itis2010me / SimpleShell

A practice of exploring how Unix shells are made.

SimpleShell |'L'|

A practice of exploring how Unix shells are made.

Please do no distribute this code!

Logo|'L'|

The prompt for the user to enter an command will be |'L'|$ . The design is based on the finder app on MacOS.

Makefile

Makefile is included. Supporting functions might be migrated into a different .cpp file in the future. For version 1.0, they are kept in the same file.

Built-in Commands

Currently, there are the following built-in commands supported:

• cd - Change directory.

  • Always take one argument.

• path - Overwrites old paths with newly specified paths.

  • Takes 0 or more arguments. Each should be separated with whitespace from the others.
  • If called without any arguments, the shell cannot execute any commands excepts the built-in ones.

• exit - Exit with 0.

clear - Coming soon....

Path

The default path that the shell will look for system program such as ls will be /bin/.

You can add new paths to the list of existing paths. The shell will search through all the paths when searching for a command.

|'L'|$ path /bin /usr/bin/
|'L'|$ 

Now the list of paths include both /bin/ and /usr/bin/.

Redirection

This shell supports basic redirection. If the file that the output of the redirection is pointing to already exist, it will be truncated and overwirtten with new content.

Note: Multiple redirection is not supported.

Example:

|'L'|$ ls -la
total 344
drwxr-xr-x   7 user  staff     224 Jun  8 22:19 .
drwxr-xr-x@  7 user  staff     224 Jun  8 21:38 ..
drwxr-xr-x  14 user  staff     448 Jun  8 21:45 .git
-rw-r--r--   1 user  staff     119 Jun  8 21:50 Makefile
-rw-r--r--@  1 user  staff    1269 Jun  8 22:19 README.md
-rwxr-xr-x   1 user  staff  144048 Jun  8 21:50 shell
-rw-r--r--@  1 user  staff   18525 Jun  8 21:51 shell.cpp
|'L'|$ ls -la > output.txt
|'L'|$ cat output.txt
total 344
drwxr-xr-x   8 user  staff     256 Jun  8 22:19 .
drwxr-xr-x@  7 user  staff     224 Jun  8 21:38 ..
drwxr-xr-x  14 user  staff     448 Jun  8 21:45 .git
-rw-r--r--   1 user  staff     119 Jun  8 21:50 Makefile
-rw-r--r--@  1 user  staff    1269 Jun  8 22:19 README.md
-rw-r--r--   1 user  staff       0 Jun  8 22:19 output.txt
-rwxr-xr-x   1 user  staff  144048 Jun  8 21:50 shell
-rw-r--r--@  1 user  staff   18525 Jun  8 21:51 shell.cpp
|'L'|$ 

Parallel Commands

This shell also supports parallel commands for up to 3.

This is simulated with fork() and waitpidin c++. The order of execution of the commands cannot be guaranteed.

Commands should be separated with &.

Error

In most cases of error, the shell will continue and prints out the error message An error has occurred.

For fatal errors such as cannot open files or passed wrong number of command line arguments, the shell will exist with exit(1).

The error message of system commands such as ls or pwd will still be printed out.

Modes

There are two modes supported.

• Interactive mode.

  • Similar to an actual Unix shell, just type the commands following the prompt |'L'|$.

  • $ ./shell
    |'L'|$ 

• Batch mode.

  • This mode will take in exactly one command line argument, a file. The file should contain a list of commands for the shell to execute, each occupying a single line.
  • There will not be the cool |'L'|$ printed in this mode.

Version

This is currently version 1.0. More features might be added in the future to further simulate a real Unix shell.

History

• June 8, 2021 - Initial version added to github. Initial README as well.