Build your own Shell

Stage 1: Shell prompt
Stage 2: Missing commands
Stage 3: REPL
Stage 4: The exit built-in
Stage 5: The echo built-in
Stage 6: The type built-in: built-ins (1/2)
Stage 7: The type built-in: executable files using PATH (2/2)
Stage 8: Run a program

Extension 1, Stage 1: The pwd built-in
Extension 1, Stage 2: The cd built-in: absolute paths
Extension 1, Stage 3: The cd built-in: relative paths
Extension 1, Stage 4: The cd built-in: reading HOME

Stage 1: Shell prompt

Program must print $ and wait for input (it should not exit after printing the prompt).

$ ./myshell
$

Stage 2: Missing commands

Program must read input and print <command>: Command not found. if the command is not found.

We won't test whether the prompt is printed again, we'll just check if <command>: Command not found. is printed to stdout.

$ nonexistent
nonexistent: Command not found.

Stage 3: REPL

Now that a user has implemented the response for one command (the "unknown command" response), we can get to implementing a REPL.

Send one command, assert <command>: Command not found. is printed to stdout. Send another command, assert <command>: Command not found. is printed to stdout. Repeat this process 3-5 (random) times. The program should not exit after the last command is sent.

$ ./myshell
$ nonexistent_1
nonexistent_1: Command not found.
$ nonexistent_2
nonexistent_2: Command not found.
$ nonexistent_3
nonexistent_3: Command not found.

Stage 4: The exit built-in

When exit is received as a command, the program should exit with code 0. We'll issue two regular commands first, then invoke exit

$ nonexistent_1
nonexistent_1: Command not found.
$ nonexistent_2
nonexistent_2: Command not found.
$ exit

Stage 5: The echo built-in

When echo is received as a command, the program should print the rest of the line to stdout.

$ echo hello
hello
$ echo world
world
$ echo hello world
hello world

Note: echo can also be a binary, so we should prevent shelling out to it.

Stage 6: The type built-in: built-ins (1/3)

The type command should be implemented as a built-in.

type echo should print echo is a shell built-in. type type should print type is a shell built-in. type nonexistent should print nonexistent: Command not found.

$ type echo
echo is a shell built-in
$ type exit
exit is a shell built-in
$ type type
type is a shell built-in
$ type nonexistent
nonexistent: Command not found.

Stage 7: The type built-in: Executable files using PATH (2/3)

When a command is received, the program should search for the command in the PATH environment variable. If the command is found, the program should print the path to the command. If the command is not found, the program should print nonexistent: Command not found..

Setting PATH to /usr/bin:/usr/local/bin
$ type ls
/usr/bin/ls
$ type abcd
/usr/local/bin/abcd
$ type missing
missing: Command not found.

Stage 8: Run a program

A valid program is received as input. The program should be executed with the arguments provided.

$ mkdir -p /tmp/test1/test2
$ touch /tmp/test1/test2/testing
$ ls /tmp/test1/test2
testing

Extension: Navigation

Extension 1, Stage 1: The pwd built-in

When pwd is received as a command, the program should print the current working directory.

We've run into problems with changing the working directory in the past (the Git challenge has a lot of hacks around this), so let's always execute the user's program in the default directory. We can use cd (later stages) to change the working directory and actually test this command.

Extension 1, Stage 2: The cd built-in: absolute paths (1/3)

When cd is received with an absolute path to a directory that exists, it should change the current working directory. If the directory doesn't exist, it should say "no such file or directory". We'll test this using pwd.

$ cd /usr/local/bin
$ pwd
/usr/local/bin
$ cd /does_not_exist
cd: /does_not_exist: No such file or directory

Extension 1, Stage 3: The cd built-in: relative paths (2/3)

When cd is received with a relative path, it should change the current working directory. Verify using pwd.

Note that most of this is implemented in the chdir system call, so we dont' have to split this into ., .. etc. - it should be handled implicitly

$ cd /usr
$ pwd
/usr
$ cd ./local/bin
$ pwd
/usr/local/bin
$ cd ../../
$ pwd
/usr
$ cd missing
cd: missing: No such file or directory

Extension 1, Stage 4: The cd built-in: reading HOME (3/3)

When cd is received with ~, it should change the current working directory to the user's home directory. Verify using pwd.

$ cd /usr/local/bin
$ pwd
/usr/local/bin
$ cd ~
$ pwd
/home/user

rohitpaulk / shell-poc