@@ -116,10 +116,14 @@ First let's just clear out something about **standard streams**. Standard stream
### Stdin
Stdin is the data that gets inputed into the program. An example of a program that requires stdin data is `cp`. In order for the program to do anything it needs input data. For example `cp file1 copy_of_file1`. Here `file1` and `copy_of_file1` is the stdin.
The file descriptor for **stdin** is: **0**
### Stdout
Stdout is the data that get ouputed from the program.
For example, when you use the command `cat file1` that data/text that gets outputed is the stdout. Not all programs have stdout. For example when you use `mv` or `cp` successfully you get no stdout back from the program.
The file descriptor for **stdout** is: **1**
### Stderr
Stderr is the stream used for outputting error messages. So if a program fails for whatever reason. For example, if we try to copy a file that does not exist, this will be the stdrr output:
cp: cannot stat 'thisfiledoesnotexist': No such file or directory
```
This is a common way for stderr to present itself, just straight out into the terminal. But sometimes stderr gets sent to a log file.
Stderr is useful because with it we can separate between **stdout** and **stderr**. However, to the eye it might be difficult to distinguish what output is **stdout** and what output is **stderr**.
One easy way to determine is the output is **stderr** or **stdout** is to simply redirect it into a file. Because by default you only redirect **stdout**, and not **stderr**.
```bash
cp thisfiledoesnotexist aaaaaaaaaa > result.txt
cp: cannot stat'thisfiledoesnotexist': No such file or directory
# If we now look at result.txt we will find that it is empty. Since the error-text we recieved could not be redirected into the textfile, since it is stderr and not stdout.
