Minishell

Description:

Implement a minimalistic shell, mini$-$shell$($msh$)$ as part of the Linux Internal module.

Objective:

The objective is to understand and use the system calls w$.$r$.$t process creation, signal handling, process synchronization, exit status, text parsing etc..

Requirement details:

$1.$Provide a prompt for the user to enter commands

$1.$ Display the default prompt as msh$>$

$2.$ Prompt should be customizable using environmental variable PS$1$

$$ To change the prompt user will do PS$1=$NEW$\_$PROMPT

$$ Make sure that you do not allow whitespaces between $=$ i$.$e$.,$ do not allow PS$1=$ NEW$\_$PROMPT

$$ In the above case, it should be treated like a normal command

$2.$ Execute the command entered by the user

$1.$ User will enter a command to execute

$2.$ If it is an external command

$$ Create a child process and execute the command

$$ Parent should wait for the child to complete

$$ Only on completion, msh prompt should be displayed

$$ If user entering without a command should show the prompt again

$3$ Special Variables:

$1.$ Exit status of the last command $($echo $$?)$

$$ After executing a command the exit status should be available

$$ echo $$?$ should print the exit status of the last command executed

$2.$ PID of msh $($echo $$$)$

$$ echo $$: should print msh$\text{'}$s PID

$3.$ Shell name $($echo $SHELL$)$

$$ echo $SHELL: should print msh executable path

$4$ Signal handling

Provide shortcuts to send signals to running program

$1.$ Ctrl$-$C $($Send SIGINT$)$

On pressing Ctrl$-$C

$$ If a programming is running in foreground, send SIGINT to the program $($child process$)$

$$ If no foreground program exists, re$-$display the msh prompt $2.$ Ctrl$+$z $($Send SIGSTP$)$

$2.$ Ctrl$-$z $($Send SIGTSTP$)$

On pressing Ctrl$+$z

$$ The program running in foreground, should stop the program and parent will display pid of child $2\mathrm{.}5$

$5.$Built$-$in commands

$1.$ exit exit: This command will terminate the msh program

$2.$ cd cd: Change directory

$3.$ pwd: show the current working directory

$6$ Background Process $/$ Job control

$1.$ Allow a program to run in background To run a program in background use ampersand $($&$)$ after the command. For eg: sleep $50$ &

$2.$ Implement fg$,$ bg and jobs commands

$$ bg will will move a stopped process to background sleep $10$ & is equallent to sleep $10$ then ctrl $+$ z and bg

After this the msh prompt should be displayed indicating it is readyto accept further commands. After a bg process ends, cleanup the process using wait.

NOTE: You may have to use SIGCHLD signal handler for this

On termination of bg process, display its exit status.User should be able to run any number of background processes.

$$ fg will bring a background process to foreground.

Only fg bring last background process, or fg will bring given pid to foreground

$$ jobs will print all background process details.

$7$ Pipe functionality

$1.$ Allow multiple processes communication by using pipes.

$2.$ Create pipes and childs dynamically as per pipes passed from command$-$line

Eg: ls $|$ wc$,$ ls $-$l $/$dev $|$ grep tty $|$ wc $-$l$....$give the code for this please $.$help please with the code