# Answers to 'Pipes and Filters' Exercises

Exercise 1: Using wildcards
---
Answer: 3

Exercise 2: What does `sort -n` do?
---

`sort` with out `-n` will assume that what you are sorting are characters. 
When sorting characters, "10" comes before "2" because "1" come before "2". In
other words, you can think of sort as sorting on the first character and then
on the second character, etc. So "300000" would come before "8" because "3"
comes before "8".  With the `-n` flag, sort considers 10 a number rather than
a character string and sorts the numbers as you'd expect.


Exercise 3: What does `<` mean?
---
They both take in `mydata.dat` as input, but one command take it as an argument and the other takes it as standard input.

Exercise 4: What does >> mean?
---
`echo hello > testfile01.txt` will always overwrite `testfile01.txt` while 
`echo hello >> testfile02.txt` will create the file if it doesn't exist and
append to the file otherwise.

Exercise 5: Piping commands together
---
`wc -l * | sort -n | head -n 3`, this is option 4

Exercise 5: Why does `uniq` only remove adjacent duplicates?
---
Removing adjacent duplicates is completely in-line with the Unix philosophy:
build small programs that do things really well and combine them to do bigger
things. Combining `uniq` and `sort` will return only the unique fish in the
list.

Exercise 6: Pipe reading comprehension
---
Copy and paste the `animals.txt` data into `nano` and save it as `animals.txt` so you can try it out yourself. 

1. `cat animals.txt`: Prints the `animals.txt` contents to `heads` std in
2. `head -n 5`: Gets the first 5 animals
3. `tail -n 3`: Gets the last 3 animals of the first 5
4. `sort -r`: Reverse sorts the last 3 animals of the first 5
5. `> final.txt`: Outputs the results to the file `final.txt`

It should contain

```
2012-11-06,rabbit
2012-11-06,deer
2012-11-05,raccoon
```

Exercise 7: Pipe construction
---
`cut -d , -f 2 animals.txt | sort  | uniq`