Shell is a project focused on building a custom shell in C, similar to bash.

Early developers found communicating with computers via binary switches tedious and impractical. The creation of shells, which allow interactive command-line communication, marked a significant milestone, bringing us closer to human-like language interactions with machines.

Shell takes you on a journey back to the challenges faced before graphical user interfaces, allowing a deeper understanding of fundamental shell functionalities.

General Guidelines

• Prompt Display: Shows a prompt when waiting for a new command.
• Command History: Maintains a history of executed commands.
• Executable Search: Finds and launches the correct executable using the PATH variable or relative/absolute paths.
• Quote Handling: Properly manages unclosed quotes and special characters, excluding unnecessary ones like \ or ;.
• Single Quotes ’: Prevents interpretation of meta-characters within single quotes.
• **Double Quotes ": Prevents interpretation of meta-characters within double quotes except for $.

Redirections

• Input Redirection <: Redirects input from a file.
• Output Redirection >: Redirects output to a file.
• Here Document <<: Reads input until a specified delimiter is reached.
• Append Redirection >>: Redirects output in append mode.

Signals

• In interactive mode
  • ctrl-C: Displays a new prompt on a new line.
  • ctrl-D: Exits the shell.
  • ctrl-\: Does nothing, similar to bash behavior.

Builtins

• echo: Supports -n option.
• cd: Changes directory with relative or absolute paths.
• pwd: Prints the current working directory.
• export: Sets environment variables.
• unset: Unsets environment variables.
• env: Prints the environment.
• exit: Exits the shell.

Others

• Pipes |: Connects the output of one command to the input of the next.
• Environment Variables: Expands environment variables to their values.
• Exit Status $?: Expands to the exit status of the most recent foreground pipeline.

&& (AND operator):

• AND Operator &&: Executes the right-hand command only if the left-hand command succeeds.

|| (OR operator):

• OR Operator ||: Executes the right-hand command only if the left-hand command fails.

() (Subshells)

• Subshells (): Runs commands in a separate shell instance.

Example of using parentheses to create a subshell:

(export A="hi" && echo $A) && echo $A

In this case, the output will be

hi

The second line is empty because the export happened in a subshell.

* (Wildcard)

• Wildcard *: Matches any sequence of characters in filenames or paths.
• It can be used in commands like ls, cp, mv, rm, and others to perform operations on multiple files or directories that match a specific pattern.
• For example, *.txt matches all .txt files.