 3.16, dd from GNU coreutils 8.23), I get a highly variable number of bytes, ranging from about 3000 to almost 4000. Change the input block size to a divisor of 6, and the output is consistently 4000 bytes as one would naively expect — the input to dd arrives in bursts of 6 bytes, and as long as a block doesn't span multiple bursts, dd gets to read a complete block.

This suggests a solution: use an input block size of 1. No matter how the input is produced, there's no way for dd to read a partial block if the input block size is 1. (This is not completely obvious: dd could read a block of size 0 if it's interrupted by a signal — but if it's interrupted by a signal, the read system call returns -1. A read returning 0 is only possible if the file is opened in non-blocking mode, and in that case a read had better not be considered to have been performed at all. In blocking mode, read only returns 0 at the end of the file.)

dd ibs=1 count="$number_of_bytes"

The problem with this approach is that it can be slow (but not shockingly slow: only about 4 times slower than head -c in my quick benchmark).

POSIX defines other tools that read binary data and convert it to a text format: uuencode (outputs in historical uuencode format or in Base64), od (outputs an octal or hexadecimal dump). Neither is well-suited to the task at hand. uuencode can be undone by uudecode, but counting bytes in the output is awkward because the number of bytes per line of output is not standardized. It's possible to get well-defined output from od, but unfortunately there's no POSIX tool to go the other way round (it can be done but only through slow loops in sh or awk, which defeats the purpose here).

Newer version of dd have a count_bytes iflag. eg:

cat /dev/zero | dd count=1234 iflag=count_bytes | wc -c

will output something like

2+1 records in
2+1 records out
1234 bytes (1.2 kB, 1.2 KiB) copied, 0.000161684 s, 7.6 MB/s
1234

Part of the point of using dd at all is that the user gets to pick the block size it uses. If dd fails for too large block sizes, IMO it's the user's responsibility to try smaller block sizes. I could ask for a TB from dd in one block, but that doesn't mean I'll get it.

If you want an exact number of bytes, this will be horrendously slow, but should work:

dd bs=1 count=1000000

If even a block size of 1 results in partial reads, …