Unix-shell

This program acts as a command interpreter, designed and implemented in C within the Unix operating system environment.

Key Functionalities

• Utilizes standard input (file descriptor 0) for reading and interpreting command lines.
• Utilizes standard output (file descriptor 1) to display results of internal commands.
• Uses standard error (file descriptor 2) to present special variables such as "prompt" and "bgpid," as well as for error notifications.
• Manages errors through the perror library function in the event of system call failures.

Components and Concepts

• Command: A sequence of texts separated by whitespace, where the first text specifies the command to execute, and subsequent texts represent its arguments.
• Sequence: A series of two or more commands separated by the | character. The standard output of each command is connected via a pipe to the standard input of the next command in the sequence.
• Redirection: The input and/or output of a command or sequence can be redirected using < and > notations.

Additional Features

• Background Execution: Commands or sequences terminated with & are executed asynchronously in the background, updating the bgpid variable.
• Signal Handling: Ensures that neither the "minishell" nor background-launched commands terminate due to keyboard-generated signals.
• Internal Commands: Includes internal commands like cd, umask, limit, and set, executed either directly or as complements provided by the "minishell."
• Filename Expansion: Text containing specific characters (e.g., ~ or $) undergoes special character substitution before execution, expanding to filenames that match the specified patterns.

Name Expansion and Constraints

Furthermore, the program features name expansion for filenames, expanding to lists of matching filenames based on specified patterns. This applies to characters such as ? and adheres to certain restrictions, particularly when combined with the / character.

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Note: The following files are not included, as they were created by the professor of the "Operating Systems" course:

• Makefile: Facilitates automatic recompilation of modified source files.
• scanner.l: Automatically generates C code for a lexical analyzer (scanner) capable of recognizing the TXT token, considering possible separators (\t␣|<>&\n).
• parser.y: Automatically generates C code for a grammar analyzer (parser) capable of recognizing valid statements based on the input grammar of the minishell.