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https://github.com/anny85-code/simple_shell

0x16. C - Simple Shell
https://github.com/anny85-code/simple_shell

c shell

Last synced: 28 days ago
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0x16. C - Simple Shell

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README 

          
# Simple Shell project 0x16.c 



This is a simple UNIX command interpreter based on bash and Sh.



## Overview



**Simple-shell** is a sh-compatible command language interpreter that executes commands read from the standard input or from a file.



### Invocation



Usage: **Simple-shell** 

Simple-shell is started with the standard input connected to the terminal. To start, compile all .c located in this repository by using this command: 

```

gcc -Wall -Werror -Wextra -pedantic *.c -o simple_shell

./simple_shell

```



**Simple-shell** is allowed to be invoked 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 'alx'" | ./simple-shell

'africa'

$

```



When **simple-shell** is invoked with standard input connected to a terminal (determined by isatty(3), the interactive mode is opened. **simple-shell** Will be using the following prompt `^-^ `.



Example:

```

$./simple-shell

^-^

```



If a command line argument is invoked, **simple-shell** will take that first argument as a file from which to read commands.



Example:

```

$ cat text

echo 'alx'

$ ./simple_shell text

'africa'

$

```



### Environment



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:



#### HOME

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



```

$ echo "echo $HOME" | ./simple_shell

/home/vagrant

```



#### PWD

The current working directory as set by the **cd** command.



```

$ echo "echo $PWD" | ./simple_shell

/home/vagrant/alx/simple_shell

```



#### OLDPWD

The previous working directory as set by the **cd** command.



```

$ echo "echo $OLDPWD" | ./simple_shell

/home/vagrant/holberton/bog-062019-test_suite

```



#### 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" | ./simple_shell

/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, **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, **sodash** 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 



**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).



### Signals



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 foutrh command.

```

$ ./simple_shell

^-^ ^C

^-^ ^C

^-^ ^C

^-^

```



### Variable Replacement



**simple-shell** interprets the `$` character for variable replacement.



#### $ENV_VARIABLE

`ENV_VARIABLE` is substituted with its value.



Example:

```

$ echo "echo $PWD" | ./simple_shell

/home/vagrant/alx/simple_shell

```



#### $?

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



Example:

```

$ echo "echo $?" | ./simple_shell

0

```



#### $$

The second `$` is substitued with the current process ID.



Example:

```

$ echo "echo $$" | ./simple_shell

3855

```



### Comments



**simple-shell** ignores all words and characters preceeded by a `#` character on a line.



Example:

```

$ echo "echo 'alx' #this will be ignored!" | ./simple_shell

'africa'

```



### Operators



**simple-shell** specially interprets the following operator characters:



#### ; - Command separator

Commands separated by a `;` are executed sequentially.



Example:

```

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

'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 'alx'" | ./simple_shell

./shellby: 1: error!: not found

$ echo "echo 'my name is' && echo 'alx'" | ./simpe_shell

'my name is'

'africa'

```



#### || - OR logical operator

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



Example:

```

$ echo "error! || echo 'wait for it'" | ./simple_shell

./simple_shell: 1: error!: not found

'wait for it'

```



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



### 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:

```

$ ./simple_shell

^-^ pwd

/home/vagrant/alx/simple_shell

$ cd ../

^-^ pwd

/home/vagrant/alx

^-^ cd -

^-^ pwd

/home/vagrant/alx/simple_shell

```



#### 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:

```

$ ./simple_shell

$ exit

```



#### env

  * Usage: `env`

  * Prints the current environment.



Example:

```

$ ./simple_shell

$ 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:

```

$ ./simple_shell

$ setenv NAME Holberton

$ echo $NAME

Holberton

```



#### unsetenv

  * Usage: `unsetenv [VARIABLE]`

  * Removes an environmental variable.

  * Upon failure, prints a message to `stderr`.



Example:

```

$ ./simple_shell

$ setenv NAME Alx Africa

$ unsetenv NAME

$ echo $NAME



$

```



## Authors & Copyrights



* Aniekan Udo <[Aniekan Udo](https://www.linkedin.com/in/aniekan-udo)>

* Olibrahim <[Olibrahim](https://github.com/Olibrahim)>



## More information



**Simple-shell** is a simple shell unix command interpreter that is part of the alx-africa low level programming module at Alx Africa and is intended to emulate the basics **sh** shell. All the information given in this README is based on the **simple-shell** and **bash** man (1) pages.