hsh — A Custom Unix Shell

A custom Unix-like shell written in C, designed to interpret and execute user commands in both interactive and non-interactive modes, and the shell replicates core functionality of /bin/sh, demonstrating proficiency in process management, file I/O, memory handling, and environment manipulation.

It's a real-world simulation of how a minimal shell works under the hood. Developed as part of a systems-level programming sprint at ALX Africa

Key Concepts Demonstrated

Through this implementation, we explored and applied the following systems programming concepts:

• Architecture and behavior of traditional Unix-based shells.
• The role of process IDs (PID) and parent processes (PPID).
• Execution of external programs using execve.
• Handling environment variables with setenv, unsetenv, and PATH resolution.
• Differentiating between system calls and C standard library functions.
• Building a responsive prompt that supports both interactive and piped commands.
• Graceful handling of signals and end-of-file (EOF) input.

Features

hsh currently supports:

• ✅ Interactive shell prompt and non-interactive mode (via piping or file input).
• ✅ Execution of commands with or without arguments.
• ✅ Environment variable resolution using the PATH.
• ✅ Built-in commands: exit, setenv, and unsetenv.
• ✅ Basic signal handling (e.g., Ctrl+D, Ctrl+C).

Built-in Commands:

• exit – Close the shell.
• setenv VARIABLE VALUE – Set or update environment variables.
• unsetenv VARIABLE – Remove environment variables.

All built-ins print a clear error message to stderr on failure.

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Planned Enhancements

The shell is actively evolving. Key improvements in the pipeline include:

• Built-in cd command with full spec support (cd, cd -, update PWD).
• Input/output redirection (<, >) and pipelines (|).
• Support for command chaining using ;, e.g., ls /dir1; ls /dir2.
• Command history and basic autocomplete.

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Sample Usage

Interactive Mode:

elgibbor@ubuntu~$ ./hsh
($) /bin/ls
file1  hsh  main.c  shell.c
($) exit

Non Interactive Mode:

elgibbor@ubuntu~$ echo "/bin/ls" | ./hsh
file1  hsh  main.c  shell.c
elgibbor@ubuntu~$ cat file1
/bin/pwd
elgibbor@ubuntu~$ cat file1 | ./hsh
/home/elgibbor

Output Consistency

hsh mirrors /bin/sh in output and error messaging. The only difference is the name of the executable shown in error messages (i.e., ./hsh vs /bin/sh):

/bin/sh error:

$ echo "qwerty" | /bin/sh
/bin/sh: 1: qwerty: not found

hsh error:

$ echo "qwerty" | ./hsh
./hsh: 1: qwerty: not found

Contributors

Built with passion and frustration by:

• Chiagoziem Eke (El-Gibbor)
• Bernard Frimpong (Teflon)

We welcome feedback and collaboration. Feel free to fork, open issues, or submit pull requests!