Project 4: Unix Shell

Create a new version of vssh1 that is enhanced in the following ways. This new version should be a separate executable called vssh2.

1. If the line ends with the & symbol, it should run in the background. That is, your shell should not wait for it to terminate; the command line should immediately return. Your shell should print the PID of the process, so that the user may later manage it as needed. This is typically used for long-running programs that perform a lot of computation. It is most often used in conjunction with output redirection, as described in step 3.
2. The line may contain two or more commands connected with the pipe symbol (|). If this happens, start a process for each command, setting up pipes to send the output of each left-hand command to the input of the following right-hand command.
3. The last command in the pipeline (or the only command, if there is no pipeline) may be followed by the > symbol and a filename. The command’s output should be stored in the designated file. If the file does not exist, it should be created.
4. The first command in the pipeline (or the only command, if there is no pipeline) may be followed by the < symbol and a filename. The command’s input should be taken from the designated file. If the file does not exist, the command should abort.

Example Execution

/home/gjf2a/solutions320$ ls -l | grep Cargo | sort
-rw-r--r-- 1 gjf2a gjf2a  262 Feb 12 12:31 Cargo.toml
-rw-r--r-- 1 gjf2a gjf2a 5264 Feb 12 12:31 Cargo.lock
/home/gjf2a/solutions320$ ls -l | grep Cargo > output.txt
/home/gjf2a/solutions320$ cat < output.txt
-rw-r--r-- 1 gjf2a gjf2a 5264 Feb 12 12:31 Cargo.lock
-rw-r--r-- 1 gjf2a gjf2a  262 Feb 12 12:31 Cargo.toml
/home/gjf2a/solutions320$ ls -l
total 32
-rw-r--r-- 1 gjf2a gjf2a 5264 Feb 12 12:31 Cargo.lock
-rw-r--r-- 1 gjf2a gjf2a  262 Feb 12 12:31 Cargo.toml
-rw------- 1 gjf2a gjf2a  108 Feb 11 10:21 grep_test.out
-rw------- 1 gjf2a gjf2a  108 Feb 15 09:08 output.txt
drwxr-xr-x 3 gjf2a gjf2a 4096 Feb 10 13:21 src
drwxr-xr-x 3 gjf2a gjf2a 4096 Feb 10 13:21 target
-rw------- 1 gjf2a gjf2a  106 Feb 11 10:22 toml.out
/home/gjf2a/solutions320$ ls -l &
Starting background process 1871
/home/gjf2a/solutions320$ total 32
-rw-r--r-- 1 gjf2a gjf2a 5264 Feb 12 12:31 Cargo.lock
-rw-r--r-- 1 gjf2a gjf2a  262 Feb 12 12:31 Cargo.toml
-rw------- 1 gjf2a gjf2a  108 Feb 11 10:21 grep_test.out
-rw------- 1 gjf2a gjf2a  108 Feb 15 09:08 output.txt
drwxr-xr-x 3 gjf2a gjf2a 4096 Feb 10 13:21 src
drwxr-xr-x 3 gjf2a gjf2a 4096 Feb 10 13:21 target
-rw------- 1 gjf2a gjf2a  106 Feb 11 10:22 toml.out
cd src
/home/gjf2a/solutions320/src$ cd bin
/home/gjf2a/solutions320/src/bin$ grep fn vssh2.rs | sort
    fn default() -> Self {
    fn execute_pipeline(&self) -> anyhow::Result<()> {
    fn final_input_fd(&self) -> anyhow::Result<i32> {
    fn from_str(s: &str) -> Result<Self, Self::Err> {
    fn initial_output_fd(&self) -> anyhow::Result<i32> {
fn disassemble_redirect(pipes: &mut Vec<String>, i: usize, redirector: char) -> Option<String> {
fn execute(cmd: &str) {
fn externalize(command: &str) -> Vec<CString> {
fn main() {
fn process_next_line() -> anyhow::Result<Status> {
fn run_command(command: &str) -> anyhow::Result<()> {
fn run_stage(cmd: &str, output_fd: i32) -> anyhow::Result<i32> {
/home/gjf2a/solutions320/src/bin$ cat < vssh2.rs | sort | tail -8 > eight.out
/home/gjf2a/solutions320/src/bin$ cat eight.out
}
}
}
}
}
}
}
}
/home/gjf2a/solutions320/src/bin$ cd ../..
/home/gjf2a/solutions320$ cat < Cargo.toml > toml2.out
/home/gjf2a/solutions320$ diff Cargo.toml toml2.out 
/home/gjf2a/solutions320$ exit

Design Hints

• Implementing a shell is simplified by separating the parsing of the command line from its execution. I recommend creating a struct to represent the different components of the parsed command line:
  • Whether it is a background command.
  • Its output file, if redirected.
  • Its input file, if redirected.
  • A vector of its pipelined commands.
• Here is one approach to parsing the command:
  • Check if it ends with an ampersand (&). If so, set the background command flag to true, and shave the & from the end.
  • Split the string based on the | symbol, and place the results into a vector.
    • Be sure to use .to_string() to convert them to fully owned objects, not borrowed objects.
  • Check the last element to see if it contains the > symbol. If so, put the output filename into your struct, then remove it from the command entry.
  • Check the first element to see if it contains the < symbol. If so, put the input filename into your struct, then remove it from the command entry.
• Redirecting standard output can make debugging difficult; I recommend using eprintln!, which prints to standard error.
• Break the execution of the command line into a series of functions:
  • The top-level execution function forks.
    • Parent waits, unless it is a background process.
    • Child calls a function to execute a pipeline.
  • The pipeline execution function:
    • Sets up a variable to track the current output file descriptor.
      • If output has been redirected to a file, open that file and use its file descriptor.
      • Otherwise, set it to 1, the file descriptor for standard output.
    • Loops through the commands in the pipeline, starting with the last one.
      • The last command waits for input from its predecessor, so it has to start first.
      • The first command in the pipeline is a special case; deal with it after the loop ends.
    • For each command in the loop:
      • Call a pipeline stage function, sending it the current command and current output file descriptor and receiving from it an updated output file descriptor.
    • When the loop is over:
      • Redirect input to be received from a file, if necessary.
      • Redirect output to the current output file descriptor.
      • Execute the first command in the pipeline.
  • The pipeline stage function:
    • Creates a pipe. The pipe will receive data from the current command’s predecessor, and send data to the current command.
    • Uses fork to create the child process.
      • In the parent (corresponding to the current command):
        • Close the pipe’s input. It is unnecessary here, as the pipe will receive data from the preceding command.
        • Use dup2 to redirect the process input to the pipe output.
        • Use dup2 to redirect the process output to the current output file descriptor.
        • Execute the current command.
      • In the child (corresponding to all commands in the pipeline that precede the current command):
        • Close the pipe’s output. It is unnecessary here, as the pipe will send data to the current command.
        • Return the pipe’s input, so it can receive output from the preceding command in the pipeline.

Submissions

• Push your enhancements to the same GitHub repository you used for vssh in the previous assignment.

Assessment

• Partial: Correctly complete any two enhancements.
• Complete: All four enhancements correctly completed.

Acknowledgement

This assignment was adapted from materials developed by David Evans at the University of Virginia.