In an answer to a question about piping and redirection, robert mentions that piping also captures the stdout of substituted processes in the pipeline, whilst redirection doesn't. Why is this so? What exactly is going on, that results in this behavior:
bash-4.1$ echo -e '1\n2' | tee >(head -n1) >redirect
1
bash-4.1$ cat redirect
1
2
bash-4.1$ echo -e '1\n2' | tee >(head -n1) | cat >pipe
bash-4.1$ cat pipe
1
2
1
I would've thought that both forms would produce the same result -- the latter one.
Reading an answer to a different question, it seemed plausible that reordering the redirect in the command might produce the desired result, but no matter the order, the result is always the same:
bash-4.1$ echo -e '1\n2' | tee >redirect >(head -n1)
1
bash-4.1$ cat redirect
1
2
bash-4.1$ echo -e '1\n2' | >redirect tee >(head -n1)
1
bash-4.1$ cat redirect
1
2
Why does the stdout redirect only affect tee, but pipe captures the substituted process head as well? Simply "By design"?
Just a thought related to the above question: I thought that redirecting to a file and piping the output would never make sense, but it does make sense with process substitution:
bash-4.1$ echo -e '1\n2\n3' | tee >(head -n1) >(tail -n1) >tee_out | cat >subst_out
bash-4.1$ cat tee_out
1
2
3
bash-4.1$ cat subst_out
1
3
The shell that runs head is spawned by the same shell that runs tee, which means tee and head both inherit the same file descriptor for standard output, which file descriptor is connected to the pipe to cat. That means both tee and head have their output piped to cat, resulting in the behavior you see.