SHELL IN C

Make instructions:

• From the directory where the Makefile is present, run: make -f MakeFile
• Running the shell: `./main

Display:

• Once the code is executed, the shell prompt would be displayed in the format: <username@system_name:curr_dir>
• Some color code has been used to display the shell prompt and some messages.
• The functionality and the output/errors displayed for all the commands have been made to resemble bash.
• '~' corresponds to the present directory(i.e the directory from which the script is executed.)

Processing the input:

• Getting line as input
• Separating the line by semicolon(), to get each separate command.
• Processing each command using strtok(), with some DELIMITERS specified, and identifying the command, background or not etc. Also checking if two commands are separated by '&'. At this step, the command and the arguments, options are separated.
• For each command identified,
• Firstly, check if it is a shell builtin or not.
• If not, execute the command using exec, otherwise send it to the appropriate function.

cd:

• When the function corresponding to cd is invoked, the directory is changed using chdir.
• Cases like '~' etc. have been replaced with appropriate strings.
• Appropriate error message will be printed if the operation fails using perror.

ls :

• Various flags are identified, [al] in all possible combinations.
• In case of no flags, all files in the directory are printed(excluding the hidden ones), allincase of -a.
• In case of -l, file type,file permissions,file modified time,file size are fetched using stat.
• When a directory is specified, files in that directory are specified.
• In case of symlinks, link is presented in ls -l (like in bash).
• The printing format used is to emulate 'ls -l' in bash.

pwd:

• Current working directory will be printed.(fetched using getcwd()).

echo:

• The input is passed through a 'state-machine' type structure, so as to efficiently handle all the possible input types.
• All the cases when characters such as "", '', spacing, multiple occurrences of quotes etc. have been handled properly including cases when newline is printed after opening quotes.

pinfo:

• Necessary information is fetched from '/proc/pid/status' and executable path from 'proc/pid/exe'
• When no pid is specified, pid of the current process is brought from getpid().
• Appropriate error message printed when the pid could not be identified or false string has been specified.

Foreground/Background:

• Identified with '&' as described above.
• fork() command is used and the process is run in background.
• Also, once the process has exited(using waitpid()), an exit message is printed after the execution of the next command or when a new line is printed(like in bash).
• SIGTTIN signal is used to ensure the bg processes do not take input from the terminal.

Piping:

• ‘pipe’ command is used to create a read end and a write end.
• ‘dup2’ command is used to redirect stidin and stdout wherever necessary.
• Commands are seperated using ‘|’ symbol and the commands are written at one end of the pipe, read from the other.

Redirection:

• File pointers are replaced with STDIN/STDOUT using dup2.
• When “<” is given, the input is read from the given filename.
• For “>” & “>>” the files are opened appropriately to append/overwrite and to create one if it doesnt exist.
• Errors Handled: Wrong/no input files, no output files.

Other Builtin commands:

setenv:

• setenv is used to set a new variable or redifie an existing one.
• If no value for variable given, sets to empty.
• Errors handled: Too few or high arguments, setenv returns an error.

unsetenv:

• unsetenv is used to unset an existing environment variable.
• Errors handled: Too few or high arguments, unsetenv fails.

jobs:

• All the jobs present in the linked list of currently in background jobs maintained will be printed along with their running/stopped status.

kjob:

• Corresponding block in the linked list is identified to retreive the pid from the getjob function implemented..
• The signal is sent to the process with kill.
• Errors handled: Wrong pid, extra arguments, or no job id specified.

fg:

• SIGCONT signal will be issued so that the process will be run if it is stopped and is sound to input and output.
• The shell control is directed to the process using waitpid.
• The process is removed from the list of backgound processes.
• Errors handled: Wrong pid, extra arguments, or no job id specified.

bg:

• The process is continued with SIGCONT signal.
• SIGTTIN signal is also been used to ensure no input is taken from terminal(as it is a background prcoess).
• The state of the process is updated to Running from Stopped.
• Errors handled: Wrong pid, extra arguments, or no job id specified.

killallbg:

• The function implemented for kjob is used.
• All the background processes will take the above function with SIGINT signal.

quit:

• The shell loop will be stopped.
• When “quit” command is identified, a function “run_exit” is called which returns a zero value requesting shell loop to stop.

CTRL+Z:

• SIGTSTP signal handler being used.
• catchCTRL_Z is called when the CTRL+Z is pressed.
• The current fg process is stopped and is added to the list of background processes.

CTRL+C:

• SIGINT signal handler being used.
• catchCTRL_C is called when the CTRL+C is pressed.
• The action taken is that the current fg process is killed.