Assignment 2 — POSIX Shell

A small interactive shell for Linux, written in C++17. It provides a command prompt, a set of built-in commands, external command execution (foreground and background), I/O redirection, pipelines, job signals, and a persistent command history.

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Build

make            # optimised build  -> ./a.out
make debug      # -O0 -g with AddressSanitizer + UBSan
make run        # build and start the shell
make clean      # remove objects and the binary

Requirements: g++ with C++17 support and a Linux system (the pinfo command reads /proc, so it is Linux-only).

The build is incremental — each .cpp compiles to its own .o, so editing one file recompiles only that file. Override the toolchain or flags from the command line if needed, e.g. make CXX=clang++ or make CXXFLAGS="-O3 -march=native".

Run

./a.out

The prompt has the form:

<user>@<host>:~<path>>

where <path> is the working directory shown relative to the directory the shell was started in (that directory itself is shown as ~). Directories outside it are shown by their absolute path.

Type exit (or press Ctrl-D) to leave the shell.

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Features

Prompt

Redrawn before every command. Shows the effective user, the hostname, and the working directory relative to the shell's launch directory.

Built-in commands

Command	Behaviour
cd [dir]	Change directory. Supports no argument and ~ (launch dir), - (previous dir, prints the new path), ., .., ~/sub, and absolute/relative paths. Reports the reason on failure.
pwd	Print the absolute working directory.
echo [args...]	Print the arguments separated by single spaces. Single and double quotes group text and are removed.
ls [-a] [-l] [path...]	List directory contents (or a single file). -a includes dotfiles, -l shows the long format. Flags may be combined (-al). Entries are sorted. Multiple paths get a path: header.
pinfo [pid]	Show pid, state, virtual memory, and executable path for a process (defaults to the shell). Reads /proc.
search <name>	Recursively look for a file or directory of that name under the current directory. Prints True or False.
history [n]	Show the last n commands (0–20, default 10). History is de-duplicated for consecutive repeats and capped at 20 entries.
exit	Terminate the shell.

External commands

Anything that is not a built-in is run via fork + execvp.

• Foreground: the shell waits for the process and stays responsive to Ctrl-C / Ctrl-Z, which are forwarded to the child.
• Background: append &. The shell prints [bg] <pid> and continues. The job runs in its own process group so it does not steal the terminal, and its completion (<pid> done) is reported before the next prompt.

I/O redirection

<, >, and >> are supported and may be combined:

cat < input.txt
ls -l > listing.txt
echo more >> listing.txt
sort < data.txt > sorted.txt

Output files are created with mode 0644 (> truncates, >> appends). A missing input file or a missing filename after an operator is reported and the command is skipped.

Pipelines

Any number of stages separated by |:

cat file.txt | head -20 | tail -5 | wc -l

All stages run concurrently, connected by pipes; the shell waits for every stage. Redirection may be used inside a pipeline stage.

Multiple commands

Separate commands on one line with ;:

cd src ; ls -l ; pwd

Signals

• Ctrl-C (SIGINT) — interrupts the foreground job, not the shell.
• Ctrl-Z (SIGTSTP) — stops the foreground job and returns to the prompt.
• Ctrl-D (EOF) — logs out.

History persistence

Commands are stored in history.txt in the shell's launch directory. The file is kept trimmed to the last 20 entries and is reloaded on the next startup.

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Source layout

File	Responsibility
main.cpp	Startup, signal setup, the read–eval loop.
headers.h	Shared includes and function declarations.
prompt.cpp	Prompt rendering.
tokenize.cpp	Splitting input by whitespace, ;, `
executor.cpp	Dispatch: ; splitting, pipeline/redirection/built-in routing, signal handlers.
cd.cpp, pwd.cpp, echo.cpp, ls.cpp	Built-ins.
background.cpp	fork/exec for external commands, background job tracking and reaping.
redirection.cpp	< / > / >> handling.
pipe.cpp	Pipeline setup and execution.
pinfo.cpp	/proc inspection.
search.cpp	Recursive file search.
history.cpp	History ring buffer and file persistence.
reference.cpp	Earlier standalone reference implementation. Not part of the build.

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Notes and limitations

• pinfo requires /proc and therefore Linux.
• Built-ins (cd, history, …) inside a pipeline run in a child process, so cd in a pipeline does not affect the shell — this matches standard shells.
• Environment-variable expansion ($VAR), globbing (*), and command substitution are not implemented.