Redirecting Multiple Commands as Input

Question

What is the correct way to redirect the output of multiple commands as input for another command?

e.g.

$ command < (command2 | grep pattern)

Answer 1

(This refers to bash in case of doubt)

The output of one command or one pipeline can be made the input of another command by creating a (or extending the existing) pipeline:

command1 | command2
command1 | command2 | command3

Several commands (including pipelines) can be combined with a subshell or a list (group command). This combination becomes the first part of the pipeline then:

(command1; command2 | command3; command4) | command5
{command1; command2 | command3; command4;} | command5 # note the ; before }

Another possibility is a "here string" (or even a "here document"):

command2 <<< $(command1)

command2 <<EOT
First input line
$(command1)
Last input line
EOT

Other cases

Command substitution is used when the output shall be part of a command line i.e. if one command shall see the output of another as its own parameter:

echo $(date)

echo sees the output of date as its parameter; as if it had been typed in the command line.

Process substitution makes the output of another process appearing as the content of a (non-seekable) file given as parameter on the command line.

grep bar <(echo $'foo\nbar\nbaz')

looks to grep like

echo $'foo\nbar\nbaz' > /path/to/file
grep bar /path/to/file

Answer 2

You have to execute the commands using the command substitution syntax.

$ command < $(command2 | grep pattern)