SKP - A unix based shell written in C.

Github Link

SKP is a Linux BASH like shell written in C. It implements all shell commands and supports pipelining. It also supports foreground and background processes

Requirements

1. gcc compiler to run C program.
2. unix based OS/Environment

How to Run?

1. Compile shell.c file using gcc shell.c -o shell command to compile.
2. Run ./shell to execute the shell.

Features of the shell

1. Many commands are inbuilt which are explained in the following section. All the other commands which are to be executed in foreground are implemented using execvp. So, all the commands available in Bash can be executed in SKP.

2. The prompt for taking the next command displays the username, hostname and the present working directory.

3. SKP provides option to run processes in background by appeding & to the end of your command.

4. Piping is provided and handled manually from scratch.

5. STATUS (success or failure) of the previous command is displayed along the prompt for the next command. :) of green color denotes success and :( of red color denotes failure.

6. SKP provides colorful display giving it a modern look. It also display different color for directories and files when ls command is executed.

Inbuilt Commands

1. clear
   

   • Clears the screen of the shell.

2. pwd
   

   • Prints the current working directory.
   • Uses the getcwd() system call.

3. cd [location]
   

   • Changes the current working directory to the mentioned directory.
   • Implemented using chdir() system call.

4. ls [-l] [Directory]
   

   • Lists all the files and directories in the mentioned directory/directories. If no parameters are passed, lists the contents of current directory.
   • -l flag lists the long format of ls, providing additional details such as permissions, owner, time of creation etc.
   • The flags and directories can be provided in any order.
   • The output is color coded to distinguish between directories and files.
   • Uses the opendir() and readdir system call

5. history [num]
   

   • Gives the num number of previous commands run. If num is not mentioned, prints entire history of commands.
   • The content is stored in history.txt. It is created when shell is started if it's not existing already.

6. env

   • Lists all environment variables present in the current session of the shell.

7. mkdir <path>/directoryname

   • Creates a new empty directory in the path given with the specified name.
   • Throws error if folder with same name already exist in that path.
   • Implemented using mkdir() system call.

8. rmdir <path>/directoryname

   • Removes the directory in the path given with the specified name.
   • Throws error if directory with the specified name doesn't exist in that path.
   • Implemented using rmdir() system call.

9. exit

   • Quits the terminal.
   • Use this command to ensure proper closing (killing all persisting background processes).
   • Ctrl + D provides the same functionality.
   • If the first command is exit, all other arguments are ignored and shell is killed.

Executional Details

1. Any command ending with & is treated as a background process the shell does not wait for its execution. If such a process requests terminal control, it will be automatically suspended in the background. The shell keeps track of all background processes and alerts the user on their completion.

2. | is used for piping of commands, i.e, stdout of one command serves as stdin for the next.
   Example: pwd | grep -o "/" | wc -l

3. Ctrl + D is an EOF character and terminates the shell.

Implementation Details

• piping
  Contains the implementation of piping using pipe system call and dup system call.

• background
  Implementation of background processes indentified by &. Creates a new child process using fork() and the shell does not wait for its completion.

• trim
  Trailing and leading whitespaces are trimmed from the commands. The quotes are also removed from the commands.

• history.txt
  This file contains all previously executed command. The file is opened in read and append mode to read from and write to it respectively. This ensures that history is preserved across sessions.

To Do

1. Arrow keys for command-recall
2. Tab for auto completion
3. stdout/stdin/stderr redirection using <and >.
4. Support multiple commands separted by ;.
5. Chaining of commands using logical operators like AND, OR,etc.
6. Aliasing
   

References

• Code Vault YT Playlist
• StackOverflow - for helping in fixing every error I got.
• Linux Man Page

Screenshots

• Start Screen
• ls command with color coding
• Error handling
• Piping (counts number of '/')