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In Linux/Unix command line, when using a command with multiple inputs, how can I redirect one of them?

For example, say I'm using cat to concatenate multiple files, but I only want the last few lines of one file, so my inputs are testinput1, testinput2, and tail -n 4 testinput3.

How can I do this in one line without any temporary files?

I tried tail -n 4 testinput3 | cat testinput1 testinput2, but this seems to just take in input 1 and 2.

Sorry for the bad title, I wasn't sure how to phrase it exactly.

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  • you can use process substutution Commented Feb 7, 2018 at 3:06
  • @ymonad I tried cat testinput1 testinput2 <tail -n 4 testinput3, but as expected, that told me -bash: tail: No such file or directory. Commented Feb 7, 2018 at 3:12
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    You forgot the parents <(process) Commented Feb 7, 2018 at 3:13
  • @DavidC.Rankin I tried cat testinput1 testinput2 < <(tail -n 4 testinput3), but that only gave me the output of the first two, just like when I tried to use piping. Commented Feb 7, 2018 at 3:15
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    Sorry, see updated comment, you do not need the extra redirection cat file1 file2 <(tail -n4 file3) Commented Feb 7, 2018 at 3:18

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Rather than trying to pipe the output of tail to cat, bash provides process substitution where the process substitution is run with its input or output connected to a FIFO or a file in /dev/fd (like your terminal tty). This allows you to treat the output of a process as if it were a file.

In the normal case you will generally redirect the output of the process substitution into a loop, e.g, while read -r line; do ##stuff; done < <(process). However, in your case, cat takes the file itself as an argument rather than reading from stdin, so you omit the initial redirection, e.g. 

cat file1 file2 <(tail -n4 file3)

So be familiar with both forms, < <(process) if you need to redirect a process as input or simply <(process) if you need the result of process to be treated as a file.

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