Hi again UNIX shell lovers, as we saw in our [last course](https://steemit.com/linux/@imil/command-line-101-feel-the-power-of-the-unix-shell), a _shell command_ outputs non-errors messages to a special file called `stdout`, which number is `1` and reads from another special file called `stdin` which number is `0`. Error messages are sent to `stderr` which number is `2`.

To be clear, this means that the following command:

```
$ ls
. .. foo bar baz
```

displayed its result to the special file `stdout`, or `1`.  
This output can actually be _captured_ by another program by using the `|` (pipe) symbol. To illustrate its usage, we will use the `grep` command, which filters the output by matching a _pattern_. For example, if I would like to output only the lines that contain the word `match` in a file called `myfile.txt`, I would do:

```
$ grep match myfile.txt
beautiful match
matching
those matches
```

Instead of using a file as the input stream, we could use `stdout` from a previous command, which the `|` (pipe) command will transform to `grep`'s `stdin`!

```
$ echo 'I like to move it move it'|grep 'move'
I like to move it move it
$ echo 'this phrase won't match'|grep 'not present'
```

Think of `|` as a convenient way of filtering big outputs, for example you could very easily see if a directory contains a `.jpg` file by using `|grep '\.jpg'`. We won't go deep into `grep` patterns just yet to keep those courses nice and short :)

In our next shell tip, we will speak about redirections. Stay tuned!

Read more about standard streams in [Wikipedia's very well written documentation](https://en.wikipedia.org/wiki/Standard_streams).