ECS 150 – Project 1
Prof. Joël Porquet-Lupine
UC Davis – 2020/2021
Copyright © 2017-2021 Joël Porquet-Lupine – CC BY-NC-SA 4.0 International License /
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Shell, an introduction
What’s a shell?
User interface to the Operating System’s services
Gets input from user, interpret the input, and launch the desired action(s)
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Shell, an introduction
Some big names
Name
Comment
First released
Thompson shell
First Unix shell
1971
Bourne shell
Default shell for Unix 7
1977
Bash
Default on most Linux distributions
1989
Zsh
My favorite shell 😀 (now default on MacOS!)
1990
Fish
New kid in town — tries to be more user friendly than other shells
2005
Big and old pieces of software
Bash: 30 years and ~200,000 lines of code!
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Simple shell
Goal
Understand important UNIX system calls Implementing a simple shell called sshell.
Specifications
Execute commands with arguments
sshell@ucd$ date -u
Redirect standard output of command to file
sshell@ucd$ date -u > file
Pipe the output of commands to other commands
sshell@ucd$ cat /etc/passwd | grep root Offer a selection of builtin commands
And some extra feature(s)…
sshell@ucd$ cd directory sshell@ucd$ pwd /home/jporquet/directory
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Simple shell
Commands
sshell@ucd$ echo Hello world
Hello world
+ completed ‘echo Hello world’ [0] sshell@ucd$ sleep 5
+ completed ‘sleep 5’ [0]
1. Display prompt
2. Read command from input
Potentially composed of multiple arguments (up to 16 total) 3. Execute command
4. Wait for completion
5. Display information message
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Simple shell
Builtin commands
sshell@ucd$ pwd /home/jporquet/ecs150/ + completed ‘pwd’ [0] sshell@ucd$ cd ..
+ completed ‘cd ..’ [0] sshell@ucd$ pwd /home/jporquet/
+ completed ‘pwd’ [0] sshell@ucd$ exit Bye…
+ completed ‘exit’ [0]
Most commands are provided by external executables
/bin/echo, /bin/ls, etc.
Some commands have to be provided by the shell directory
cd, exit, etc.
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Simple shell
Standard output redirection: >
sshell@ucd$ echo Hello world>file
+ completed ‘echo Hello world>file’ [0] sshell@ucd$ cat file
Hello world
+ completed ‘cat file’ [0]
Output redirection means that the process’s output will be written to a file instead of to the terminal
Spacing shouldn’t matter
echo Hello world>file is equivalent to echo Hello world > file
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Simple shell
Pipeline of commands: |
sshell@ucd$ echo Hello world | grep Hello|wc -l
1
+ completed ‘echo Hello world | grep Hello|wc -l’ [0][0][0]
Interconnection of multiple commands into a job
Output of command before ‘|’ is redirected as the input of the command located right
after
Up to three pipes on the same command line
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Simple shell
Extra feature: variables
sshell@ucd$ set j toto
+ completed ‘set j toto’ [0] sshell@ucd$ set i $j
+ completed ‘set i $j’ [0] sshell@ucd$ echo $i
toto
+ completed ‘echo $i’ [0]
26 variables, from a to z
set to initialize or reset a variable $ to use variable
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General information
Project assignment
Project assignment was published this morning
Read assignments multiple times and follow the instructions
Suggested phases to help with your progression
Recommended to follow but you don’t have to Stay up-to-date
Extra information given during lecture or discussion Class announcements
Piazza
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General information
Group work
Teams of exactly two partners
Find a partner on Piazza (post @5)
Find a partner with whom you can work well
Define what kind of collaboration you’re looking for before pairing up How to meet? How regularly? Etc.
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General information
Deadline
Project is due by Friday, January 22nd, 2021, at 11:59pm No extension will be given
5% penalty for each hour late
Prorated by minutes
Start early, this project is considered to be intense!
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General information
Academic integrity
On your end
Projects are to be written from scratch Even if you already took (part of) this class
Projects are to be written in equal proportion by both partners Avoid using snippets of code you find online (e.g., stackoverflow)
Instead rewrite them yourself Cite your sources
On my end
Use of MOSS on all submissions and comparison with previous quarters If you find existing source code available online (e.g., Github)
Will most likely appear via MOSS!
Transfer to SJA in case of suspected misconduct
At best, fail the project
At worst, fail the class (and even get suspended or dismissed if not first offense)
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Git
Introduction
Version control system for tracking changes in computer files and coordinating work on those files among multiple people.
Unlimited private repositories on github and gitlab. Initial configuration
$ git config –global user.name “Firstname Lastname” $ git config –global user.email “name@ucdavis.edu”
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Git
How to start?
