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>
• Olibrahim <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.