NYASH

NYASH: Not Your Average Shell - A simple shell written in C for Linux systems

Usage:

Run the following commands

make
./nyash

Features:

`;` separated commands

Concatenate multiple commands in a single line by separating them with a semicolon ;

`&` terminated background processes

Run processes in background by appending an ampersand & at the end of the command
NYASH displays the pid of such processes before executing them in the background
Child handling: NYASH displays the exit status of such processes when they are terminated, suspended or continued.

File redirection

< [FILE] can be used to redirect input from the specified [FILE] for a command
> [FILE] and >> [FILE] can be used to redirect the output to the specified [FILE] for a command. > Overwrites the output file and >> appends to the output file
Both input and output redirections can be used on a single command
- Example: cat < input.txt > output.txt

Piping commands

<command> | <command> can be used to pipe the output of the command on the left side of the pipe to the command on the right side of the pipe
Two or more commands can be chained together by piping
Piping can be used alongside input and output redirections
- Example: cat < in.txt | wc -l > lines.txt
Note that in case of a conflict between taking input/output from the pipe or through a redirection, the redirection is given higher preference
- Example: cat in.txt | sort > out.txt | wc -l
- This shall output 0 since the sort command redirects the output to out.txt and not the pipe

Signals

<ctrl>C or SIGINT does not kill NYASH
<ctrl>Z or SIGTSTP does not kill NYASH
<ctrl>D or an EOF safely logs out of the shell (functionally equivalent to the exit command)

Prompt

NYASH comes with a sleek prompt displaying the username and the hostname of the machine, and the current working directory

cd - Change current working directory

Usage: cd [DIR]

Here, [DIR] can take on any of the following values
- ~ Changes directory to the home directory i.e. the directory from which the shell was executed
- - Changes directory to the previous working directory
- Any relative path from the home directory is valid
- Any absolute path (i.e. from the root directory) is valid
- Passing no arguments changes directory to the home directory
- Note that these are valid as long as the user has the appropriate permissions for the directory passed

pwd - Display current working directory

Usage: pwd

echo - Echoes the passed string into terminal

Usage: echo [STRING]

Note that quotes and escape characters are not handled. The passed string is tokenized based on whitespace before echoing.

ls - List files/folders and their information from one or multiple directories

Usage: ls [OPTIONS]... [FILE]...

Here, [OPTIONS] can take any of the following values
- Specifying no options prints only the names of the non-hidden files/folders from the FILEs passed
- -a (all) Behaves similar to no options but additionally prints hidden files
- -l (long format) Prints additional info such as permissions, date modified, size for each non-hidden file
- Note that multiple options can be used at the same time
The [FILE] argument can be
- A relative or absolute path to a file or a folder
- Note that the user is required to have appropriate permissions to read the details/contents of the file/folder

pinfo - Prints the process related info of a shell program

Usage: pinfo [pid]

Displays the pid, process status, memory consumption and executable path for the specified process
Passing a valid pid displays the info related to the specified pid
If no arguments are passed, instead displays the info related to the current NYASH process

repeat - Repeats a command specified number of times

Usage: repeat [N >= 0] [command]

history - Displays a specific number of most recently used commands

Usage: history [0 <= N <= 20]

If no argument is passed, displays the 10 most recently used commands
This data is stored in /home/user/.nyash_history to allow for persistent usage, deleting which may lead to loss of all history data

jobs - Displays the processes spawned by the shell which are either running or suspended along with a unique job number

Usage: jobs [OPTIONS]...

Here [OPTIONS] can take any of the following values
- Specifying no options prints all spawned processes which are either running or stopped
- -r prints only the currently running spawned processes
- -s prints only the currently stopped spawned processes
NYASH assigns a positive integer which is greater than all the current job numbers in use as the job number to a spawned process. Hence, multiple processes may have the same job number overall (due to the counter resetting by emptying the job list), but they will not share the same job number simultaneously. This feature was implemented in this way to avoid large job numbers through usage of the shell for a longer period.

sig - Sends a specified signal to the process with the specified job number

Usage: sig [job_number] [signal_number]

The job_number can be any of the job numbers listed using the command jobs
The signal_number can be any of the signals listed under the manpage signal(7)

fg - Brings the running or stopped process specified by the job number to the foreground

Usage: fg [job_number]

The job_number can be any of the job numbers listed using the command jobs

bg - Sends the running or stopped process specified by the job number to the background

Usage: bg [job_number]

The job_number can be any of the job numbers listed using the command jobs

replay - Executes a given command and repeats execution every specified interval of time for a total of a specified time period

Usage: replay [OPTIONS]...

Here, the [OPTIONS] must include exactly one instance of the following
- -i (or alternatively, --interval and -interval) Followed by a positive integer [N > 0] which indicates the frequency of the replay command
- -p (or alternatively, --period and -period) Followed by a positive integer [N > 0] which indicates the total time period for which the replay command can be executed
- -c (or alternatively, --command and -command) Followed by the command to be replayed <command>
An additional feature of the replay command is that, it can be suspended via sending a signal <ctrl>Z like any other system command and it is displayed as a job via the jobs command

baywatch - Prints a specific system data periodically for a specified period

Usage: baywatch [OPTION] <command>

Here, the [OPTION] must include exactly one instance of
- -n followed by a positive integer [N > 0] which specifies the interval for printing the command output
Here, the <command> must be exactly one among the following
- interrupt which prints the number of times the CPU(s) has been interrupted by the keyboard-controller
- newborn which prints the PID of the most recently created process on the system
- dirty which prints the size of the part of the memory which is dirty
A keypress of q stops execution of the command

exit: Exits out of the shell

File organization

The main function is contained in main.c
A few global variables used across multiple files are declared as extern variables in the file globals.h
The commands folder contains the .c and .h files related to the commands implemented in NYASH named appropriately as the command which they are used in implementing
The processor folder contains the .c and .h files related to the child signal handling, prompt feature and execution of system commands in foreground and background
The utils folder contains the .c and .h files implementing various data-structres and functionality that are used throughout various other files
- Including a vector of strings, linked list of strings, linked list of job struct and a tokenizer

| Hope you have an above average experience! :D

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