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Simple Shell - Posh

This is a simple UNIX command interpreter based on bash and sh

Overview

Posh is a sh-compatible command language interpreter that executes commands read from the standard input or from a file

Invocation

Usage: Posh Posh 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 posh
./posh

Posh is allowed to be invoked interactively and non-interactively. If posh is invoked with standard input not connected to a terminal, it reads and executes received commands in order.

Example:

$ echo "echo 'best school'" | ./posh
'best school'
$

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

Example:

$./posh
[posh]-$

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

Example:

$ cat text
echo 'best school'
$ ./posh text
'best school'
$

Environment

Upon invocation, posh 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" | ./posh
/home/vagrant

PWD

The current working directory as set by the cd command.

$ echo "echo $PWD" | ./posh
/home/vagrant/alx/simple_shell

OLDPWD

The previous working directory as set by the cd command.

$ echo "echo $OLDPWD" | ./posh
/home/vagrant/alx/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" | ./posh
/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, posh 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. posh 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, posh 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

posh 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, posh 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 fourth command.

$ ./posh
[posh]-$ ^C
[posh]-$ ^C
[posh]-$ ^C
[posh]-$

Variable Replacement

posh interprets the $ character for variable replacement.

$ENV_VARIABLE

ENV_VARIABLE is substituted with its value.

Example:

$ echo "echo $PWD" | ./posh
/home/vagrant/alx/simple_shell

$?

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

Example:

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

$$

The second $ is substitued with the current process ID.

Example:

$ echo "echo $$" | ./posh
3855

Comments

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

Example:

$ echo "echo 'best school' #this will be ignored!" | ./posh
'best school'

Operators

posh specially interprets the following operator characters:

; - Command separator

Commands separated by a ; are executed sequentially.

Example:

$ echo "echo 'hello' ; echo 'world'" | ./posh
'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'" | ./posh
./shellby: 1: error!: not found
$ echo "echo 'my name is' && echo 'alx'" | ./posh
'my name is'
'alx'

|| - 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'" | ./posh
./posh: 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:

$ ./posh
[posh]-$ pwd
/home/vagrant/alx/simple_shell
$ cd ../
[posh]-$ pwd
/home/vagrant/alx
[posh]-$ cd -
[posh]-$ 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:

$ ./posh
$ exit

env

• Usage: env
• Prints the current environment.

Example:

$ ./posh
$ 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:

$ ./posh
$ setenv NAME Alx
$ echo $NAME
Alx

unsetenv

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

Example:

$ ./posh
$ setenv NAME Alx
$ unsetenv NAME
$ echo $NAME

$

Authors & Copyrights

• Kelechi Egbuta <sekaycee>
• Jo Karachi <devjokar>

More information

Posh is a simple shell unix command interpreter that is part of the low level programming module at ALX and is intended to emulate the basic sh shell. All the information given in this README is based on the posh and bash man (1) pages.