Simple Shell is a simple UNIX command language interpreter that reads commands from either a file or standard input and executes them.

Usage: hsh [filename]

To invoke Simple Shell, compile all .c files in the repository and run the resulting executable:

gcc *.c -o hsh
./hsh

Simple Shell can be invoked both interactively and non-interactively. If Simple Shell is invoked with standard input not connected to a terminal, it reads and executes received commands in order.

Example:

$ echo "echo 'hello'" | ./hsh
'hello'
$

If Simple Shell is invoked with standard input connected to a terminal (determined by isatty(3)), an interactive shell is opened. When executing interactively, Simple Shell displays the prompt $ when it is ready to read a command.

Example:

$./hsh
$

Alternatively, if command line arguments are supplied upon invocation, Simple Shell treats the first argument as a file from which to read commands. The supplied file should contain one command per line. Simple Shell runs each of the commands contained in the file in order before exiting.

Example:

$ cat test
echo 'hello'
$ ./hsh test
'hello'
$

Upon invocation, Simple Shell receives and copies the environment of the parent process in which it was executed. This environment is an array of name-value strings describing variables in the format NAME=VALUE. A few key environmental variables are:

The home directory of the current user and the default directory argument for the cd builtin command.

$ echo "echo $HOME" | ./hsh
/home/vagrant

The current working directory as set by the cd command.

$ echo "echo $PWD" | ./hsh
/home/vagrant/simple_shell

The previous working directory as set by the cd command.

$ echo "echo $OLDPWD" | ./hsh
/home/vagrant/_printf

A colon-separated list of directories in which the shell looks for commands. A null directory name in the path (represented by any of two adjacent colons, an initial colon, or a trailing colon) indicates the current directory.

$ echo "echo $PATH" | ./hsh
/home/vagrant/.cargo/bin:/home/vagrant/.local/bin:/home/vagrant/.rbenv/plugins/ruby-build/bin:/home/vagrant/.rbenv/shims:/home/vagrant/.rbenv/bin:/home/vagrant/.nvm/versions/node/v10.15.3/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin:/home/vagrant/.cargo/bin:/home/vagrant/workflow:/home/vagrant/.local/bin

After receiving a command, Simple Shell tokenizes it into words using " " as a delimiter. The first word is considered the command and all remaining words are considered arguments to that command. Simple Shell then proceeds with the following actions:

1. If the first character of the command is neither a slash (\) nor dot (.), the shell searches for it in the list of shell builtins. If there exists a builtin by that name, the builtin is invoked.
2. If the first character of the command is none of a slash (\), dot (.), nor builtin, Simple Shell searches each element of the PATH environmental variable for a directory containing an executable file by that name.
3. If the first character of the command is a slash (\) or dot (.) or either of the above searches was successful, the shell executes the named program with any remaining given arguments in a separate execution environment.

Simple Shell returns the exit status of the last command executed, with zero indicating success and non-zero indicating failure.

If a command is not found, the return status is 127; if a command is found but is not executable, the return status is 126.

All builtins return zero on success and one or two on incorrect usage (indicated by a corresponding error message).

While running in interactive mode, Simple Shell ignores the keyboard input Ctrl+c. Alternatively, an input of end-of-file (Ctrl+d) will exit the program.

User hits Ctrl+d in the third line.

$ ./hsh
$ ^C
$ ^C
$

Simple Shell interprets the $ character for variable replacement.

ENV_VARIABLE is substituted with its value.

Example:

$ echo "echo $PWD" | ./hsh
/home/vagrant/simple_shell

? is substitued with the return value of the last program executed.

Example:

$ echo "echo $?" | ./hsh
0

The second $ is substitued with the current process ID.

Example:

$ echo "echo $$" | ./hsh
6494

Simple Shell ignores all words and characters preceeded by a # character on a line.

Example:

$ echo "echo 'hello' #this will be ignored!" | ./hsh
'hello'

Simple Shell specially interprets the following operator characters:

Commands separated by a ; are executed sequentially.

Example:

$ echo "echo 'hello' ; echo 'world'" | ./hsh
'hello'
'world'

command1 && command2: command2 is executed if, and only if, command1 returns an exit status of zero.

Example:

$ echo "error! && echo 'hello'" | ./hsh
./hsh: 1: error!: not found
$ echo "echo 'all good' && echo 'hello'" | ./hsh
'all good'
'hello'

command1 || command2: command2 is executed if, and only if, command1 returns a non-zero exit status.

Example:

$ echo "error! || echo 'but still runs'" | ./hsh
./hsh: 1: error!: not found
'but still runs'

The operators && and || have equal precedence, followed by ;.

• Usage: cd [DIRECTORY]
• Changes the current directory of the process to DIRECTORY.
• If no argument is given, the command is interpreted as cd $HOME.
• If the argument - is given, the command is interpreted as cd $OLDPWD and the pathname of the new working directory is printed to standad output.
• If the argument, -- is given, the command is interpreted as cd $OLDPWD but the pathname of the new working directory is not printed.
• The environment variables PWD and OLDPWD are updated after a change of directory.

Example:

$ ./hsh
$ pwd
/home/vagrant/simple_shell
$ cd ../
$ pwd
/home/vagrant/
$ cd -
$ pwd
/home/vagrant/simple_shell

• Usage: alias [NAME[='VALUE'] ...]
• Handles aliases.
• alias: Prints a list of all aliases, one per line, in the form NAME='VALUE'.
• alias NAME [NAME2 ...]: Prints the aliases NAME, NAME2, etc. one per line, in the form NAME='VALUE'.
• alias NAME='VALUE' [...]: Defines an alias for each NAME whose VALUE is given. If name is already an alias, its value is replaced with VALUE.

Example:

$ ./hsh
$ alias show=ls
$ show
README.md	environ.c	getenv.c	list1.c
_atoi.c         errors.c	getinfo.c	shell.h
builtin.c	errors1.c       getline.c
builtin1.c	exits.c		history.c

• Usage: exit [STATUS]
• Exits the shell.
• The STATUS argument is the integer used to exit the shell.
• If no argument is given, the command is interpreted as exit 0.

Example:

$ ./hsh
$ exit

• Usage: env
• Prints the current environment.

Example:

$ ./hsh
$ env
NVM_DIR=/home/vagrant/.nvm
...

• Usage: setenv [VARIABLE] [VALUE]
• Initializes a new environment variable, or modifies an existing one.
• Upon failure, prints a message to stderr.

Example:

$ ./hsh
$ setenv NAME Poppy
$ echo $NAME
Poppy

• Usage: unsetenv [VARIABLE]
• Removes an environmental variable.
• Upon failure, prints a message to stderr.

Example:

$ ./hsh
$ setenv NAME Poppy
$ unsetenv NAME
$ echo $NAME

$

• <Joel050>
• <PhoenixClix>