• Perl

open my $fh, '<', $f;
my $lines = do { local $/; <$fh> };
close $fh;

• C

FILE *file;
size_t len=0;
char *lines;
assert(file=fopen(f,"rb"));
assert(lines=malloc(sizeof(char)));

while(!feof(file))
{
	assert(lines=realloc(lines,(len+0x1000)*sizeof(char)));
	len+=fread(lines,1,0x1000,file);
}

assert(lines=realloc(lines,len*sizeof(char)));

• C

int err = 0;
int fd = 0;
void * ptr = NULL;
struct stat st;
if ((fd = open (f, O_RDONLY))
&& (err = fstat (fd, &st)) == 0
&& (ptr = mmap (NULL, st.st_size, PROT_READ, MAP_PRIVATE, fd, 0)) != -1) {
    const char * lines = ptr;
    puts (lines);
    munmap (ptr, st.st_size);
    close (fd);
}

• Clojure

(def lines (slurp f))

• C++

std::string fromFile(std::string _f)
{
    std::ifstream t(_f);
    t.seekg(0, std::ios::end);
    size_t size = t.tellg();
    std::string buffer(size, ' ');
    t.seekg(0);
    t.read(&buffer[0], size); 
}

• C#

string lines = File.ReadAllText(f);

• D

string lines = cast(string) read(f, size_t.max);

• D

auto lines = f.readText;

• Dart

var lines = new File(f).readAsStringSync();

• Elixir

lines = File.read!(f)

• Erlang

{ok, Lines} = file:read_file(F).

• Fortran

program p
   character(len=:),allocatable :: s
   open(unit=10,file='myfile',access='stream')
   inquire(10,size=i)
   allocate(character(len=i) :: s)
   read(10)(s(j:j),j=1,i)
   write(*,*)s
end program p

• Go

b, err := os.ReadFile(f)
if err != nil {
	// Handle error...
}
lines := string(b)

• Haskell

do lines <- readFile f; putStr lines

• JS

fs.readFile(f, (err, lines) => {
    if (err) {
        // Handle error...
    }

    // Work with `lines` here.
}

• Java

byte[] encoded = Files.readAllBytes(Paths.get(f));
String lines = new String(encoded, StandardCharsets.UTF_8);

• Java

File F = new File(f);
try (Scanner s = new Scanner(F)) {
    s.useDelimiter("\\R");
    String n = lineSeparator(),
           lines = s.tokens()
                    .collect(joining(n));
}

• Kotlin

File(f).readText()

• Lisp

(with-open-file (stream f)
  (uiop:slurp-stream-string stream))

• Lisp

(defvar *lines*
   (with-open-file (stream f)
      (let ((contents (make-string (file-length stream))))
            (read-sequence contents stream)
      :return contents)))

• Lua

lines = io.input(f):read('a')

• Obj-C

NSString *lines=[NSString stringWithContentsOfFile:f encoding:e error:NULL];

• PHP

$lines = file_get_contents('f');
if ($lines === false) {
    // handle error...
}

• Pascal

var
 _lines, _f: String;
 SL: TStringList;
begin
  SL := TStringList.Create;
  SL.LoadFromFile(_f);
  _lines := SL.Text;
  SL.Free;
end;

• Python

with open(f) as fo:
    lines = fo.read()

• Python

lines = open(f).read()

• Ruby

lines = File.read(f)

• Rust

let lines = fs::read_to_string(f).expect("Can't read file.");

• Rust

let mut file = File::open(f)?;
let mut lines = String::new();
file.read_to_string(&mut lines)?;

• Scala

val lines = Source.fromFile(filename).getLines().mkString("\n")

• Smalltalk

| lines |
lines := f asFilename readStream upToEnd.

• VB

Dim lines = IO.File.ReadAllText(f)