### Notes about command lines functions from the "Data Science at the Command Line"


`echo` is a command that outputs the strings it is being passed as arguments

```
echo 'Hello World'
Hello World
```

`wc` - word count

```
echo 'Hello World' | wc
 1       2      12
```

`alias` set an alias

```
alias gp = 'git push origin master'

```

`grep` used for regular expression

```
seq 30 | grep 3
3
13
23
30
```

Or

```
seq 30 | grep 3 | wc -l
4
```

Creating a new file using `>` operator
Append for existing fill using the `>>` operator

```
echo -n "Hello" > hello-world.txt
echo " World" >> hello-world.txt
```

`cat` print the conent of a file

```
cat hello-world.txt | wc -w
```

Move a file for a different directory with the `mv` command

```
mv hello-world.txt ~/Downloads
```

`cp` command for copy a file

The `man` provides the command documentation

```
man cat
```
To show only the first 20 lines:

```
man cat | head -n 20
```

Similarly, the `help` command returns the help documentation of the command (not always the `man` is available)

```
help cd | head -n 20
```