OS-mimic

A simple, mainly educational shell/OS simulator written in C. Implements a command-line shell with in-memory variables, batch script execution, a toy multiprocessing model (PCBs + scheduling queue), and CPU scheduling policies (FCFS, SJF, Round Robin, Aging).

Features

• Interactive shell -- reads commands from stdin and dispatches them.
• Batch mode -- automatically detected when stdin is not a TTY (e.g. ./mysh < script.txt).
• Shell variables -- set, print, echo $VAR backed by a simple key/value store (shellmemory.c).
• File system helpers -- my_ls, my_mkdir, my_touch, my_cd.
• Script execution
  • source SCRIPT -- run a single script to completion (FCFS).
  • exec P1 [P2] [P3] [POLICY] [#] --— load 1–3 programs as separate "processes" (PCBs), schedule them under a chosen policy, and run them. The trailing # puts the shell itself into "background mode," so the rest of stdin becomes an additional process in the schedule.
• Real process execution -- run PROGRAM [ARGS...] forks and execvps an actual external program, waiting for it to finish.
• Command chaining -- commands separated by ; on one line are run in sequence.

Building

make

This produces the mysh executable. To clean build artifacts:

make clean

Running

Interactively:

./mysh
$ help
$ set X hello world
$ print X
$ quit

As a batch script:

./mysh < myscript.txt

Command reference

Command	Description
help	Lists available commands.
quit	Prints "Bye!" and exits.
set VAR VAL...	Assigns (possibly multi-word) value to a shell variable.
print VAR	Prints the value of a variable, or "Variable does not exist".
echo TOKEN	Prints a literal token, or the value of $VAR if prefixed with $.
my_ls	Lists files in the current directory (dotfiles hidden, sorted).
my_mkdir NAME	Creates a directory (name must be alphanumeric, or $VAR resolving to one).
my_touch PATH	Creates an empty file at PATH.
my_cd PATH	Changes the current working directory.
source SCRIPT	Runs SCRIPT to completion under FCFS.
exec P1 [P2] [P3] POLICY [#]	Schedules 1–3 scripts under the given policy; # backgrounds the shell.
run PROG [ARGS...]	Forks and execs a real external program.

Scheduling policies [POLICY]

Implemented in schedule_policy.c / queue.c, selected by name when calling exec:

Policy	Behavior
FCFS	Runs each process to completion, in arrival order.
SJF	Shortest-Job-First; runs to completion; ties broken by FCFS.
RR	Round Robin; each process gets a 2-instruction time slice.
RR30	Round Robin with a 30-instruction time slice.
AGING	Priority scheduling with aging (1-instruction slices); durations of waiting processes decrease each dequeue, and ties at the head are handled specially so equal-length programs preserve arrival order on initial scheduling.