Description
| Objective |
| The objective of this part of the homework is to develop 1 C++ program to:
•     Understand internals of the bash shell •     Gain familiarity with fork and exec system calls •     Continue to build strength with I/O streams & string processing •     Review problem solving strategies. |
Develop a custom Linux shell
Background
Operating Systems essentially provide system calls to run programs, but an OS does not start running programs by itself. This task is delegated to another program, called a “shell”. The shell (in our case we are using bash shell) accepts inputs from the user and based on the user-input executes different commands. Note that shells can be graphical (one desktop on Linux is called Gnome shell) and permit users to double-click on an icon to indicate the program to run.
Homework requirements
This homework requires developing a textual shell program analogous to bash used in Linux. Note that the shell you will be developing will be rather simple when compared to bash, but will help you obtain a good understanding of how a shell works. Your shell must operate in the following manner:
Repeatedly prompt (via a simple “> “) and obtain a line of input (via std::getline) from the user and process each line of input in the following order:
- If the line is empty or the line begins with a pound (#) sign, ignore those lines.
- The 1st word in the line is assumed to be a command (case sensitive) and must be processed as:
- If the command is exit your shell must terminate
- If the command is SERIAL then the 2nd word is name of a file that contains the actual commands to be executed one at a time. The shell waits for each command to finish.
- If the command is PARALLEL then the 2nd word is name of a file that contains the actual commands to be executed. In this case, all the commands are first exec’d. Then wait for all the processes to finish in the same order they were listed.
- If the 1st word is not one of the above 3, then it is assumed to be the name of the program to be executed and rest of the words on the line are interpreted as a command-line arguments for the program.
Note: For every command run (including SERIAL and PARALLEL runs), the shell must print the command and command line arguments. Once the command has finished, the shell must print the exit code. See sample outputs for wording and spacing.
Tips & suggestions
- This homework deals with concepts from Exam #1. In prior semesters it was assigned before
Exam #1. However, this semester we are going a bit slow and this program is coming after Exam #1. Nevertheless, this homework helps solidify concepts from the course. Importantly, it is a good program to showcase on Git hub and build your resume.
- A lot of the code you need is already in lecture slides. So ensure you review some of the material on string processing, vector, fork, and exec before starting on the program.
- When calling exec don’t forget the nullptr at the end of command-line arguments
- Get base case working first. The string processing is trivial (almost all the code in slides) with std::quoted. Do not overcomplicate the string processing part.
- If you code the base case method to use generic I/O streams then processing commands from files becomes straightforward.
- Use a std::vector to hold PIDs of child processes so that you can call waitpid on each one when running them in parallel.
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Sample input and outputs
In the sample outputs below, the following convention is used:
- Text in bold are inputs (logically) typed-in by the user (↵ is for pressing enter key)
- Text in blue are additional outputs printed by your program
- Other text are outputs from various commands exec’d by your program
Base case (10 points)
| > # Lines starting with pound signs are to be ignored.↵
> echo “hello, world!” ↵ Running: echo hello, world! hello, world! Exit code: 0 > ↵ > head -2 /proc/cpuinfo↵ Running: head -2 /proc/cpuinfo processor    : 0 vendor_id    : GenuineIntel Exit code: 0 > ↵ > # A regular sleep test. ↵ > sleep 1↵ Running: sleep 1 Exit code: 0 > ↵ > # Finally exit out↵ > exit↵ |
SERIAL test (7 points)
| > echo “serial test take about 5 seconds”↵ Running: echo serial test take about 5 seconds serial test take about 5 seconds
Exit code: 0 > SERIAL simple.sh↵ Running: sleep 1 Exit code: 0 Running: sleep 1s Exit code: 0 Running: sleep 1.01 Exit code: 0 Running: sleep 0.99 Exit code: 0 Running: sleep 1s Exit code: 0 > exit↵ |
PARALLEL test (8 points)
> # echo “parallel test take about 1 second”↵
> PARALLEL simple.sh↵
Running: sleep 1
Running: sleep 1.01
Running: sleep 1s
Running: sleep 0.99
Running: sleep 1s
Exit code: 0
Exit code: 0
Exit code: 0
Exit code: 0
Exit code: 0
> exit↵
 Organization and structure verification
In order to earn the full 5 points in this category, the program should strive to reuse code as much as possible. Specifically, processing lines from files (in SERIAL or PARALLEL command) should reuse code from the method used for processing individual commands. This kind of code reuse will be an important expectation in your future jobs.
