README

• This project was completed for an Operating Systems course at Oregon State University as a Computer Science undergraduate student.

• Note: the "Overview" and "Program Functionality" portions were taken from the written project guidelines that students were provided in the course. Feel free to skip to the "Execution Instructions" section to see how to run the program.

1. Overview
2. Program Functionality
3. Execution Instructions

In this assignment you will write smallsh your own shell in C. smallsh will implement a subset of features of well-known shells, such as bash. Your program will

• Provide a prompt for running commands
• Handle blank lines and comments, which are lines beginning with the # character
• Provide expansion for the variable $$
• Execute 3 commands exit, cd, and status via code built into the shell
• Execute other commands by creating new processes using a function from the exec family of functions
• Support input and output redirection
• Support running commands in foreground and background processes
• Implement custom handlers for 2 signals, SIGINT and SIGTSTP

Use the colon : symbol as a prompt for each command line.

The general syntax of a command line is:

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

…where items in square brackets are optional.

• You can assume that a command is made up of words separated by spaces.
• The special symbols <, > and & are recognized, but they must be surrounded by spaces like other words.
• If the command is to be executed in the background, the last word must be &. If the & character appears anywhere else, just treat it as normal text.
• 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.
• Your shell does not need to support any quoting; so arguments with spaces inside them are not possible. We are also not implementing the pipe "|" operator.
• Your shell must support command lines with a maximum length of 2048 characters, and a maximum of 512 arguments.
• You do not need to do any error checking on the syntax of the command line.

Your shell should allow blank lines and comments.

• Any line that begins with the # character is a comment line and should be ignored. Mid-line comments, such as the C-style //, will not be supported.
• A blank line (one without any commands) should also do nothing.
• Your shell should just re-prompt for another command when it receives either a blank line or a comment line.

Your program must expand any instance of "$$" in a command into the process ID of the smallsh itself. Your shell does not otherwise perform variable expansion.

Your shell will support three built-in commands: exit, cd, and status. These three built-in commands are the only ones that your shell will handle itself - all others are simply passed on to a member of the exec() family of functions.

• You do not have to support input/output redirection for these built in commands
• These commands do not have to set any exit status.
• If the user tries to run one of these built-in commands in the background with the & option, ignore that option and run the command in the foreground anyway (i.e. don't display an error, just run the command in the foreground).

The exit command exits your shell. It takes no arguments. When this command is run, your shell must kill any other processes or jobs that your shell has started before it terminates itself.

The cd command changes the working directory of smallsh.

• By itself - with no arguments - it changes to the directory specified in the HOME environment variable
  • This is typically not the location where smallsh was executed from, unless your shell executable is located in the HOME directory, in which case these are the same.
• This command can also take one argument: the path of a directory to change to. Your cd command should support both absolute and relative paths.

The status command prints out either the exit status or the terminating signal of the last foreground process ran by your shell.

• If this command is run before any foreground command is run, then it should simply return the exit status 0.
• The three built-in shell commands do not count as foreground processes for the purposes of this built-in command - i.e., status should ignore built-in commands.

Your shell will execute any commands other than the 3 built-in command by using fork(), exec() and waitpid()

• Whenever a non-built in command is received, the parent (i.e., smallsh) will fork off a child.
• The child will use a function from the exec() family of functions to run the command.
• Your shell should use the PATH variable to look for non-built in commands, and it should allow shell scripts to be executed
• If a command fails because the shell could not find the command to run, then the shell will print an error message and set the exit status to 1
• A child process must terminate after running a command (whether the command is successful or it fails).

You must do any input and/or output redirection using dup2(). The redirection must be done before using exec() to run the command.

• An input file redirected via stdin should be opened for reading only; if your shell cannot open the file for reading, it should print an error message and set the exit status to 1 (but don't exit the shell).
• Similarly, an output file redirected via stdout should be opened for writing only; it should be truncated if it already exists or created if it does not exist. If your shell cannot open the output file it should print an error message and set the exit status to 1 (but don't exit the shell).
• Both stdin and stdout for a command can be redirected at the same time (see example below).

