Shell

Assignment 2/3 - Operating Systems And Networks, Monsoon 2022

Building

Tested on fedora 35 running on kernel 5.19.4. Tested compilation on gnu make 4.3 and gcc 11.3.1. Run make all in the root directory to build. Run ./batak to start the shell.

Navigating the source

The src folder contains all the source code of the project. The source is mainly divided into three directory:

• src Contains the main file, and utilities required by components all over the project.
• src/core Contains the core components of the shell, like execute.c which is executes commands, foreground and background processes.
• src/builtins Contains the builtin commands.

Files

File	Description	File	Description
main.c	contains the main function and signal handling code	utils.(c/h)	contains utility functions
logger.(c/h)	contains utilities for info and error logging	globals.h	contains global macros and variables, defined by extern
builtins/builtins.h	declares all the builtin command and functions for initialising them	builtins/builtins.c	defines functions for initialising builtin commands
builtins/*.c	contains definitions of builtin commands
core/execute.(c/h)	declares and defines functions for executing commands	core/history.(c/h)	contains code for loading, saving and recording commands in history
core/parse.(c/h)	declares and defines functions for parsing commands, then executes them using functions declared in execute.h	core/process_list.(c/h)	defines a linked list for storing all the background processes started by the shell
core/prompt.(c/h)	declares and defines function for printing the prompt, uses state variables from globals.h to show last command status and time	core/io.(c/h)	defines a print function which prints to stdout and keeps track of number of lines printed, written for fixing the double prompt bug
core/autocomplete.(c/h)	contains code for handling autocompletion and displaying the possible completions

Usage

The working directory of the shell at startup, that is the directory where it was executed from, is considered to be the home.

Commands can be chained with ; or &. ; will run the previous command in foreground, whereas & will run it in background.

tab can be pressed to trigger autocompletion.

Pipe And Redirection

Processes can have their input output redirected to files or to another process. You can pipe output of one process to another by using | and it can be chained any number of times.

You can take input from a file for the first process in a pipeline and redirect the last processes output to a file.

• cat file.txt | wc
• ls | cat | wc
• cat < file.txt | wc > out.txt

Note: For now pipelines cannot run in background. They can be suspended and continued but they cannot run in background. Attempting to do so will result in a parsing error.

Inbuilt Commands

• exit: exits the shell
• echo: <space-tab-separated-strings> prints the strings as it as, while ignring the spacing.
• cd: -, ., .., <relative-path>, <absolute-path>
• pwd: prints the current working directory. Takes no arguements.
• ls: -a, -l, <path(s)>
• pinfo: <optional-pid>
• history: prints the last 10 commands
• discover: <target-dir> -d -f <filter> Search for files/directories recursively in the given directory
• jobs: -r, -s prints all stopped or running background jobs started by the shell
• sig: <index> <signal> raises <signal signal for the job with index <index> in jobs list
• fg: <index> brings the job with index <index> to the foreground and continues it if it is stopped
• bg: <index> continues a background job with index index if it is stopped

More info

cd : change directory

• Flags: -
  • -: Goes to previous directory and prints its path. By default the directory where the shell was executed from is considered to be the previous directory.

ls : list files/directories

• Flags: a, l
  • a: display hidden files
  • l: display extra info

Accepts paths to both directories and files. For directories it will list all the files and directories inside it.

pinfo : process information

Gives information like status and virtual memory size for the current process or process with the given PID.

Sample Output

pid : 205888
process status : R+
memory : 1980
executable path : ~/batak

history : command history

Records commands in a file and saved across sessions. Currently, the values of how many commands to store and display are fixed.

discover : find files

• Flags: d, f
  • d: search for directories
  • f: search for files
• Example:
  • discover -d src "build" this will output all folders named in build inside src
  • discover -f src ".c" this will output all files with extension .c inside src
  • discover -d -f src "tmp" this will output all folders/files with named tmp inside src
  • discover -d -f ~ thi will output all files/folders in ~
  • discover ~ thi will output all files/folders in ~

Lists all files/folders(depending on flags) matching the filter, in the given target directory. For now <filter> just accepts normal strings.