smallsh - c shell

Custom bash-like shell written in C as the portfolio project for OSU's Operating Systems I class. Demonstrates proficiency in the use of the Unix process API, signal handling, and I/O redirection.
Grade received: 100

Features / Description

• Provides prompt for running commands using bash-like syntax
• Supports execution of shell scripts
• Handles blank lines and comments (defined as lines beginning with the # character)
• Provides variable expansion for any instance of $$ in a given command, replacing with the shell Process ID
• Provides 3 built-in commands: -exit: kills any processes / jobs started by shell before terminating shell itself -cd: provides bash-like functionality using the chdir() system call; supports both relative and absolute directory paths -status: prints either the exit status or the terminating signal of the last foreground process run the shell, excluding the 3 built-in commands
• Executes all other commands by creating new processes using the fork(), exec(), and waitpid() system calls
• Supports input and output redirection using the dup2() system call
• Supports execution of commands in both the foreground and background
• Implements custom signal handlers for the SIGINT and SIGTSTP signals

Functionality Demo

Installation

(requires gcc compiler)

• download or clone project source files, navigate to directory and run:

make

Usage

starting shell:

./smallsh

quitting shell:

• input the exit command

general syntax:

command [arg1 arg2 ...] [< input_file] [> output_file] [&]

restrictions / bash differences:

• command is assumed to be comprised of words separated by spaces
• special symbols <, >, and & are recognized, but must be surrounded by spaces like other words
• if standard input or output is to be redirected, the > or < words followed by a filename word must appear after all the arguments. input redirection can appear before or after output redirection
• the pipe operator, |, is not implemented
• a command line has max length of 2048 characters, and 512 max arguments
• lines beginning with the # character (comments) or blank lines are both ignored (re-prompt for a new command)
• any instance of $$ in a command is expanded into the process ID of the shell itself. this is the only variable expansion supported by the shell
• any command, with the exception of exit cd and status, can be run in either foreground or background
  • if command is to be executed in background, last word must be &

signal handling:

• SIGINT (CTRL-C)
  • ignored by shell itself, or any children running as background processes
  • any child process running in foreground terminates itself upon receiving SIGINT
• SIGTSTP (CTRL-Z)
  • ignored by a child process running in foreground, or any children running as background processes
  • if received by parent processes running shell, shell enters state in which commands cannot be run in background (& operator is ignored; all commands are run in foreground)
  • if received again, shell toggles back to normal mode (background processes enabled)