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I need to log stdout and stderr to logfiles, but only show the error messages on screen. I can do this with:
cp -rpv a/* b 1> copyLog.txt 2> >(tee copyError.txt >&2)
Which I found somewhere in the web.
I just want to know how this >(tee copyError.txt >&2) thing is called? I can't google for it, since Google ignores characters like angle brackets and the parentheses..
From man bash:
Process Substitution
Process substitution is supported on systems that support
named pipes (FIFOs) or the /dev/fd method of naming open
files. It takes the form of <(list) or >(list). The process
list is run with its input or output connected to a FIFO or
some file in /dev/fd. The name of this file is passed as an
argument to the current command as the result of the expan‐
sion. If the >(list) form is used, writing to the file will
provide input for list. If the <(list) form is used, the
file passed as an argument should be read to obtain the out‐
put of list.
You can search manpages by pressing / and then typing your search string, which is a good way of finding information like this. It does of course require that you know in which manpage to search :)
You have to quote the ( though, because it has a special meaning when searching. To find the relevant section in the bash manpage, type />\(.
>(tee copyError.txt >&2) is actually a few different features:
>(...) is called 'process substitution'. It creates a named pipe file in /dev/fd and writing to that file will provide input to the process in the parentheses.
> : In general, this is called 'output redirection' and allows you to send standard output (> or 1>) or standard error (2>) to a file or process. >&2 is output redirection but in this case, the output of tee is sent to standard error (that's what &2 is, &1 is standard output)
Without >, parentheses (()) will start a subshell. Running commands in parentheses will spawn another shell which will only exist for as long as those commands are running. You can see how this works if you declare a variable in the subshell:
$ foo='Tom';(foo='Dick'; echo "Sub: $foo"); echo "Orig: $foo"
Sub: Dick
Orig: Tom
As you can see, the $foo that was defined in the subshell is separate from the one defined in the parent shell.
>(...) is not a redirection. >(...) is expanded to a file name. If you want to redirect output to that, you need > >(...) but >(...) is more generally used where redirections can't be used. The OP's command can be achieved with traditional pipes, there's not need for process substitution there.
Sep 21, 2013 at 17:11
> can denote output-redirection and that () can denote a subshell, but >(...) is actually a single, unitary feature that does not consist of > and ().