In this exercise we will implement a simple shell in the C language under the Linux operating system. The shell will display a prompt to the user, read the commands and send them to the operating system for execution. and the three characteristics detailed in "Introduction and background" and they are $(1)$ alias $(2)$ prompt and $(3)$ running

script file

General requirements from the shell will be as follows:

$1.$ Input of a command from the user

$$ The maximum length of the command including the arguments is $1024$ characters.

$$ The maximum number of arguments is $4.$ If there are more than $4$ arguments, the command will be defined as invalid

$$ No limit can be placed on the number of characters of each argument $($or of the template name$)$ but their sum is less than $1024!$

$$ There can be several spaces between one argument and another, but it can be assumed that there are no other characters between them $($such as $(\text{'}$r$\text{'},\text{'}\backslash$t$\backslash \text{'}$

$2.$ Running the command

The commands will be sent to the operating system for execution using the execvp $|$ commands fork. The lecture will be by having the program generate a child process. The son will be the one who will actually run the command using the execvp command $-$ details given in class and practice

$3.$ The shell will exit when the string exit$\_$shell is typed. When the shell exits $-$ it will print to the screen the number of commands including dashes $($only a number will be printed without any additions$)$ for example if one of the commands was "echo $"$hi assaf $1$ will be printed $4.$ The structure of the prompt required in the exercise

#cmd: $|$ #alias: $|$ #script lines:$>$

The details of the elements in:prompt$-$

#cmd::

Represents the number of legal commands $($commands$)$ that have been successfully executed in the shell since the start of the run. Valid commands are commands that the system was able to run without errors.

#alias::

Represents the number of aliases currently active in the system. An active alias is a shortcut to a command that was defined by the user using the alias command and was not removed using the unalias command.

#script lines::

Represents the total number of command lines $-$ valid or invalid. $($which have been run from scripts $($source$)$ since the start of the shell's run.

prompt$-$example for

In this example:

Five valid commands were executed successfully.

#cmd:$5]$#alias:$31$#script lines:$20>$

Three active aliases are currently configured.

Twenty command lines were run from scripts.

The prompt must be updated every time a valid command is executed, an alias is set or removed, or a command line is executed from a script.

$5.$ Alias $$definition

Defining an alias when the user types a command in the following structure:

alias $=\text{'}$

The program must define as a shorthand for the command, for example:

will define $||$ In short for the command $|-$ Is run alias when the user types a command defined in alias on the program

alias ll$=\text{'}$ls $-$l$\text{'}$

Replace the command with the full command before executing it$.$ for example:

will cause a run

Is $-|$

Displaying an alias When the user types only an alias, the program must display all the

$-$aliases defined as a list. for example:

Introduce:

Removing an alias when the user types a command in the following structure:

The program should remove the abbreviation II

$6.$ Script files

Reading a script file:

As for the user typing a command in the following structure:

II$=\text{'}$ls $-$l$\text{'}$

alias

unalias $11$

source

The program must read the file