🐚 TinyShell

A Unix shell implemented in C, built as a semester-long university systems programming project. TinyShell supports command execution, I/O redirection, pipelines, and full job control — developed incrementally across three phases.

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Features

✅ Phase 1 — Command Execution

• Interactive prompt with fgets-based input reading
• Custom tokenizer supporting single and double quotes
• PATH-aware executable resolution (find_executable)
• fork / execv execution model with proper exit-status reporting
• Built-in exit command and EOF (Ctrl+D) handling

✅ Phase 2 — Redirections & Pipelines

Operator	Behaviour
<	Redirect stdin from file
>	Redirect stdout to file (truncate)
>>	Redirect stdout to file (append)
2>	Redirect stderr to file
|	Pipe stdout of one command to stdin of the next

• Supports multiple pipes (cmd1 | cmd2 | cmd3 | ..., up to 16 commands)
• Redirections and pipes can be freely combined

✅ Phase 3 — Job Control

• Background execution with &
• Signal handling
  • Ctrl-C (SIGINT): terminates the foreground job; shell ignores it
  • Ctrl-Z (SIGTSTP): stops the foreground job and adds it to the job list
  • SIGCHLD: automatically reaps zombie processes
• Built-in job management commands

Command	Description
jobs	List all jobs and their status (Running / Stopped / Done)
fg [%n]	Bring job to foreground
bg [%n]	Resume stopped job in background

• Job specifiers: %n, %+, %-, %%
• Race condition prevention via SIGCHLD blocking during job list updates
• Process groups for proper signal isolation (each job gets its own PGID)
• Terminal control properly transferred between shell and foreground jobs

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Project Structure

.
├── tinyshell.c      # Main shell loop, built-ins, fork/exec logic
├── utilities.c      # Parser, redirections, pipeline, job control implementation
├── utilities.h      # Structs, constants, function declarations
└── Makefile         # Build configuration

Key data structures

Command — holds parsed redirection targets for a single command:

typedef struct {
    char *inputFile;   // <
    char *outputFile;  // >
    char *appendFile;  // >>
    char *errorFile;   // 2>
    int   appendMode;
} Command;

job_t — linked-list node representing a background/stopped job:

typedef struct job {
    int      jobId;
    pid_t    pgid;
    char     command[BUFFER_SIZE];
    JobState state;   // RUNNING | STOPPED | DONE
    struct job *next;
} job_t;

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Build & Run

Prerequisites

A C compiler (gcc) and make. On Debian/Ubuntu or WSL:

sudo apt update && sudo apt install build-essential
gcc --version   # verify installation

Build

make all

Run

./tinyshell

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Usage Examples

# Basic commands
tinyshell> ls -la
tinyshell> pwd

# I/O Redirection
tinyshell> ls > output.txt
tinyshell> sort < input.txt >> sorted.txt
tinyshell> make 2> errors.log

# Pipelines
tinyshell> ls -la | grep ".c" | wc -l
tinyshell> cat file.txt | sort | uniq

# Background execution
tinyshell> sleep 10 &
[1] 4821

# Job control
tinyshell> jobs
[1]  RUNNING    sleep 10
tinyshell> fg %1
tinyshell> bg %1

# Quoted arguments
tinyshell> echo "Hello, World!"
tinyshell> grep 'search term' file.txt

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Implementation Notes

• The shell ignores SIGINT, SIGTSTP, SIGTTIN, and SIGTTOU; child processes reset these to SIG_DFL.
• SIGCHLD is blocked during fork() and job-list mutations to prevent race conditions.
• find_executable handles both absolute/relative paths (containing /) and PATH-resolution; the returned string is heap-allocated and must be freed by the caller.
• splitCommands is quote-aware, so pipes inside quoted strings are not treated as delimiters.

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Platform

Developed and tested on Linux (Ubuntu 24 / WSL2). Not tested on macOS — __strtok_r is used internally and may require adjustment on non-GNU platforms.