Operating Systems-Program 3: Smallsh Solved

Overview
In this assignment you will write your own shell in C. The shell will run command line instructions and return the results similar to other shells you have used, but without many of their fancier features.

In this assignment you will write your own shell, called smallsh.  This will work like the bash shell you are used to using, prompting for a command line and running commands, but it will not have many of the special features of the bash shell.

Your shell will allow for the redirection of standard input and standard output and it will support both foreground and background processes (controllable by the command line and by receiving signals).

Your shell will support three built in commands: `exit`, `cd`, and `status`. It will also support comments, which are lines beginning with the # character.

## Specifications
### The Prompt

Use the colon : symbol as a prompt for each command line. Be sure you flush out the output buffers each time you print, as the text that you're outputting may not reach the screen until you do in this kind of interactive program. To do this, call fflush() immediately after each and every time you output text.

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 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.

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.

Finally, 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.

### Command Execution

You will use fork(), exec(), and waitpid() to execute commands. From a conceptual perspective, consider setting up your shell to run in this manner: let the parent process (your shell) continue running. Whenever a non-built in command is received, have the parent fork() off a child. This child then does any needed input/output redirection before running exec() on the command given. Note that when doing redirection, that after using dup2() to set up the redirection, the redirection symbol and redirection destination/source are NOT passed into the following exec command (i.e., if the command given is `ls > more`, then you do the redirection and then simply pass `ls` into exec() ).

Note that exec() will fail, and return the reason why, if it is told to execute something that it cannot do, like a program that doesn't exist. In this case, your shell should indicate to the user that a command could not be executed, and set the value retrieved by the built-in status command to 1.

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.

As above, after the fork() but before the exec() you must do any input and/or output redirection with dup2(). 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).

Your program must expand any instance of "$$" in a command into the process ID of the shell itself. Your shell does not otherwise perform variable expansion. This feature makes it easier to create a grading script that keeps your work separate.

### Background and Foreground

The shell should wait() for completion of foreground commands (commands without the &) before prompting for the next command. If the command given was a foreground command, then the parent shell does NOT return command line access and control to the user until the child terminates. It is recommend to have the parent simply call waitpid() on the child, while it waits.

The shell will not wait for background commands to complete. If the command given was a background process, then the parent returns command line access and control to the user immediately after forking off the child. In this scenario, your parent shell will need to periodically check for the background child processes to complete (with waitpid(...NOHANG...)), so that they can be cleaned up, as the shell continues to run and process commands. Consider storing the PIDs of non-completed background processes in an array, so that they can periodically be checked for. Alternatively, you may use a signal handler to immediately wait() for child processes that terminate, as opposed to periodically checking a list of started background processes. The time to print out when these background processes have completed is just BEFORE command line access and control are returned to the user, every time that happens.

Background commands should have their standard input redirected from /dev/null if the user did not specify some other file to take standard input from. What happens to background commands that read from standard input if you forget this? Background commands should also not send their standard output to the screen (again, redirect to /dev/null).

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. You should check to see if any background processes completed just before you prompt for a new command and print this message then. In this way the messages about completed background processes will not appear during other running commands, though the user will have to wait until they complete some other command to see these messages (this is the way the C shell and Bourne shells work; see example below). You will probably want to use waitpid() to check for completed background processes.

### Signals

A CTRL-C command from the keyboard will send a SIGINT signal to your parent process and all children at the same time. Make sure that SIGINT does not terminate your shell, but only terminates the foreground command if one is running. To do this, you'll have to create the appropriate signal handlers with sigaction(). The parent should not attempt to terminate the foreground child process when the parent receives a SIGINT signal: 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.

Background processes should also not be terminated by a SIGINT signal. They will terminate themselves, continue running, or be terminated when the shell exits (see below).

A CTRL-Z command from the keyboard will send a SIGTSTP signal to your shell. When this signal is received, your shell will display an informative message (see below) and then enter a state where new commands can no longer be run in the background. In this state, the & operator should simply be ignored - run all such commands as if they were foreground processes. If the user sends SIGTSTP again, display another informative message (see below), then return back to the normal condition where the & operator is once again honored, allowing commands to be placed in the background. See the example below for usage and the exact syntax which you must use for these two informative messages.

### Built-in Commands

Your shell will support three built in commands: `exit`, `cd`, and `status`. You do not have to support input/output redirection for these built in commands and they do not have to set any exit 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 (which member is up to you) as described above.

If the user tries to run one of these built-in commands in the background with the & option, ignore that option and run it in the foreground anyway (i.e. don't display an error, just run the command in the foreground).

The `exit` command exits the 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 directories.  By itself, it changes to the directory specified in the HOME environment variable (not to the location where smallsh was executed from, unless your shell is located in the HOME directory).  It can also take one argument, the path of the directory to change to. Note that this is a working directory: when smallsh exits, the pwd will be the original pwd when smallsh was launched. 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 (not both, processes killed by signals do not have exit statuses!).

### Example

Here is an example:

```
$ 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/faculty/b/brewsteb/CS344/prog3
: cd
: pwd
/nfs/stak/faculty/b/brewsteb
: cd CS344
: pwd
/nfs/stak/faculty/b/brewsteb/CS344
: echo 4867
4867
: echo $$
4867
: ^C
: ^Z
Entering foreground-only mode (& is now ignored)
: date

: sleep 5 &
: date

: ^Z
Exiting foreground-only mode
: date
: sleep 5 &
background pid is 4963
: date

: exit $
```