A simple UNIX command interpreter written as part of the low-level programming and algorithm track at Alx.
Shellby is a simple UNIX command language interpreter that reads commands from either a file or standard input and executes them.
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](https://linux.die.net/man/3/isatty)(3)), an *interactive* shell is opened. When executing interactively, **shellby** displays the prompt `$ ` when it is ready to read a command.
Example:
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 :deciduous_tree:
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 :hocho:
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 :wave:
**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 :exclamation:
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 :heavy_dollar_sign:
**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 :hash:
**Shellby** ignores all words and characters preceeded by a `#` character on a line.
Example:
$ echo "echo 'hello' #this will be ignored!" | ./shellby 'hello'
### Operators :guitar:
**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 :nut_and_bolt:
#### 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 :black_nib:
* Falila Abdul-Razak <[Alx-Falila](https://github.com/Alx-Falila)>
* Wesley Brandon <[RatwebTech](https://github.com/RatwebTech)>
## Acknowledgements :pray:
**Shellby** emulates basic functionality of the **sh** shell. This README borrows form the Linux man pages [sh(1)](https://linux.die.net/man/1/sh) and [dash(1)](https://linux.die.net/man/1/dash).