smallsh

smallsh is a mini shell written in C. It implements a subset of features of well-known shells, such as bash. It handles variable expansion, comments, blank lines, input and output redirection, foreground and background processes, and more.

Table of Contents

1. About The Project
   • Program Functionality
   • Sample Execution
   • Built With
2. Getting Started
   • Prerequisites
   • Installation
3. License
4. Contact
5. Acknowledgments

About The Project

Program Functionality

1. The Command Prompt
   • Format:

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

   • Use & at the end of a command to execute it in the background
   • Use > and < for standard input or output redirection
   • Maximum line length is 2048 characters with a maximum of 512 arguments
2. Comments and Blank Lines
   • Lines that begin with # are treated as comments and are ignored
   • Blank lines (one without any commands) are ignored
   • The shell will re-prompt for another command if it receives a blank line or comment line
3. Expansion of the Variable $$
   • The variable $$ will be expanded into the process ID of smallsh itself
4. Built-in Commands
   • Three built-in commands are supported: exit, cd, and status
   • All other commands are passed on to a member of the exec() family of functions
5. Executing Other Commands
   • Non-built-in commands will be forked off into a child process
   • Non-built-in commands must be located in the PATH variable in order for smallsh to execute them
6. Input and Output Redirection
   • Input and output redirection uses dup2()
   • Input files redirected via stdin are opened for reading only
   • Output files redirected via stdout are opened for writing only
   • stdin and stdout for a command can be redirected at the same time
7. Executing Commands in Foreground and Background
   • Commands without & at the end will run in the foreground
   • Commands with & at the end will run in the background (except built-in commands which will run in the foreground regardless of an & at the end of the command)
   • Background commands will print a process ID of the background process when it begins and a message with the process ID and exit status when the process terminates
8. Signals SIGINT and SIGTSTP - A Ctrl-C command from the keyboard will send a SIGINT signal to smallsh - A child process running in the foreground will terminate itself when it receives a SIGINT - smallsh itself will ignore SIGINT - A Ctrl-Z command from the keyboard will send a SIGTSTP signal to smallsh - Subsequent commands will no longer run in the background - & operator will be ignored - A subsequent SIGTSTP signal will return smallsh to its normal condition

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Sample Execution

$ smallsh
: ls
junk   smallsh    smallsh.c
: ls > junk
: status
exit value 0
: cat junk
junk
smallsh
smallsh.c
: wc < junk > junk2
: wc < junk
       3       3      23
: test -f badfile
: status
exit value 1
: wc < badfile
cannot open badfile for input
: status
exit value 1
: badfile
badfile: no such file or directory
: sleep 5
^Cterminated by signal 2
: status &
terminated by signal 2
: sleep 15 &
background pid is 4923
: ps
  PID TTY          TIME CMD
 4923 pts/0    00:00:00 sleep
 4564 pts/0    00:00:03 bash
 4867 pts/0    00:01:32 smallsh
 4927 pts/0    00:00:00 ps
:
: # that was a blank command line, this is a comment line
:
background pid 4923 is done: exit value 0
: # the background sleep finally finished
: sleep 30 &
background pid is 4941
: kill -15 4941
background pid 4941 is done: terminated by signal 15
: pwd
/nfs/stak/users/chaudhrn/CS344/prog3
: cd
: pwd
/nfs/stak/users/chaudhrn
: cd CS344
: pwd
/nfs/stak/users/chaudhrn/CS344
: echo 4867
4867
: echo $$
4867
: ^C^Z
Entering foreground-only mode (& is now ignored)
: date
 Mon Jan  2 11:24:33 PST 2017
: sleep 5 &
: date
 Mon Jan  2 11:24:38 PST 2017
: ^Z
Exiting foreground-only mode
: date
 Mon Jan  2 11:24:39 PST 2017
: sleep 5 &
background pid is 4963
: date
 Mon Jan 2 11:24:39 PST 2017
: exit
$

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Built With

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Getting Started

Prerequisites

• GCC Compiler: https://gcc.gnu.org/

Installation

1. Clone the repo

   git clone https://github.com/cjsidler/smallsh.git

2. Build an executable

   gcc -std=gnu99 main.c -o smallsh

3. Run the executable.

   .\smallsh

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License

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Contact

Contact me via email, LinkedIn, or GitHub:

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Acknowledgments

• README Template
• Link Badges

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