DESCRIPTION

I have designed a simple shell that can handle three internal commands – cd, echo and pwd and five external commands – ls, cat, date, rm and mkdir. I have implemented a shell using C library functions and Linux system calls. My code has a while loop in the main function, that runs until “exit” is typed on the command line. In the main() function, I have taken user input by using fgets() function. Then I split/parse the user input(string) using the strtok() function, that tokenises a string based on a delimiter. Then interpret() function is called, that interprets the command entered by the user, and calls the internal_commands() function or external_commands() function depending on the user input. This function also checks for “&t” at the end of the command and subsequently creates a thread using pthread_create() function.

Internal Commands

Internal commands are those which are interpreted by the shell program itself without requiring a different program to handle the expected operations. Internal commands implemented are as follows-

cd

It is used to change the current working directory to a specified directory. cd has been implemented using getcwd() and chdir().

Flags implemented :

1. cd.. --> This command is used to move to the parent directory of current directory
2. cd~ --> This command is used to change directory to the home directory.

echo

It is used to display line of text/string that are passed as an argument.

Flags implemented :

1. -n --> When this flag is used, it doesn’t append a new line.
2. -e --> This flag enables interpretation of backslash escapes.

pwd

This command prints the current working directory of the shell.

Flags implemented :

1. -L --> This flag uses PWD from the environment. It is used for showing the symbolic positioning of current directory.
2. -P --> This flag is used for showing the physical positioning of current directory.

exit

This command exits the user from the shell.

External Commands

External commands are commands that aren’t implemented by the shell itself. The shell creates a new child process which executes these commands. The following system calls have been used to run all external commands :

• fork()
• execvp()
• waitpid()

In order to handle the external commands, a child process is created every time any of the external commands are used on the shell. It creates a child process using fork() system call and using execvp() system call which is used to replace the current process with a new process, we pass the arguments to the external commands program and access them. We use waitpid() system call to allow the parent process to wait for the termination of the child process. Error handling is done for system calls using perror().

date

This command prints the system date and time.

Flags implemented :

1. -u --> This flag prints the time in Universal Time Coordinated (UTC) / Greenwich Mean Time (GMT).
2. -R --> This flag prints date and time in RFC 5322 format.

cat

This command is used to concatenate files and print them on the standard output.

Flags implemented :

1. -E --> This flag adds $ at the end of each line of the file.
2. -n --> This flag numbers all the lines in the file.

ls

This command is used to list all files in a directory. The ls command implementation makes use of the dirent structure in C, which stores directory entries. Firstly, a directory is opened using opendir(), and then individual entries are read using the readdir() function. At last the directory is closed using closedir().

Flags implemented :

1. -a --> Using this flag, the command does not ignore entries starting with .
2. -m --> This flag displays all files and directories inside current directory separated by commas

rm

This command is be used to remove files or directories.

Flags implemented :

1. -i --> This flag takes confirmation from the user before deleting a file.
2. -v --> This flag displays a message after removing a file.

mkdir

This command is used to create new directories.

Flags implemented :

1. -p --> Takes a path and creates all of the necessary directories on the path that don’t exist.
2. -v --> This flag prints a message after creating a directory.