Week3

class: center, middle

# Shells, Environment, Scripting, and Bash
## (in 80 minutes[!])

.copyright[
<a rel="license" href="http://creativecommons.org/licenses/by/4.0/"><img alt="Creative Commons License" style="border-width:0" src="https://i.creativecommons.org/l/by/4.0/88x31.png" /></a>
** [Pat Pannuto](http://patpannuto.com) / Marcus Darden **
]

---

# Q: How does a program start?

???

Lots of answers to this
 - "double click" something
 - spotlight something (or magic ubuntu menu that's the same)
 - type it into a terminal

## The jobs of a _shell_

- Spawn (launch) new programs
 - Handle input and output to programs
 - Kill and clean up old programs

### What shells have you used?

???

- All the text ones: bash, csh, zsh, fish, (I'm sure there are more)
 - KEY POINT: The desktop is also a "shell", it does all these jobs!
              Indeed, the desktop in Gnome is called the "unity shell"

---

# Let's poke around how the [Desktop] shell works

```bash
$ cp /usr/share/applications/firefox.desktop ~/Desktop/
$ chmod +x ~/Desktop/firefox.desktop
```

## What makes `firefox.desktop` work?

???

- Change "Name=..."
 - What's the point of Name[ar] and friends?
 - What might "Keywords=..." do?
 --> click ubuntu icon in top corner, try typing each keyword, what's first?
 --> can't change this as easily, doesn't read from files on ~/Desktop/
 - What about this "Exec=..."
 --> Exec=gnome-calculator

## How does the [desktop] shell:

- Spawn (launch) new programs
 - Handle input and output to programs
 - Kill and clean up old programs

---

# Let's poke around how the [bash] shell works

```bash
$ firefox
<Ctrl-C>
$ firefox &
$ jobs
$ fg
<Ctrl-Z>
$ bg

$ echo "hello" > test
$ cat test

$ true && echo "hello"
$ false && echo "nope" || echo "whaaaat?"
```
--

## How does the [bash] shell:

- Spawn (launch) new programs
 - Handle input and output to programs
 - Kill and clean up old programs

---

# Where's `firefox` anyway?

```bash
$ firefox              # This works
$ gcc hello.c -o hello # This works
$ hello                # This doesn't
$ ./hello              # This works
```

## Your _environment_ affects program behavior

- Even shells! (they're a program too)

---

# Changing the environment will change program behavior

- In this case, how a shell performs the search for programs
```bash
$ PATH=$PATH:/home/username/    # Assuming "hello" is in this folder
$ hello                         # Now this works!
$ PATH=/home/username           # What if you'd done this instead?
```
  - Also saw a brief example of environment variables in last week's homework

---

# Your programs can use the environment too

## Exercise for your own time:

```c
#include <stdio.h>
int main(int argc, char **argv, char **envp) {
  printf("argc: %d\n", argc);
  printf("envp[0]: %s\n", envp[0]);
  // while (*envp++ != NULL) {          // Try me too!
  //   printf("%s\n", *envp);           // Don't uncomment that
  // }                                  // first printf... (why?)
}
```

```bash
$ ./a.out
$ HELLO=world ./a.out
$ lower=fine many=okaytoo ./a.out
$ export IamPermanent=ish
$ ./a.out
$ # Try uncommenting the while loop, did you find the missing ones?
$ ./a.out | less      # This may explain some of the funny colors
```

---

# Now what about _scripting_?

## Surprise! You've been scripting this whole time!

- Typing commands into the bash shell and running a bash script are _the same_
```bash
$ cat test.sh
echo "hello" > test
cat test
true && echo "hello"
false && echo "nope" || echo "whaaaat?"
$ chmod +x test.sh # What is this doing?
$ ./test.sh
```

- How to write a bash script?

- Try things out in the terminal
    - Copy things that work into a file
    - Run that file
    - Repeat

---

# Bash is old...

## But useful, especially for really short things

## But has ugly and finicky syntax

- `VARIABLE=test` != `VARIABLE = test` :(

## But running programs is really easy

- (it's what it was built for after all)
  - `g++ -O3 -m32 thread.o libinterrupt.a test1.cpp -ldl -o test1`
  - `./test1`

## But doing much more is tricky

- Validate program output (`diff`?), what if it varies?
  - Rule of thumb: More than 50-100 lines, more than a shell script

---

# Live Python exercises

`git clone https://gitlab.eecs.umich.edu/c4cs/rpn`

???

## Anatomy of a Python script

- Syntax
   - #comments
   - if: def:
   - whitespace

- Fundamental data types
   - numbers
   - strings
   - (), [], {}

- Dynamic Typing

- Walk through how it works -- add prints, show off raise, make mistakes

- Improvements to make:

- Division support
      o Show off help(operator)

- Add `except EOFError` to main while loop (ctrl-d support)

### Scripts vs modules

>>> import rpn_basic as rpn
    >>> rpn.calculate("1 1 +")
    2.0

---

# Closing remarks

- **Try one of the Advanced Exercises**

- Reminder: You must submit to staff at OH