Bash – How to redirect stdout to a file and read from the same file simultaneously with another task

The Bash documentation is using the term 'file handle' as a synonym for 'file descriptor'.

There's no requirement for programs to use the C Standard Library for I/O. Obviously, if they do, they can use fdopen() to obtain a (pointer to a) FILE structure from one of the file descriptors.

The redirection operator to open a file in read+write mode without truncation is <> in all Bourne-like shells (that maps to open(file, O_RDWR|O_CREAT) (though zsh also throws in a O_NOCTTY) or fopen(file, "w+")):

exec 3<> "$file"

opens the $file on file descriptor 3 in read+write mode (without truncating it and creating it if it didn't exist).

However, only ksh93 and zsh have seeking operators. dd can seek, but not backwards. And note no shell except zsh can have NUL bytes in their variables.

In zsh:

zmodload zsh/system
exec 3<> $file
sysread -i 3 -c 2 var # a read() of 2 bytes
sysseek -u 3 0 # seek back to beginning
# or sysseek -u 3 -w current -2 # to seek back 2 bytes
syswrite -o 3 something-else
exec 3<&- # close

In ksh93:

exec 3<> "$file"
var=$(dd bs=2 count=1 <&3 2>/dev/null; echo .)
var=${var%?}
exec 3<#((0)) # seek to beginning
# or exec 3<#((CUR-2)) # to seek back 2 bytes
print -ru3 something-else

Portably, you could still open the file several times, for each offset you want, like here to read and write 2 bytes at offset 2 (provided they're not bytes with value 0 if not using zsh):

var=$(dd bs=2 count=1 skip=1 < "$file"; echo .)
var=${var%?}
printf %s something-else | dd bs=2 seek=1 1<> "$file"

Or:

printf %s something-else | dd bs=2 seek=1 of="$file" conv=notrunc

To read and write to the same file, ksh93 has two other interesting redirection operators:

tr 01 10 < file >; file

Would store the output of tr in a temporary file and if tr is successful, rename that to file (beware the file is created anew, so with possibly different permissions and ownership).

tr -d 0 < file 1<>; file

Same as the standard/Bourne tr -d 0 < file 1<> file except that if tr succeeds, file is truncated where tr finished writing. You can use that for filter commands that produce less output than they read input, or more precisely commands that would not read data that they've previously written.

And zsh has the =(...) form of process substitution which you can use as:

 mv =(tr 01 10 < file) file

(with similar effect and caveats as ksh93's >;). Or:

 cp =(tr 01 10 < file) file

which would preserve attributes of file but means an extra copy.

Now if you need to read and write at the same offset using the same file descriptor and neither zsh nor ksh93 are available, you could always revert to perl/python/ruby...

perl -e '
  open F, "<>", "file" or die "open: $!";
  read F, $var, 1;
  seek F, 0, 0;
  print F "something-else"'

Now, after re-reading the updated version of your question, it looks like your file is behaving more like a socket or bidirectional pipe, and not like a regular, seekable file.

In which case, it could be just a matter of:

socat - file:your-file

or:

(cat >&3 3>&- & cat <&3 3<&-) 3<> your-file

to feed data from and to that file as read from/to stdin/stdout.

Note that each cat reads/writes to its own copy of the file descriptor 3 open by the shell, but they share the same open file description so it should be equivalent.