Double redirection test

What does this print your shel开发者_运维百科l?

echo foo | while read line; do echo $line; done < <(echo bar)

I would expect it to evaluate to echo foo | bar or foo < <(bar), both of which would result in an error message.

In Bash 4.1.5 it looks like the pipe is simply discarded:

bar

In Dash:

sh: Syntax error: redirection unexpected

Dash doesn't support process substitution (<()).

The behavior you're seeing is consistent if you use syntax that's supported by each of the shells you're comparing. Try this:

echo hello | cat < inputfile

You should see the contents of "inputfile" and not "hello". Of several shells I tried, only Z shell showed both.

This is what POSIX says regarding pipelines and redirection:

The standard output of command1 shall be connected to the standard input of command2. The standard input, standard output, or both of a command shall be considered to be assigned by the pipeline before any redirection specified by redirection operators that are part of the command (see Redirection ).

I interpret this to mean that in the case of the example above, the pipeline assigns stdin to cat then the redirection overrides it.