Q: How does a program start?

Q: How does a program start?

The jobs of a shell

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

Q: How does a program start?

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?

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

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

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

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

What makes firefox.desktop work?

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

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

What makes firefox.desktop work?

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

$ firefox
<Ctrl-C>
$ firefox &
$ jobs
$ fg
<Ctrl-Z>
$ bg
$ echo "hello" > test
$ cat test
$ true && echo "hello"
$ false && echo "nope" || echo "whaaaat?"

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

$ 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?

$ 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

  $ 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:

#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?)
}

$ ./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?

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

  $ 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

Closing remarks

• Try one of the Advanced Exercises

• Reminder: You must submit to staff at OH