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


$ command < (command2 | grep pattern)
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Do you really mean the output of multiple commands ot the output of one pipeline? And which shell are we talking about, or are you looking for the perfectly portable solution? –  Hauke Laging Apr 28 '13 at 8:49
@HaukeLaging No; the output of one pipeline as the input to one command. –  Thomas Apr 28 '13 at 9:13
How is this different from extending the pipeline? command2 | grep pattern | command –  Hauke Laging Apr 28 '13 at 9:17
@HaukeLaging Ah, I didn't know they were the same! Thank you. Would you like to answer so I can mark it as accepted? –  Thomas Apr 28 '13 at 9:28
The <( .... ) syntax is primarily used in those cases where a simple pipeline does not work, either because some program refuses to read from standard input and requires a file argument, or because there are several commands whose output should be fed into one other command, i.e., cmd <(cmd1) <(cmd2). –  Uwe Apr 28 '13 at 9:42

2 Answers 2

up vote 2 down vote accepted

(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
Last input line

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

$ command < $(command2 | grep pattern)
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Tried it? pastebin.com/686v9iQr –  manatwork Apr 28 '13 at 8:46
@manatwork Would you mind making your findings a formal answer so that this question can be successfully closed? –  Hauke Laging Apr 28 '13 at 8:54
@HaukeLaging, I prefer to let the answer's owner to make the necessary corrections, instead of posting a concurrent answer. After all, Spack seems to be new here and this answer's age is less than half hour. –  manatwork Apr 28 '13 at 9:09

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