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 ```c++ #include <iostream> using namespace std; int main(int argc, char *argv[], char *envp[]) { cout << "argc: " << argc << endl; cout << "envp[0]: " << envp[0] << endl; // while (*envp++) { // Try this one, too! // cout << *envp << endl; // } return 0; } ``` ```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 ($ history) - 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 --- # Closing remarks - **Try one of the Advanced Exercises** - We're at the end of Segment 1 - Reminder: You must submit to staff at OH