Ondrej Spanik (c) iairu.com 2022

Program parts

syscall.S, syscall.h

• Custom syscall calling using Assembly 0x80 interrupt method in syscall.S and interface with C using syscall.h alongside helpful comments regarding origin of other system calls.

main.c

Contents of the main function explain the flow pretty well:

1. Process any external arguments using processArgs
2. Prepare the socket if requested in arguments
3. Inform the user about how the shell is ran (on socket as client or server / without socket)
4. Prepare buffer for user input
5. Interactive shell loop consisting of:
   1. Print up-to-date prompt
   2. Retrieve user input until new line
   3. If the user input contains a built-in command, execute it internally (note: no support for arguments for now as it wasn't deemed necessary)
   4. Else proceed to argument parsing for external commands / programs
   5. Handle program execution (forking, waiting, ...)
   6. Free processed arguments from dynamic memory
6. Free buffers from dynamic memory

Additional documentation

processArgs

External arguments handling. Defines internal behavior.

parseArgs

Internal user input argument parsing into array.

Special characters

Character	Expected behavior	Implementation notes
#	comment	rest of the input is ignored
;	next input
<
>
`	`	pipe
\	escape	literal treatment of ", space, #, others NYI

Additional processing behavior:

• Double quotes " are treated as special characters unless escaped with \. They allow for (multiple) spaces to be inserted without the need for individual escapes.
• Spaces are allowed to be escaped for literal treatment.
• Check for matching pair of double quotes.
• Processing of n> (stream redirection) is not considered because it is viewed as a separate operator from >

Improvement suggestions

• Major improvements are flagged with // todo within code
• Minor improvements are flagged with // _todo within code