shellby - Simple Shell 🐚

A simple UNIX command interpreter written as part of the low-level programming and algorithm sprint at ALX Software Engineering Bootcamp.

Description 💬

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

Invocation 🏃

Usage: shellby [filename]

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

gcc *.c -o shellby
./shellby

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

Example:

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

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

Example:

$./shellby
$

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

Example:

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

Environment 🌳

Upon invocation, shellby 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:

HOME

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

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

PWD

The current working directory as set by the cd command.

$ echo "echo $PWD" | ./shellby
/home/vagrant/holberton/simple_shell

OLDPWD

The previous working directory as set by the cd command.

$ echo "echo $OLDPWD" | ./shellby
/home/vagrant/holberton/printf

PATH

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" | ./shellby
/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

Command Execution 🔪

After receiving a command, shellby 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. Shellby 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, shellby 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.

Exit Status 👋

Shellby 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).

Signals ❗

While running in interactive mode, shellby 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.

$ ./shellby
$ ^C
$ ^C
$

Variable Replacement 💲

Shellby interprets the $ character for variable replacement.

$ENV_VARIABLE

ENV_VARIABLE is substituted with its value.

Example:

$ echo "echo $PWD" | ./shellby
/home/vagrant/holberton/simple_shell

$?

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

Example:

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

$$

The second $ is substitued with the current process ID.

Example:

$ echo "echo $$" | ./shellby
6494

Comments #️⃣

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

Example:

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

Operators 🎸

Shellby specially interprets the following operator characters:

; - Command separator

Commands separated by a ; are executed sequentially.

Example:

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

&& - AND logical operator

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

Example:

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

|| - OR logical operator

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

Example:

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

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

Shellby Builtin Commands 🔩

cd

• 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:

$ ./shellby
$ pwd
/home/vagrant/holberton/simple_shell
$ cd ../
$ pwd
/home/vagrant/holberton
$ cd -
$ pwd
/home/vagrant/holberton/simple_shell

alias

• 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:

$ ./shellby
$ alias show=ls
$ show
AUTHORS            builtins_help_2.c  errors.c         linkedlist.c        shell.h       test
README.md          env_builtins.c     getline.c        locate.c            shellby
alias_builtins.c   environ.c          helper.c         main.c              split.c
builtin.c          err_msgs1.c        helpers_2.c      man_1_simple_shell  str_funcs1.c
builtins_help_1.c  err_msgs2.c        input_helpers.c  proc_file_comm.c    str_funcs2.c

exit

• 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:

$ ./shellby
$ exit

env

• Usage: env
• Prints the current environment.

Example:

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

setenv

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

Example:

$ ./shellby
$ setenv NAME Poppy
$ echo $NAME
Poppy

unsetenv

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

Example:

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

$

Authors ✒️

• Chinonso Ogwehi <nonsoflex>
• Femi Ajanaku <stanwukong>

License 🔒

This project is licensed under the MIT License - see the LICENSE file for details.

Acknowledgements 🙏

Shellby emulates basic functionality of the sh shell. This README borrows form the Linux man pages sh(1) and dash(1).

This project was written as part of the curriculum for ALX Software Engineering. ALX SE is a remote campus-styled full-stack software engineering program that prepares students for careers in the tech industry using project-based peer learning. For more information, visit this link.