Linux Shell in C

Make Instructions

In the directory containing the source code, execute:

$ make
$ ./a.out

Functionality

Prompt

Prompt is displayed in the same format as in BASH with username, system name and current working directory. Home directory ("~") is considered as the directory from which the shell a.out was executed.
Errors while getting the current working directory (home directory), username or system name leads to termination of the shell.

Input : multiple commands can be given separated by ;. Extra whitespaces given in commands or arguments is ignored.

Error checking : If any command fails to execute due to mulitple reasons, the relevant error message will be printed to the terminal by use of perror and in some cases without perror. That command will be skipped and shell will continue to run.

Commands

pwd - prints the absolute path of the current working directory.
echo - prints whatever is given as argument as it is
cd - changes the cwd to the mentioned directory, if no argument, changes to the home (~) directory.
- arguments: ., .., -, ~, any relative/absolute path to a directory
history
- by default prints the last 10 typed commands of shell, or if lesser than 10 available, then print them.
- with flag num. Prints last num commands from history. 1 < num < 21.
- Maximum of 20 commands are stored in the history.
- On closing the shell by using CTRL+D or exit or quit, the history is saved to a file .shell_history in the home directory. History is loaded from the same file for other shell sessions.
- If the file .shell_history is not formed yet, or is unreadable, it is ignored. Errors are generated if writing to this file is problematic.
- Up Arrow - on pressing the up arrow, recent commands can be seen in the order newer to older.
- Down Arrow - on pressing the down arrow, recent commands can be seen in the order older to newer.
pinfo - displays the details of a process with pid passed as argument.
- If no argument passed, the pid of the running shell is considered.
Other commands from Bash - executed with the help of execvp command.
jobs - List all the background processes of the shell, with Job_Num, status (either Running or Stopped), Name, PID.
- The result is sorted albhabetically by the Name of the command, and then by the Job Num.
- The Job Num is alloted by the shell and changed dynamically as the processes are killed/terminated from the background.
- The Job Num is given sequentially, a process starting earlier will have lower Job Num.
bg - Takes the argument job_num which is made by the shell. It continues any stopped process which is in the background, if its already running, it will be ignored.

Processes and Other functionalities

Foreground process - execution of process that happen in the foreground. These take away control from shell and must be finished/terminated to regain control of the shell.
Background process - any command passed with & flag is treated as background process. The user can use the shell normally without waiting for completion of command. The pid of child process is printed to terminal when command is executed.
Finishing of Background process - a message is printed whenever a background process is completed/terminated, expressing whether is was normal or abnormal termination.
Piping - The shell supports | piping, which transfers the input and output from one command to another instead of the terminal and thus allows interprocess communication. Implemented using dup and dup2.
Signal Handling-
- CTRL+Z - converts any foreground process to background and stops it. Signal used - SIGTSTP. no effect on the shell if no foreground process running. Implemented using signal() function.
- CTRL+C - Interrupts any foreground process. Signal used - SIGINT. No effect on the shell if no foreground process running. Implemented using signal() function.
- CTRL+D - Closes this shell, but not the actual terminal of the computer. Implemented by considering the EOF (end of file) ASCII character.

File Structure

Files	Functionality
cd.c	`cd` command
echo.c	`echo` command
history.c	`history` command
pinfo.c	`pinfo` command
pwd.c	`pwd` command
jobs.c	`jobs` command
sig.c	`sig` command
bg.c	`bg` command
shell_commands.c	Executing foreground/background processes, and the commands which were not implemented exclusively for this shell
piping.c	Implmentation of Piping feature of the shell
arrow.c	For taking input from the shell through a lower level code
commands.h	Header file for declarations of all the commands related .c files
prompt.c	Prompt functionality in shell
utils.c	Various utility functions
utils.h	Header File for declaring functions in utils.c
main.c	The main file containing the `main` function for running the shell
headers.h	Header file for declaring all global variables and including other .h files
makefile	make instructions for compiling the code