Re: Backticked, nested command will steal piped stdin

[Dale R. Worley]

Justus <justus.kohl@gmail.com> writes:
> I found a bug in my beloved bash and hope that you can fix it!

>       $ seq 3 | head -n $(calc -dp "1+1")

This actually isn't a bug; a command substitution inherits the fd's
except for 1 (stdout) (which is captured by bash to create the command
substitution).

       Command substitution allows the output of a command to replace the com‐
       mand name.  There are two forms:

              $(command)
       or
              `command`

       Bash performs the expansion by executing command in a subshell environ‐
       ment and replacing the command substitution with the standard output of
       the command, with any trailing newlines deleted.

Though I don't know exactly how command substitution interacts with the
redirections that are applied to the containing command.

In particular, I've been bitten occasionally by the construct

    $ (command) | while read FOO BAR BAZ ; do (commands) ; done

when one of (commands) sucks on stdin.

In your case, you probably want

>       $ seq 3 | head -n $(</dev/null calc -dp "1+1")

Dale