# Applications

ModuOS includes several built-in and userland applications.

## Built-in Applications

### Zenith4 Shell
The main shell interface for ModuOS.

**Features**:
- Command execution
- File operations
- Process management
- Drive mounting

**Usage**: Automatically started at boot

### Real-Time Clock (rl-clock)
Display current date and time from RTC.

**Command**: `clock`

## Userland Programs

### cat.sqr
Display file contents to console.

**Usage**: `cat <filename>`

**Example**:
```
# cat /ModuOS/System64/pcname.txt
ModuOS-PC
```

### echo.sqr
Print text to console.

**Usage**: `echo <text>`

**Example**:
```
# echo Hello, World!
Hello, World!
```

### sh.sqr
Alternative shell implementation.

**Usage**: `sh`

### memtest.sqr
Memory testing utility.

**Usage**: `memtest`

**Tests**:
- Memory allocation/deallocation
- Read/write verification
- Heap integrity

### zsfetch.sqr
System information display (similar to neofetch).

**Usage**: `zsfetch`

**Displays**:
- OS name and version
- CPU model
- Memory usage
- Storage devices
- Uptime

## Developing Applications

> See also: [SQR / SQRM / SQRL Formats](SQRELf-Formats.md)

### ELF Format
User programs must be in ELF64 format.

### Compilation
```bash
cd userland
x86_64-elf-gcc -c program.c -o program.o
x86_64-elf-ld -T user.ld program.o -o program.sqr
```

### Entry Point
```c
void _start(void) {
    // Program entry point
    main();
    syscall_exit(0);
}
```

## Next Steps

- [Games](Games.md) - Built-in games
- [Shell Commands](Shell-Commands.md) - Shell usage
- [System Calls](System-Calls.md) - Programming interface