Linux Shell in Language C

(NOTE) UNIVERSAL Error shown:

• If the user give a command other than the 10 commands given in the assignment, the program will show command not found.

Commands:

1. ls: Prints the contents of directories.

flags used: ls -1, ls -m

-1: prints every file/dir in a newline.
-a: prints directory comma seperated.
Errors: None

2. echo: Prints whatever after echo

flags used: -n, -E

-n:  this option is used to omit echoing trailing newline 
-E:  gives same output as echo without flags.
Errors: None

3. rm: Remove files and using -r can also delete directories

flags used: -r, -f

-r: remove directories and their contents recursively
-f:  ignore nonexistent files and arguments, never prompt
 Errors Shown:
    - if no file/dir is given after rm it throws an error: missing operand.

4. cat: Reads and prints files.

flags used: -n, -E

-n:  number all output lines
-E: display $ at end of each line
Errors Shown:
    - If file does not exits it will throw an error: no such file or directory

5. mkdir: Create directories

flags used: --help, -v

--help: shows help for mkdir.
-v:  print a message for each created directory
Errors Show:
    - if no file/dir is given after rm it throws an error: missing operand.
    - if the user does mkdir dir1/dir2 and dir1 doesn't exist it will show an error: directory does not exist.
    - if the user does mkdir hello and hello exits it will again give the same error: File exists.

6. date: Shows date

flags used: -u, -R

-u: print or set Coordinated Universal Time (UTC)
-R: output date and time in RFC 5322 format.
Example: Mon, 14 Aug 2006 02:34:56 -0600
Errors: None

7. cd: Goes into the directory which we specify

flags used: --help, -P

--help: shows help for cd.
-P: use the physical directory structure without following
    symbolic links: resolve symbolic links in DIR before
    processing instances of `..'
Errors: None

8. pwd: Shows current directory

flags used: --help, -P

--help: shows help for pwd.
-P:  print the physical directory, without any symbolic links
Errors: None

9. history: Showing history of commands.

flags used: -a, -c

    -a: append that command in history.
    -c: clears the history.
Erros: None

10. exit: For exiting program.

flags: None

Error: None

System Calls Used:

1)Fork(): Used for creating a new process, known as child process and it concurrently runs with the parent process.
2)execvp(): the created child process does not have to run the same program as the parent process does. The execvp system call allow a 
process to run any C file, whose executable file can be made.
3)chdir():It changes the current directory of the calling process to the directory specified in path.
4)readdir():This function returns a pointer to a dirent struct variable
pointing the next directory
5)opendir():This function opens a directory corresponding to the directory name given and returns a pointer of that directory
6)wait(): It blocks the calling process until one of the child processes exits. After child process 
exits or ends, parent continues its execution after wait call instruction.
7)unlink(): unlink() deletes a file from the filesystem.
8)remove(): used to delete a file or directory.
9)mkdir(): creates a new directory of the name passed in it as argument.
10)getcwd(): used to get current working directory
11)exit(): exits out of the program.
>>exit