1. Create account and private repository online 2. Add partner as collaborator
3. Clone it locally
$ git clone git@github.com:nickname/ecs150-sshell.git && cd ecs150-sshell 4. Start coding
$ vim sshell.c
5. Commit and push
6. Your partner can now pull your commit
$ git pull
More resources
https://guides.github.com/activities/hello-world/ https://www.atlassian.com/git/tutorials
$ git add sshell.c
$ git commit -m “Initial commit” $ git push
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Git
Workflow
Commit often
One logical change per commit
Write meaningful commit messages
Study and understand what your partner commits
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Makefile
Intro
A Makefile is a file containing a set of rules
Represents the various steps to follow in order to build a program Building recipe
Used with the build automation tool make Anatomy of a rule
For target to be generated, the prerequisites must all exists (or be themselves generated if necessary)
is generated by executing the specified command
is generated only if it does not exist, or if one of the prerequisites is more recent
Prevents from building everything each time, but only what is necessary
target: [list of prerequisites]
[
target
target
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Makefile
Simple Makefile
# Generate executable
myprog: prog.o utils.o
gcc -Wall -Wextra -Werror -o myprog prog.o utils.o
# Generate objects files from C files
prog.o: prog.c utils.h
gcc -Wall -Wextra -Werror -c -o prog.o prog.c
utils.o: utils.c utils.h
gcc -Wall -Wextra -Werror -c -o utils.o utils.c
# Clean generated files
clean:
rm -f myprog prog.o utils.o
Adapt this code to your needs
(Could be actually a lot simpler than this: intermediate object generation is not necessary if only one C file)
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Best practices
Implementation quality
Writing some code that implements certain specs is not enough A good code is easy to understand, manipulate, and extend
Use the right data structures
If you’re using char *** in your code, that’s probably the wrong approach Split your functions the right way
Ideally, one function per logical functionality
Don’t over-complicate your design/code
Simple is always the best option
Don’t be scared to rewrite big chunks of your code at some point That’s how any large project works!
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Best practices
Coding style
Consistent
Don’t mix tab and spaces
Keep the same indentation (at least 4) Comment your code (with meaningful comments)
Example of poor comment: i++; // increment variable i Name your variable properly:
Example of poor variable name: int temp; Remove dead code
If your want to become a pro, start acting like one now: Don’t submit a draft!
Submit a program that works and is nice to read
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ECS 150 – Project 1
Prof. Joël Porquet-Lupine
UC Davis – 2020/2021
Copyright © 2017-2021 Joël Porquet-Lupine – CC BY-NC-SA 4.0 International License /
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Automatic grading
Overview
Automatic grading is ~60% (Partially) captures correctness
Does your code implement the given specifications?
Specs
Testing script
Execution
Complete grading test script has more than 20 test cases
Covers all the features, with multiple scenarios, error management, etc.
Partial test script will be published later today 7 test cases, one per main feature
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Automatic grading
Manual test cases
All the test cases can be reproduced manually
run_cmd_no_arg() { …
touch titi toto
sshell_test “ls\nexit\n”
…
local line_array=()
line_array+=(“$(select_line “${STDOUT}” “2”)”)
line_array+=(“$(select_line “${STDOUT}” “3”)”)
local corr_array=()
corr_array+=(“titi”)
corr_array+=(“toto”)
…
}
$ mkdir test && cd test
$ touch titi toto
$ echo -e “ls\nexit\n” | ../sshell sshell@ucd$ ls
titi toto
+ completed ‘ls’ [0]
sshell@ucd$ exit
Bye…
$ echo -e “ls\nexit\n” | ../sshell 2>/dev/null sshell@ucd$ ls
titi toto
sshell@ucd$ exit
$
Script
Advice
Make sure that you pass the given test script
Manual test case
Do it right away, before you continue adding any new features!
Don’t assume that the grading test script will work differently Re-read the assignment prompt entire and carefully
Reproduce all the examples shown Especially regarding error handling…
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Manual review
Overview
Manual review is ~40%
(Partially) captures quality of project
Various aspects related to real-life programming projects
Is the project well-packaged?
Is the code properly designed and implemented? Is the code well-explained?
Does the code follow good coding style?
Score breakdown
Easy points (~30%)
Submission : ~10% Makefile: ~10% Code style: ~10%
Harder points (~70%)
Report file: ~40%
Quality of implementation: ~30%
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Manual review
Submission AUTHORS.csv
File containing information about partners
Compilation
Successful compilation of project when typing make
Contents
Include only what’s necessary, nothing more
Source code (*.c, *.h)
Makefile
Report file
Optionally: custom testing script or other relevant files (Git specific files such as .gitignore are fine)
Example of clutter that will affect your submission grade
Core dumps, backup files, object or executable files, hidden macOS folders (.__MACOSX, .DS_Store), etc.
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Manual review
Makefile
gcc with -Wall -Wextra -Werror
And compile without any warnings or errors
Properly constructed rules
No use of implicit rules
Simple rules whenever possible
Inclusion of functional clean rule
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Manual review
Report
Rationale
Ability to explain code from a high-level point of view Important for many aspects of software development
In-team communication Out-group communication Problem solving
Help guides and documentation Software design and empathy Career value
https://www.computer.org/publications/tech-news/trends/why-every-developer-should- know-a-bit-of-technical-writing
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Manual review
Report
Dos and dont’s
Explain how you transformed the assignment into code
Not “What does your code do?”
