Overview

A POSIX compliant shell (in the near future) capable of interpreting shell commands, running external programs and builtin commands like cd, pwd, echo and more.

Java Version

You will require Java 25.

Getting Started

Run ./your_program.sh

Current Features

1. Prompt: The shell starts with a prompt ($) that signals it’s ready for your command. As of now, only a few commands are supported.

2. The exit command terminates the shell.

   1. Control + C terminates the shell.

3. The echo builtin prints its arguments to stdout, with spaces between them, and a newline at the end. 1.

    $ echo hello world
    hello world
    $ echo one two three
    one two three
   

4. The type builtin is used to determine how a command would be interpreted if it were used. It checks whether a command is a builtin, an executable file, or unrecognized.

   1. The type builtin can search for executable files using PATH. For example, if the PATH is set to /dir1:/dir2:/dir3, the shell would search for executables in /dir1, then /dir2, and finally /dir3, in that order.

       $ type echo
       echo is a shell builtin
       $ type exit
       exit is a shell builtin
       $ type invalid_command
       invalid_command: not found
       $ type grep
       grep is /usr/bin/grep
       $ type invalid_command
       invalid_command: not found
       $ type echo
       echo is a shell builtin
      

5. Support for running external programs with arguments

6. When a command isn't a builtin, the shell would:

   1. Search for an executable with the given name in the directories listed in PATH (just like type does)

    $ type echo
    echo is a shell builtin
    $ type ls
    ls is /bin/ls
    $ type grep
    grep is /usr/bin/grep
   

7. pwd (print working directory) builtin prints the full, absolute path of the current working directory to stdout.

8. cd builtin is used to change the current working directory.

Quotes

Single Quotes

Single quotes (') disable all special meaning for characters enclosed within them. Every character between single quotes is treated literally.

1. Characters inside single quotes (including escape characters and potential special characters like $, *, or ~) lose their special meaning and are treated as normal characters.
2. Consecutive whitespace characters (spaces, tabs) inside single quotes are preserved and are not collapsed or used as delimiters.
3. Quoted strings placed next to each other are concatenated to form a single argument.

E.G.

Command	Expected output	Explanation
echo 'hello world'	hello world	Spaces are preserved within quotes
echo hello world	hello world	Consecutive spaces are collapsed unless quoted
echo 'hello''world'	helloworld	Adjacent quoted strings 'hello' and 'world' are concatenated
echo hello''world	helloworld	Empty quotes '' are ignored

The cat command, with the file name parameter enclosed in single quotes:

$ cat '/tmp/file name' '/tmp/file name with spaces'
content1 content2

where /tmp/file name equals content1 and /tmp/file name with spaces equals content2

Double Quotes

In shell syntax, most characters within double quotes (") are treated literally. This implementation covers

1. Consecutive whitespaces (spaces, tabs) must be preserved.
2. Characters that normally act as delimiters or special characters lose their special meaning inside double quotes and are treated literally.
3. Double-quoted strings placed next to each other are concatenated to form a single argument.

E.G.

$ echo "hello    world"
hello    world         # Multiple spaces preserved

$ echo "hello""world"
helloworld             # Quoted strings next to each other are concatenated.

$ echo "hello" "world"
hello world            # Separate arguments

$ echo "shell's test"
shell's test           # Single quotes inside are literal

The cat command, with the file name parameter enclosed in single quotes:

$ cat "/tmp/file name" "/tmp/file 'name with 'spaces"
content1 content2

where /tmp/file name equals content1 and /tmp/file 'name with 'spaces equals content2