Introduction to Named Pipes
Bash uses named pipes in a really neat way. Recall that when you enclose a command in parenthesis, the command is actually run in a “subshell”; that is, the shell clones itself and the clone interprets the command(s) within the parenthesis. Since the outer shell is running only a single “command”, the output of a complete set of commands can be redirected as a unit. For example, the command:
(ls -l; ls -l) >ls.out
writes two copies of the current directory listing to the file ls.out.
Command substitution occurs when you put a < or > in front of the left parenthesis. For instance, typing the command:
cat <(ls -l)
results in the command ls -l executing in a subshell as usual, but redirects the output to a temporary named pipe, which bash creates, names and later deletes. Therefore, cat has a valid file name to read from, and we see the output of ls -l, taking one more step than usual to do so. Similarly, giving >(commands) results in Bash naming a temporary pipe, which the commands inside the parenthesis read for input.
If you want to see whether two directories contain the same file names, run the single command:
cmp <(ls /dir1) <(ls /dir2)
The compare program cmp will see the names of two files which it will read and compare.
Command substitution also makes the tee command (used to view and save the output of a command) much more useful in that you can cause a single stream of input to be read by multiple readers without resorting to temporary files—bash does all the work for you. The command:
ls | tee >(grep foo | wc >foo.count) \ >(grep bar | wc >bar.count) \ | grep baz | wc >baz.count
counts the number of occurrences of foo, bar and baz in the output of ls and writes this information to three separate files. Command substitutions can even be nested:
cat <(cat <(cat <(ls -l))))works as a very roundabout way to list the current directory.
As you can see, while the unnamed pipes allow simple commands to be strung together, named pipes, with a little help from bash, allow whole trees of pipes to be created. The possibilities are limited only by your imagination.
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