Loop through binary data chunks from stdin in Bash

Question

I'm looking for something like while IFS= read -r -n $length str; do ... done but for binary data. Is it possible to do this using dd or other tools? Is there some technique to make these tools able to see when pipe -which stdin is actually read from it- is closed and terminate loop?

Currently I encode and decode binary data and use read but It's so slow.. (base64 | while read -r -n77 str; do echo $str | base64 -d; ... done)

Answer 1

bash can't hold binary data in its variables. It's already bad enough to process text with shell loops, it would be even worse for processing binary data. The shell is the tool to run other tools.

Also note that the read buit-in command reads characters, not bytes.

Also, dd does one read system call, so a dd bs=77 count=1 won't necessarily read 77 bytes especially if stdin is a pipe (the GNU implementation of dd has iflag=fullblock for that).

Here, you want to use a data processing programming language like perl:

In perl:

perl -ne 'BEGIN{$/=\77}
  print "Do something with the 77 byte long <$_> record\n"'

With GNU awk:

LC_ALL=C awk -vRS='.{,77}' '{print "the record is in <" RT ">"}'

If you want to use a shell, your best option is probably zsh which is the only one that can store binary data in its variables:

while LC_ALL=C IFS= read -ru0 -k77 record; do
  print -r -- "you may only call builtins with $record
    anyway since you can't pass NUL bytes in arguments
    to an external command"
done

If all you want to do is pass each chunk as stdin to a new invocation of some command, then you can use GNU split and its --filter option:

split -b 77 --filter='some command'

--filter starts a new shell to evaluate some command for each chunk. Unless your sh does the optimisation already by itself, you can do:

split -b 77 --filter='exec some command'

To save a fork.

Using dd, you could parse its stderr output to find out the end of input. You'd need the GNU specific iflag=fullblock as well:

while
  {
    report=$({
      LC_ALL=C dd bs=77 iflag=fullblock count=1 2>&3 |
        some command >&4 3>&- 4>&-
    } 3>&1)
  } 4>&1
  [ "${report%%+*}" -eq 1 ]
do
  : nothing
done

If the input size is multiple of 77 though, some command will be run an extra time with an empty input.