Bash $* and $@ (2017)

Re:Bash $* and $@ (2017) by spc476 on Monday January 13, 2020 @05:17AM (#22033249)

Okay, but can you pipe stdout to one string of commands, and stderr to a different string of commands? That's something I feel should be possible but how Unix shells handle redirection is just ... alien to my way of thinking.

Yes, I know that underneath it's all calls to `pipe()` and `dup2()`, which I can do (and have done) in a language other than shell. It's the shell redirection syntax (for anything more complex than simple redirection or a pipe) that just doesn't make sense to me.

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Re:Bash $* and $@ (2017) by pwg on Monday January 13, 2020 @07:19AM (#22033773)

Yes you can (provided you have Bash and a system that supports process substitution):

    command 2>&1 > >(cat - | cat - > fd1) | cat - | cat - > fd2

Note, the extra 'cats' are just to show a "string of commands".

How to read this:

When Bash sets up the file descriptors for 'command' it initially makes descriptor 1 refer to the pipe, and descriptor 2 refer to the terminal.

So, the dup operator (2>&1) copies the fd in descriptor 1 (which is the pipe) into descriptor 2 (so after the dup operator is processed both stderr and stdout for command reference the pipe).

Next descriptor 1 is replaced (the > operator) by a reference to a fifo created by the process substitution operator (the >(...) operator).

So now command's descriptor 1 refers to the fifo created by >() (which itself contains a "string of commands").

Then, because stderr for command was made to be a copy of what was previously stdout (before modifying stdout) it continues to refer to the pipe Bash setup (the | operator), and stderr now flows out over the pipes to another "string of commands".

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