sed, 4

/./q 

Waits for a line of input that has 1 or more characters, then quits.

Try it online. But it works better on a live shell:

sed '/./q' 

Python 3, 18 bytes

while''==input():0 

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JavaScript, 37 22 17 bytes

while(!prompt()); 

while(!prompt());

Explanation

The while keyword starts the while loop. In the condition of the loop, !prompt() asks for input and checks whether it is given or not. If it isn't given, the body of the loop is executed, which in our case is empty, then the interpreter goes back to the loop condition. The same process happens over and over again until the user gives the input.

HTML5, 33 22 bytes

<form><input required> 

<form><input required>

Explanation

The required attribute on the <input> causes the browser to inform the user "this field is required"-sort-of-message, if they do not enter a value. However, when they enter a value, the value is sent to the URL of the action attribute of the <form> (which in our case is the current file itself, since we haven't specified any value explicitly).

This was tested on the latest version of Google Chrome (version 55.0). May work in other browsers and versions.

Java, 55 bytes

void f()throws Exception{while(System.in.read()==10);}} 

If I remember correctly (it's been a while since I've been active on PPCG), my program can just be a function.

This is system specific; it only works on systems where the end-of-line character is a single newline. If the end-of-line character is instead a carriage return, replace the 10 with 13. On Windows, this doesn't work, as end-of-line on windows is \r\n.

This makes use of the fact that I can read directly from System.in.

Pyth, 3 bytes

W!w 

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Translated to Python:

while Pnot(input()): pass 

Jelly, 3 bytes

ɠṆ¿ 

Not much to look at on TIO, I'm afraid.

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How it works

This is a niladic program, meaning that it takes no input arguments. The implicit argument and return value are both 0 in this case.

¿ (while) is a quick that pops two links from the link stack: a condition (Ṇ) and a body.

Ṇ is a monadic atom: flat logical NOT. If the previous return value is falsy (here, 0 or an empty string), it returns 1 and the body is called.

ɠ is a niladic atom; it reads a raw line from STDIN and returns the result.

Once ɠ reads a non-empty line, Ṇ returns 0 and we break out of the loop.

brainfuck, 26 bytes

+[>,----------[[-]<[-]>]<] 

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Perl 5, 8+1 (-p or -n flag) bytes

/./&&die 

Takes input from stdin and dies as soon as the regex matches anything except a newline.

Bash, 51 bytes (39 without "#!/bin/bash")

It's my first time participating in PPCG, so dont be to rude ;D

#!/bin/bash read a while [ -z "$a" ] do read a done 

C, 52 bytes, 33 bytes, 29 bytes

-19 bytes thanks to Justin

-4 bytes thanks to Christoph

main(){while(getchar()==10);} 

10 is equal to '\n' - In case this isn't obvious.

R, 27 24 23 22 bytes

while(!sum(scan()^0))t 

Takes input from stdin, and repeat as long as input is of length 0. Cut off some bytes due to Jarko Dubbeldam and MickyT. Replaced the {} with t to save another byte.

Java, 128 126 bytes

2 bytes thanks to Kevin Cruijssen.

import java.util.*;class a{public static void main(String[]a){for(Scanner s=new Scanner(System.in);s.nextLine().isEmpty(););}} 

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AWK, 8 11 bytes

NF{exit} 

Wait for input. If the number of fields in input is more than 0, exit.

EDIT:

I've just realized that this doesn't work for input containing whitespace characters only. IFS needs to be set to empty string using -F "" command line option. Therefore +3 bytes.

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C# (.NET Core), 66 bytes

class C{static void Main(){for(;System.Console.ReadLine()!="";);}} 

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-6 bytes thanks to raznagul.

QBasic 4.5, 15 bytes

INPUT a$:IF a$=""THEN RUN 

Asks for input, then checks if any was given. If not, RUN restarts the program.

Ruby, 13 bytes

gets until/./ 

I wish gets took a Regexp argument.

APL (Dyalog Unicode), 5 bytes

Full program.

→''≡⍞ 

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→ go to line…

''≡ the empty string matches…

⍞ get line of input

If the empty string matches, the comparison gives 1, and we go to line 1, which is the current line, causing a loop. If the empty string doesn't match, the comparison gives 0, and going to line 0 means quitting.

PHP, 18 bytes

while(!readline()) 

SpecBAS - 34 bytes

1 DO : INPUT a$: LOOP UNTIL a$<>"" 

Just loops until non-empty string is entered.

Perl, 15 12 bytes (11 + -n flag)

-3 bytes thanks to manatwork

1/0if$/ne$_ 

Explanation:

The -n flag places a read loop around the program, turning it into:

LINE: while (<>) { 1/0 if $/ ne $_ # Spaces added for clarity. } 

Where "<>" reads a line from stdin to the variable $_. If the line contained more than the ending newline (ne is string inequality, $/ is the input record separator which defaults to newline), a division by zero is performed which causes the program to output an error message and terminate.

ZX81 BASIC, 20 bytes

10 INPUT A$ 20 IF NOT LEN A$ THEN RUN 

Because ZX81 BASIC is tokenised, NOT LEN A$ is a byte shorter then A$<>"". Program lines count 5 bytes each including the line number.

Mathematica, 26 20 bytes

For[,Input[]==Null,] 

Haskell, 19 17 bytes

f=do""<-getLine;f 

Defines a function f that if it reads the empty line calls itself again. When reading a non-empty line an exception is raised and the recursion is stopped.

Edit: @Laikoni replaced the parameter with a pattern match and saved 2 bytes. Thanks!

Aceto, 9 5 4 bytes

read a value, negate it (!; implicitly casting to a boolean), and mirror horizontally if the value is truthy (|).

! r| 

Java, 66 64 60 bytes

int a(){return!System.console().readLine().isEmpty()?0:a();} 

My first Java answer, would love some tips!

C (clang), 24 bytes

f(*i){gets(i);*i||f(i);} 

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Charcoal, 4 bytes

Ｗ¬Ｓω 

Explanation

Ｗ While ¬Ｓ Logical not of string input ω Print "" 

05AB1E, 5 bytes

[IõÊ# 

Explanation:

[ Start infinite loop I One line of input Ê Not equal to õ Empty string # Break if true 

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