Gҽɳҽടιട

A simple shell written in C

Mini Project 1 - Operating Systems and Networks, Monsoon 2023

Building and running

Run make in the root directory to build. Run ./genesis to run the shell.

Navigating the directory structure

• src folder contains all the source code of the project which are mainly .c files
• include folder contains all the header files associated with the format .h.
• /include/libs.h is the main header file which imports all the other custom and system header files.

.
├── genesis
├── include
│   ├── activities.h
│   ├── buffer_sizes.h
│   ├── color.h
│   ├── error_handling.h
│   ├── execute.h
│   ├── fg_bg.h
│   ├── genesis-stdlib.h
│   ├── iman.h
│   ├── jobs.h
│   ├── libs.h
│   ├── neonate.h
│   ├── pastevents.h
│   ├── peek.h
│   ├── ping.h
│   ├── proclore.h
│   ├── raw_mode.h
│   ├── read_parse.h
│   ├── seek.h
│   ├── shell.h
│   ├── signal.h
│   └── warp.h
├── makefile
├── README.md
└── src
    ├── activities.c
    ├── error_handling.c
    ├── execute.c
    ├── fg_bg.c
    ├── genesis-stdlib.c
    ├── iman.c
    ├── jobs.c
    ├── main.c
    ├── neonate.c
    ├── pastevents.c
    ├── peek.c
    ├── ping.c
    ├── proclore.c
    ├── raw_mode.c
    ├── read_parse.c
    ├── seek.c
    ├── shell.c
    ├── signal.c
    └── warp.c

Usage

The working directory of the shell is the directory where it was executed from. Commands can be chained with ; or &. ; runs the previous command in foreground. & runs the previous command in background.

Compatibility

• sigabbrev_np and sigdescr_np are used which may not be available on all systems

Commands overview

Commands	Forked or not	Input Redirection	Output Redirection	Piping status
activities	yes	no	yes	cannot read from pipe
proclore	yes	no	yes	cannot read from pipe
bg	no	no	no	not piped
fg	no	no	no	not piped
iMan	yes	no	yes	cannot read from pipe
neonate	yes	no	yes	cannot read from pipe
ping	yes	no	yes	cannot read from pipe
warp	no	no	yes	cannot read from pipe
peek	yes	no	yes	cannot read from pipe
seek	no	no	yes	cannot read from pipe
pastevents	yes	no	yes	cannot read from pipe
pastevents purge	yes	no	yes (but no output to redirect)	cannot read from pipe
builtins	yes	yes	yes	piped

Pipe and Redirection

• prioritizing piping over redirection [bash has a proper grammar and handles things differently]
• warp(), fg() and bg() wont work with piping

Inbuilt functions

• sed will work with piping given you don't include the replace logic in quotes (this probably happens because)

echo "Hello there" | sed 's/hello/hi/' | sed 's/there/OSN/'

The above command will not work and output an error message but the below command will give a valid output.

echo "hello there" | sed s/hello/hi/ | sed s/there/OSN/
"hi OSN" # output 

Custom functions

pastevents

• commands stored in history are properly formatted with spaces. The raw string is not stored.
• regardless of a commands' validity, it will be stored in history
• history will never store the command line in history if there is even a single pastevents or pastevents purge, irrespective of the remaining command. Example:

pastevents | pastevents execute 7 | wc > output.txt

The above command won't be stored in history.

pastevents execute 7 | wc > output.txt

The above command will be stored in history after replacing pastevents execute 7 with the 7th command stored in history

proclore

• Earlier, I was forking proclore to run it but it never showed the parent shell as $R+$ but as $S/S+$ since at the moment of execution, the shell is not in foreground but the child process is. This does not mean that the implementation is wrong. But due to my implementation of piping and all, I decided to keep this so I could do piping with proclore.

warp

• will work with output redirection

iMan

• in iMan , grep is very large so can't be malloc that much space. This is my output of realloc() again and again

iMan grep
# output: cur_size is the realloc size() called
cur_size = 2048
cur_size = 4096
cur_size = 6144
cur_size = 8192
cur_size = 10240
cur_size = 12288
cur_size = 14336
cur_size = 16384
cur_size = 18432
realloc(): invalid next size

what I am doing is reading until a max size and displaying whatever information I fetched. This is to keep everything simple.

Some assumptions

• getenv() and setenv() have been used to set HOME , PWD and OLDPWD environment variables. None others have been utilized.
• to keep everything atomic, the changes are not made immediately but in the next iteration of while loop

Things to keep in mind while giving input

• ; and & can be chained in similar ways to bash

Below are a few bash specific things

• ctrl-d does not work while sleep is in foreground
• ctrl-d , ctrl-z are disabled while taking input
• ctrl-c works just fine while taking input. It moves to a newline

Some Resources

Raw mode and Terminal attributes

• Really good resource
• KEEP IN MIND : when implementing ctrl-c or anything similar, make sure in your raw mode, you disable them using raw.c_lflag &= ~(ECHO | ICANON | ISIG); and install signal handlers separately

GPT Usage

• all the code which required sorting or formatted printing, then GPT was used
• most of the code has been written by referring to either some books or man pages, specifically
  • Books
    • CSAPP
    • Unix Programming
  • Man pages
    • GNU C Library