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Basic-Linux-Unix-Shell

Designing a simple unix shell in C

Linux (and other Unix like OSes), have “shells” which provides an interface to the unix system.

The program handles the following 2 types of commands through a command line interface for the shell:-

  1. Internal Commands: Those which are interpreted by the shell program itself, without requiring a different program to handle the expected operations.
  2. External Commands: Those commands which are not handled directly by the shell program but by an external program.

This shell can handle 3 internal commands("pwd","echo","cd") and 5 external commands("ls","date","cat","rm","mkdir"). The shell and its commands have been implemented using standard C libraries and linux system calls; the external commands have been handled by executing fork(), exec() & wait() family of system calls. A thread based execution using pthread_create()/system() can also be used for running these commands. The program also throws error messages if invalid commands are entered by the user.

Each command is capable of handling 2 different options. The following options have been handled for each command:-

  1. echo: used to display any text/string that is passed as an argument after ‘echo’. Options: ● echo -n: does not print a trailing newline character after its execution. ● echo -e(\b only): removes all spaces between the text passed as argument.

  2. pwd: prints the working directory. Options: ● pwd -L: displays value of PWD environment variable along with absolute path name. ● pwd -P: displays absolute path name of the active directory.

  3. cd: used as change directory command. Options: ● cd .. : used to switch to the parent directory. ● cd / : used to change directory to the root directory.

  4. ls: used to list the contents of a directory. Options: ● ls -a: shows all the files including hidden files. ● ls -l: show files in a long list format.

  5. cat: reads data from files and displays its contents in different formats. Options: ● cat -n: displays contents of a file along with line numbers. ● cat -E: appends “$” character after every line in the file and prints the output.

  6. mkdir: used to create new directories. Options: ● mkdir -v: displays message after creating the directory. ● mkdir -p: creates parent directories as well if they do not exist.

  7. rm: used to remove objects from a directory. Options: ● rm -i: displays prompt before removing file. ● rm -f: removes a file forcefully.

  8. date: used to display system date, time and other information. Options: ● date -u: displays GMT. ● date “+%D”: displays date in dd/mm/yyyy format.

Steps to run the program:-

  1. Clone the entire repository.
  2. make; it should successfully compile all the programs.
  3. Run the following command: ./shell
  4. make clean; after exiting the program

Notes: For thread based execution; enter "&t" after the command. The program should be run preferably in unix based system