shellby - Simple Shell 🐚

A simple UNIX command interpreter written as part of the low-level programming and algorithm track at ALX Africa and done by a peer Learning.

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

$