minishell

A minimal UNIX shell built in C as part of the 42 School curriculum.
It reproduces the core behaviour of bash, supporting interactive input, environment variable expansion, quoting rules, pipes, redirections, heredoc, and the standard set of built-in commands.

Authors:aragragu · ykasmi

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Table of Contents

• Requirements
• Build
• Running
• Features
  • Prompt
  • Command History
  • Environment Variable Expansion
  • Quoting
  • Pipes
  • Redirections
  • Heredoc
  • Built-in Commands
  • Signals
• Limitations
• Project Structure

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Requirements

Dependency	Notes
cc (C compiler)	Compiled with -Wall -Wextra -Werror
readline	Provides interactive line editing and history
macOS (recommended)	Makefile paths default to Homebrew readline (~/.brew/opt/readline)

macOS users: The Makefile expects a user-local Homebrew installation at ~/.brew (i.e. /Users/$USER/.brew/opt/readline). If you installed Homebrew system-wide (the standard location is /opt/homebrew on Apple Silicon or /usr/local on Intel), update CFLAGS and LDFLAGS accordingly.
Linux users: Update CFLAGS and LDFLAGS in the Makefile to point to your system's readline installation (usually /usr).

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Build

# Compile the binary
make
# Remove object files only
make clean
# Remove object files and the binary
make fclean
# Full rebuild
make re

The resulting binary is named minishell and placed in the repository root.

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Running

./minishell

The shell does not accept file arguments – it runs in interactive mode only.

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Features

Prompt

Displays an interactive prompt on every new line:

➜ (minishell)✗ 

Command History

Previous commands are accessible via the ↑ / ↓ arrow keys, powered by the GNU readline history library.

Environment Variable Expansion

Syntax	Expands to
$VAR	Value of the environment variable VAR
$?	Exit status of the most recently executed command

Variable expansion is performed inside double-quoted strings and in unquoted words.

➜ (minishell)✗ echo$HOME
/Users/aragragu
➜ (minishell)✗ echo$?
0

Quoting

Quote type	Behaviour
'single quotes'	Preserves the literal value of every character; no expansion takes place
"double quotes"	Preserves literal values except for $VAR / $? expansion

➜ (minishell)✗ echo'$HOME'$HOME
➜ (minishell)✗ echo"$HOME"
/Users/aragragu

Pipes

Multiple commands can be chained with |. Each command's standard output is connected to the next command's standard input.

➜ (minishell)✗ ls -la | grep .c | wc -l

An arbitrary number of pipes is supported.

Redirections

Operator	Description
< file	Redirect standard input from file
> file	Redirect standard output to file (truncate)
>> file	Redirect standard output to file (append)
<< DELIM	Heredoc – read input until DELIM is encountered

➜ (minishell)✗ echo hello > out.txt
➜ (minishell)✗ cat < out.txt
➜ (minishell)✗ echo world >> out.txt

Heredoc

A heredoc reads lines from the terminal until the delimiter is matched:

➜ (minishell)✗ cat <<EOF> line one> line two $HOME> EOFline oneline two /Users/aragragu

Variable expansion ($VAR, $?) is performed inside the heredoc body.
Pressing Ctrl+C during heredoc input cancels it without executing the command.

Built-in Commands

These commands are executed directly by the shell without forking a child process:

Command	Description
echo [-n] [args…]	Print arguments to stdout; -n suppresses the trailing newline
cd [path]	Change the current working directory; updates PWD and OLDPWD
pwd	Print the current working directory
export [name[=value]…]	Set or declare environment variables
unset [name…]	Remove environment variables
env	Print all exported environment variables
exit [n]	Exit the shell with optional exit code n

Signals

Signal	Interactive prompt	During command execution
Ctrl+C (SIGINT)	Displays a new prompt on a new line	Sends SIGINT to the foreground process group
Ctrl+\ (SIGQUIT)	Ignored	Sends SIGQUIT to the foreground process group (exits with code 131)
Ctrl+D (EOF)	Exits the shell	—

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Limitations

• No logical operators – && and || are not implemented (the corresponding tokens are defined but not handled in the execution engine).
• No subshell support – parenthesised expressions (cmd) are not implemented.
• Interactive mode only – the shell cannot run a script file passed as an argument.
• User-local Homebrew path – the Makefile hard-codes readline to ~/.brew/opt/readline (user-local Homebrew). Users with a standard Homebrew installation (/opt/homebrew or /usr/local) or on Linux must edit CFLAGS/LDFLAGS before compiling.
• No job control – background execution (&), fg, bg, and jobs are not supported.
• No wildcard globbing – patterns like *.c are passed literally to commands.

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Project Structure

minishell/
├── Makefile # Build rules
├── minishell.h # Shared header: all structs, enums, and prototypes
│
├── parsing/ # Lexer, parser, and pre-execution processing
│ ├── main.c # Entry point – initialisation and the main read loop
│ ├── token.c … token4.c # Tokeniser: breaks raw input into a linked list of tokens
│ ├── expanding.c … expanding5.c # Variable expansion ($VAR, $?)
│ ├── redirections.c … redirections4.c # Redirection token handling
│ ├── syntax_error.c … syntax_error3.c # Syntax validation and error reporting
│ ├── cmd.c … cmd4.c # Builds the command list (t_cmd) from the token list
│ ├── signals.c # Signal handlers and exit-status management
│ ├── split.c # Custom string splitter
│ ├── libft.c … libft6.c # Re-implemented standard C library functions
│ └── ft_itoa.c # Integer-to-string conversion
│
├── execu/ # Execution engine and built-ins
│ ├── ft_excu.c / ft_excu2.c # Core execution logic (fork, exec, pipes)
│ ├── ft_redi.c # Applies redirections at execution time
│ ├── ft_cd.c # cd built-in
│ ├── ft_echo.c # echo built-in
│ ├── ft_env.c # env built-in
│ ├── ft_exit.c # exit built-in
│ ├── ft_export.c / ft_export_norm.c / ft_export_norm2.c # export built-in
│ ├── ft_pwd.c # pwd built-in
│ ├── ft_unset.c # unset built-in
│ └── ft_utils1.c … ft_utils8.c # Shared utility functions
│
└── ft_printf/ # Minimal custom printf implementation
├── ft_printf.c
└── ft_utils.c