| is a “pipe.” The
| takes the standard output of the command on the left, and pipes it as standard input to the command on the right. You can think of this as “command to command” redirection.
$ cat volcanoes.txt | wc
Above, the output of
cat volcanoes.txt becomes the standard input of
wc. in turn, the
wc command outputs the number of lines, words, and characters in volcanoes.txt, respectively.
$ cat volcanoes.txt | wc | cat > islands.txt
|s can be chained together. Here the standard output of
cat volcanoes.txt is “piped” to the
wc command. The standard output of
wc is then “piped” to
cat. Finally, the standard output of
cat is redirected to
You can view the output data of this chain by typing
Let’s try some more redirection commands. In the terminal, type:
cat volcanoes.txt | wc
(Tip: If you are unable to type
|, you can find help here.)
cat volcanoes.txt | wc | cat > volcanoes_count.txt
cat to output the contents of volcanoes_count.txt. Notice that three numbers appear as the output.