Unix Shell (C, Linux, Systems Programming)

A custom Unix-like shell developed in C that replicates core functionalities of standard Linux shells while providing several custom utilities for process management, file exploration, and system monitoring.

The project explores fundamental Operating Systems concepts such as process creation, signal handling, inter-process communication, terminal control, filesystem traversal, and command execution.

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Features

Shell Core

• Execute standard Linux system commands.
• Support for multiple commands in a single input using ;.
• Background execution using &.
• Dynamic shell prompt displaying:
  • Username
  • System hostname
  • Current working directory.
• Relative path handling similar to conventional Unix shells.

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Command Parsing

• Robust tokenization and parsing logic.
• Handles:
  • Multiple spaces and tabs.
  • Quoted strings.
  • Multiple commands in a single line.
• Supports combinations of:
  • Background processes
  • Pipes
  • Redirection

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Custom Commands

hop

Custom directory navigation command.

Supports:

hop .
hop ..
hop ~
hop <directory>

Allows sequential navigation through multiple directories.

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reveal

Custom file listing utility similar to ls.

Supports:

reveal
reveal -a
reveal -l
reveal -la

Features:

• Hidden file listing (-a)
• Detailed file information (-l)
• Multiple occurrences of flags handled gracefully

Examples:

reveal
reveal -aaa
reveal -l
reveal -la
reveal -alll

Behavior:

Command	Equivalent
reveal	reveal -a
reveal -aaa	reveal -a
reveal -laa	reveal -l
reveal -alll	reveal -l

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log

Command history management utility.

Features:

• Stores last 15 commands.
• Displays command history.
• Clears history.
• Re-executes previous commands.

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proclore

Displays information about a process.

Usage:

proclore
proclore <pid>

Information displayed:

• Process status
• Process group
• Memory usage
• Executable path

Uses the Linux /proc filesystem.

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seek

Recursive file and directory search utility.

Usage:

seek <flags> <search_term> <directory>

Features:

• Search files
• Search directories
• Permission checks
• Search with or without extensions

Examples:

seek main .
seek -f main .
seek -d src .

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activities

Displays all processes spawned by the shell.

Information shown:

• Command name
• PID
• Process state

Output is maintained in lexicographic order.

Example:

activities

Sample output:

sleep : 12345 : Running
vim   : 12350 : Stopped

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ping

Sends signals to processes.

Usage:

ping <pid> <signal_number>

Example:

ping 12345 9

Supported signal numbers:

0 - 31

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fg

Brings a background process to the foreground.

Usage:

fg <pid>

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bg

Resumes a stopped process in the background.

Usage:

bg <pid>

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neonate

Monitors newly created processes in the system.

Usage:

neonate -n <time_interval>

Example:

neonate -n 3

Uses:

• /proc
• readdir()
• select()

to periodically display newly created process IDs.

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iMan

Fetches Linux man pages from the internet using sockets.

Usage:

iMan <command>

Example:

iMan grep

Retrieves documentation from:

http://man.he.net

and prints the contents directly to the terminal.

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I/O Redirection

Supports standard Unix redirection operators.

Output Redirection

command > output.txt

Append Redirection

command >> output.txt

Input Redirection

command < input.txt

Combined Redirection

sort < input.txt > output.txt

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Pipes

Supports arbitrary-length pipelines.

Examples:

cat file.txt | grep error

cat file.txt | grep error | sort

cat file.txt | grep error | sort | uniq

Pipelines are executed from left to right using process chaining and inter-process communication.

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Pipes with Redirection

Supports combinations of pipes and redirection.

Examples:

cat file.txt | grep error > output.txt

sort < input.txt | uniq > output.txt

cat log.txt | grep warning | sort > warnings.txt

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Job Control

Supports:

• Foreground processes
• Background processes
• Process suspension
• Process continuation

Example:

sleep 60 &

Output:

[1] 12345

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Signal Handling

The shell correctly handles common terminal signals.

Ctrl + C (SIGINT)

• Terminates foreground process.
• Shell remains active.

Ctrl + Z (SIGTSTP)

• Stops foreground process.
• Moves process to background job list.

Ctrl + D (EOF)

• Gracefully exits the shell.
• Terminates active child processes.

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myshrc Configuration

Supports shell customization through a .myshrc file.

Aliases

Example:

alias ll="reveal -la"

Functions

Example:

mk_hop()
{
    mkdir $1
    hop $1
}

Example:

hop_seek()
{
    hop $1
    seek $2 .
}

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Operating Systems Concepts Used

This project extensively uses:

• Process creation using fork()
• Program execution using execvp()
• Process synchronization using wait() and waitpid()
• Signal handling using signal() and kill()
• Inter-process communication using pipe()
• Filesystem traversal using opendir() and readdir()
• Process introspection through /proc
• Terminal management using tcgetattr() and tcsetattr()
• Socket programming for online man-page retrieval

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Project Structure

.
├── src/
│   ├── shell.c
│   ├── parser.c
│   ├── hop.c
│   ├── reveal.c
│   ├── seek.c
│   ├── proclore.c
│   ├── activities.c
│   ├── neonate.c
│   ├── ping.c
│   ├── iman.c
│   ├── redirection.c
│   ├── pipes.c
│   └── signals.c
│
├── include/
│   └── *.h
│
├── myshrc
│
└── README.md

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Building

Compile using GCC:

gcc *.c -o mysh

Or using Make:

make

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Running

./mysh

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Example Session

user@system:~> reveal -la

user@system:~> seek main .

user@system:~> sleep 30 &

[1] 12453

user@system:~> activities

sleep : 12453 : Running

user@system:~> cat file.txt | grep error | sort > output.txt

user@system:~> proclore 12453

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Technical Highlights

• Written entirely in C.
• Modular architecture with separate source and header files.
• Linux process management and job control.
• Custom command implementation.
• Arbitrary-length pipeline support.
• Combined pipe and redirection support.
• Online man-page retrieval using sockets.
• Persistent command history management.
• Extensive error handling for robust shell execution.

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Future Improvements

• Tab completion
• Environment variable expansion
• Shell scripting support
• Command auto-suggestions
• Advanced job scheduler
• Command timing and profiling
• Plugin architecture for custom commands

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Author

Prakhar Gupta

B.Tech, Electronics and Communication Engineering

Indian Institute of Information Technology Design and Manufacturing (IIITDM) Kancheepuram