# 11. Standard Library

A tour of the built-in functions. (Networking, graphics, and database have their
own lessons: 12, 13, 14.)

## Files
```stencil
write_file("data.txt", "line 1\n");      // create/overwrite, returns bool
append_file("data.txt", "line 2\n");     // append
print(file_exists("data.txt"));          // true
print(read_file("data.txt"));            // whole file as a string (null if missing)
print(list_dir("."));                    // list of names in a folder
make_dir("output");                      // create folder(s)
delete_file("data.txt");                 // remove a file
```

## OS / system
```stencil
print(args());        // command-line args after the script: stencil app.st X Y -> [X, Y]
print(env("PATH"));   // an environment variable (null if unset)
print(cwd());         // current working directory
let out = exec("echo hello");   // run a shell command, return its stdout
print(out);
// exit(0);           // stop the program with an exit code
```

## JSON
```stencil
let data = { "name": "Ember", "langs": ["STencil", "Python"], "age": 20 };
let text = json_str(data);            // compact JSON string
print(text);                          // {"age":20,"langs":["STencil","Python"],"name":"Ember"}
print(json_pretty(data));             // indented JSON
let back = json_parse(text);          // parse JSON -> dict/list/...
print(back["name"], back["langs"][0]);  // Ember STencil
```

## Regular expressions
```stencil
print(regex_match("[0-9]+", "abc123"));          // true
print(regex_find("[0-9]+", "abc123def456"));     // 123
print(regex_find_all("[0-9]+", "a1b22c333"));    // [1, 22, 333]
print(regex_replace("[aeiou]", "merhaba", "*")); // m*rh*b*
```

## Date & time
```stencil
print(now());                       // milliseconds since 1970 (int)
print(timestamp());                 // seconds since 1970 (int)
print(date());                      // "2026-06-30 11:29:53" (local)
print(date_format("%d/%m/%Y"));     // custom format
```

## Math
```stencil
print(abs(-7), round(3.6), floor(3.9), ceil(3.1));  // 7 4 3 4
print(sqrt(144.0), pow(2, 10));                     // 12.0 1024
print(sin(0.0), cos(0.0), log(2.718281828));        // 0.0 1.0 ~1
print(min([5,2,8]), max([5,2,8]), sum([1,2,3]));    // 2 8 6
```

## Random (for games & simulations)
```stencil
print(random());                       // float 0..1
print(randint(1, 6));                  // dice roll, inclusive
print(choice(["rock","paper","scissors"]));
print(shuffle([1,2,3,4,5]));           // a shuffled copy
```

## Input
```stencil
let name = input("Your name: ");   // read a line from the user
print("Hi " + name);
```

Next: [Networking & Web](12_networking_and_web.md)