But “How does it do it?”
Focus on how your program works at the time you submit it
Don’t go too much into the details of everything you tried to get there Mention the limitations if any
Don’t repeat the assignment
Don’t write a book and don’t explain every line of code…
See example documentation published for Project #0
Important reminder
The report file is 40% of the manual review grade (so ~15% of the total)
Take care of the report instead of wasting time adding another feature at the last minute!
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Manual review
Report
Formatting
Name of the file is REPORT.md (not README.md, nor REPORT.txt, etc.) Formatted in Markdown
Specifications: https://guides.github.com/features/mastering-markdown/
Nothing fancy necessary, but at least: Headers, Some text formatting (italic, bold), Lists,
Code blocks
Formatted in lines of 80 characters maximum
Good formatting 🙂
Poor formatting 🙁
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Manual review
Coding style
Go through your code starting from the beginning
Reformat all the code with the same indentation (spaces vs tabs) Rename variables or functions that are poorly named
Space out your code so that it looks nice to read
Etc.
Quality of implementation
Think about your overall design
Make it as simple and straightforward as possible Use of proper data structures
Don’t over use arrays, they’re not the solution to everything
Split code into appropriate functions Avoid memory leaks
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Manual review
Submit production code
int main( void) { func1(a, b , c);
func2(a,b,c);
// printf(“Back from func2\n”);
// Still need to investigate func3()
func3();
/* Also need to make a haircut appt */
// Return value 0
return 0; }
int main(void)
{
/* Parse the command line */
func1(a, b, c);
func2(a, b, c);
/* Run requested command(s) */
func3();
return 0; }
myexec: mycode.c
gcc -g -Wall -Werror …
What happens in development “stays” in development
Production code (i.e., your submission) is a final product, not a draft Remove dead code and debugging comments
Compile for performance, not debug
myexec: mycode.c
gcc -O2 -Wall -Werror …
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Coding tips
Fixed values (1)
int main(void)
{
char cmdline[512];
fgets(cmdline, 512, stdin);
…
return 0; }
#define CMDLINE_MAX 512
int main(void)
{
char cmdline[CMDLINE_MAX];
fgets(cmdline, CMDLINE_MAX, stdin);
…
return 0; }
Avoid hardcoded values and prefer named macros Definition in one location, easy to update
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Coding tips
Fixed values (2)
void error_message(int error_code)
{
switch(error_code) {
case 0: fprintf(stderr,
“Error: missing command\n”); break;
case 1: fprintf(stderr,
“Error: command not found\n”); break;
case 2: …
} }
void func(void)
{
…
error_message(1);
… }
enum { ERR_MISSING_CMD, ERR_CMD_NOTFOUND, …
};
void error_message(int error_code)
{
switch(error_code) {
case ERR_MISSING_CMD: fprintf(stderr,
“Error: missing command\n”); break;
case ERR_CMD_NOTFOUND: fprintf(stderr,
“Error: command not found\n”); break;
… }
}
void func(void)
{
…
error_message(ERR_CMD_NOTFOUND);
… }
Again, avoid hardcoded values Use generic constructs
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Coding tips
Error checking
int myfunc(char *buffer, int len)
{
int i;
if (buffer) {
if (len >= 0) {
/* Process buffer */
for (i = 0; i < len; i++) ...
return 0; }
}
return -1; }
int myfunc(char *buffer, int len)
{
int i;
/* Error checking */
if (!buffer || len < 0) return -1;
/* Process buffer */
for (i = 0; i < len; i++) ...
return 0; }
Avoid deep nesting of code
Process error cases first, then proceed with actual code, at the same level
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Coding tips
Function order
#include
int func1(); char func2(int); int func3(char);
int main(void)
{
func3();
}
int func1()
{
func2();
func3(); }
char func2(int)
{
func3();
}
int func3(char) {
}
#include
int func3(char) {
}
char func2(int)
{
func3();
}
int func1(void)
{
func2();
func3(); }
int main(void)
{
func3();
}
Popular design pattern in big C project, such as the Linux kernel Less code, less prototypes to maintain
With a proper code editor function navigation is easy
Look into ctags or cscope (and interface with vim or emacs)
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Coding tips
Functions
#include
int main(void)
{
…
/* 2. Then we do that */
…
/* 3. Then it depends */
if (value == 42) {
/* 3a. Some code using 42 */
} else {
/* 3b. Same code not using 42 */
}
return 0; }
/* 1. First we do this */
#include
int func1(char) {
}
char func2(int) {
}
int func3(int param) {
}
int main(void)
{
func1();
func2();
func3(value);
}
Split your code into tinier chunks
If a function starts doing multiple things, time to break it down When possible, write generic and parameterizable functions
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Coding style tips
Editor configuration
What you might see in your code editor: What I see in mine:
struct linked_list {
struct linked_list_node *head;
int size; };
Ensure visual consistency between code editors
Spaces vs tabs
Settle for one convention with your partner, and configure your editors accordingly Remove unnecessary spaces
Take actual space in the source code
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