Any command without an & at the end must be run as a foreground command and the shell must wait for the completion of the command before prompting for the next command. For such commands, the parent shell does NOT return command line access and control to the user until the child terminates.

Any non built-in command with an & at the end must be run as a background command and the shell must not wait for such a command to complete. For such commands, the parent must return command line access and control to the user immediately after forking off the child.

• The shell will print the process id of a background process when it begins.
• When a background process terminates, a message showing the process id and exit status will be printed. This message must be printed just before the prompt for a new command is displayed.
• If the user doesn't redirect the standard input for a background command, then standard input should be redirected to /dev/null
• If the user doesn't redirect the standard output for a background command, then standard output should be redirected to /dev/null

A CTRL-C command from the keyboard sends a SIGINT signal to the parent process and all children at the same time (this is a built-in part of Linux).

• Your shell, i.e., the parent process, must ignore SIGINT
• Any children running as background processes must ignore SIGINT
• A child running as a foreground process must terminate itself when it receives SIGINT
  • The parent must not attempt to terminate the foreground child process; instead the foreground child (if any) must terminate itself on receipt of this signal.
  • If a child foreground process is killed by a signal, the parent must immediately print out the number of the signal that killed it's foreground child process (see the example) before prompting the user for the next command.

A CTRL-Z command from the keyboard sends a SIGTSTP signal to your parent shell process and all children at the same time (this is a built-in part of Linux).

• A child, if any, running as a foreground process must ignore SIGTSTP.
• Any children running as background process must ignore SIGTSTP.
• When the parent process running the shell receives SIGTSTP
  • The shell must display an informative message (see below) immediately if it's sitting at the prompt, or immediately after any currently running foreground process has terminated
  • The shell then enters a state where subsequent commands can no longer be run in the background.
  • In this state, the & operator should simply be ignored, i.e., all such commands are run as if they were foreground processes.
• If the user sends SIGTSTP again, then your shell will
  • Display another informative message (see below) immediately after any currently running foreground process terminates
  • The shell then returns back to the normal condition where the & operator is once again honored for subsequent commands, allowing them to be executed in the background.
• See the example below for usage and the exact syntax which you must use for these two informative messages. Sample Program Execution Here is an example run using smallsh. Note that CTRL-C has no effect towards the bottom of the example, when it's used while sitting at the command prompt:

$ 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
$

Required: The program is intended for Unix systems only. More specifically, I have only tested the program on CentOS 7 via docker and school engineering servers (CentOS as well).

Included Files:

1. main.c
2. README.md
3. makefile

Commands to enter in the command line:

1. make

• make will produce a executable named smallsh

2. ./smallsh
   • Launch the executable file by entering: ./smallsh via command line (current directory when entering the command should be the same directory as the smallsh executable file.)

Default CFLAGS values at compilation: CFLAGS = '-g -v -Wall -pedantic -std=c99' Overriding CFlags (e.g. use only -std=c99 flag or -w std=c99)

You can override the default CFLAGS variable values in the makefile and set your own CFLAGS values (flags used at command line compilation)

To do so, add CFLAGS='your flags here -std=c99' (case-sensitive) after the make command.

Note: CLFAGS values are in single quotes(e.g. CFLAGS=' ')

Note: Always include -std=c99 at the end of your CFLAGS values within the single quotes.

e.g. CFLAGS=' your flags -std=c99' Example: make CFLAGS='-w -std=c99'

• debug mode: make debug make debug will compile the files and launch the executable in valgrind

• Please run make clean in the command line if you want to do a "clean" restart of the program.

• make clean
• This cleans up all created object files

Sources and References

• The Linux Programming Interface (Chapters 5, 6, 7, 20, 21, 22, 24, 25, 26, 27, 28) by Michael Kerrisk
• Modules in coursework on Processes, Signals