First, it's a convenient way to turn around pipelines inside scripts in situations where you need to feed a command output or pipeline into a read loop but the loop needs to be part of the original script process so it can manipulate variables, etc. Meaningless example of optionally incrementing a counter in a loop based on the output of a pipeline of commands:

    while read LINE; do
        if grep -q xyz <<< "$LINE"; then
            COUNTER=$(( $COUNTER + 1 ))
        fi
    done < <(awk '{print $4,$7}' infile.txt |cut -d, -f3)

The other most common way I use it is to diff a couple of sets of pipelines or even just files that need sorting before comparison:

    diff <(sort -u file1.txt) <(sort -u file2.txt)

    diff <(cut -d, -f1 quarantined-files.csv |xargs -L1 basename |sort) <(ls ./quarantine/ |sort)

It took me a while to really grok this sort of thing, but once it clicked, it's often the first thing I think of. Basically any time you might be tempted to dump something to a temporary file, you should ask yourself "can I use process substitution?"

  zcat large_file.gz | tee >(grep -F pattern1 > pattern1_results.txt) >(grep -F pattern2 > pattern2_results.txt) > /dev/null

Example: how can you redirect stdout to syslog, a specified log file, and stdout? Easy, with tee, right?

``` $my_command | tee -a >(logger -t "$0[my_command]" -p "local0.INFO") "/var/log/my_logs/$(date +%s).log") ```

Okay, that's cool. But, I have to run it for every command in my script! How inconvenient. Is there a way we can make all stdout for the script do this?

Yes! Use `exec`!

``` exec &> tee -a >(logger -t "$0[my_command]" -p "local0.INFO") "/var/log/my_logs/$(date +%s).log")

$my_command ```

It's not too long and very well written so it's an enjoyable read.

You almost always learn something new which you can integrate into your workflow.

http://www.gnu.org/software/bash/manual/bashref.html

For example, `grep -f <pattern-file>` - grep works on standard input, but if you also want to provide patterns via process output, you need to use temporary files, fifos, or process substitution.

Here's an example:

    find | fgrep -f <(cut -f 2 tab-delimited-text.txt)

  $ echo "Process substitution is fun" | tee >(rev)
  Process substitution is fun
  nuf si noitutitsbus ssecorP

  sudo echo "fs.inotify.max_user_watches=1000000" > /etc/sysctl.conf

  echo "fs.inotify.max_user_watches=1000000" | sudo tee /etc/sysctl.conf

You could get a similar effect in other ways (e.g., tail -f, or terminal scrollback), but I find tee is convenient.

    long_running_thing | tee long_running_thing_stdout

As seen on https://unix.stackexchange.com/questions/25372/turn-off-buff... , either `unbuffer long_running_thing` or `stdbuf -oL long_running_thing` causes it to output linewise instead of 4k at a time.

  bash$ wc <(grep script /usr/share/dict/linux.words)
  fish$ wc (grep script /usr/share/dict/linux.words | psub)

[1]: http://fishshell.com/

    >(wc)

dag from GitHub came up with[0] a quick way of using psub:

    bind \es 'commandline -ij "(|psub)"; commandline -f backward-word backward-word'

moreutils http://joeyh.name/code/moreutils/