You can use the #version command as the first line of your shader to specify GLSL version:

#version 120

void main() {
    gl_FragColor = vec4(1.0);
}

GLSL versions are released alongside GL versions. See the following charts to decide which version you would like to target.

GLSL Versions

GLSL ES Versions (Android, iOS, WebGL)

OpenGL ES has its own Shading Language, and the versioning starts fresh. It is based on OpenGL Shading Language version 1.10.

So, for example, if a feature is available in GLSL 120, it probably won't be available in GLSL ES 100 unless the ES compiler specifically allows it.

Differences at a Glance

Differences between (desktop) GLSL versions.

Version 100

Vertex shader:

uniform mat4 projTrans;

attribute vec2 Position;
attribute vec2 TexCoord;

varying vec2 vTexCoord;

void main() {
    vTexCoord = TexCoord;
    gl_Position = u_projView * vec4(Position, 0.0, 1.0);
}

Fragment shader:

uniform sampler2D tex0;

varying vec2 vTexCoord;

void main() {
    vec4 color = texture2D(tex0, vTexCoord);
    gl_FragColor = color;
}

Version 330

As of GLSL 130+, in and out are used instead of attribute and varying. GLSL 330+ includes other features like layout qualifiers and changes texture2D to texture.

Vertex shader:

#version 330

uniform mat4 projTrans;

layout(location = 0) in vec2 Position;
layout(location = 1) in vec2 TexCoord;

out vec2 vTexCoord;

void main() {
    vTexCoord = TexCoord;
    gl_Position = u_projView * vec4(Position, 0, 1);
}

Fragment shader:

#version 330
uniform sampler2D tex0;

in vec2 vTexCoord;

//use your own output instead of gl_FragColor 
out vec4 fragColor;

void main() {
    //'texture' instead of 'texture2D'
    fragColor = texture(tex0, vTexCoord);
}

Other Significant Changes

GLSL 120 Additions

• You can initialize arrays within a shader, like so:

float a[5] = float[5](3.4, 4.2, 5.0, 5.2, 1.1);
float b[5] = float[](3.4, 4.2, 5.0, 5.2, 1.1);

However, the above is not supported on Mac OSX Snow Leopard, even with GLSL 120.(1)

• You can initialize uniforms in a shader, and the value will be set at link time:

uniform float val = 1.0;

• You can use built-ins like sin() when setting a const value
• Integers are implicitly converted to floats when necessary, for example:

float f = 1.0; <-- valid
float g = 1; <-- only supported in GLSL 120
vec2 v = vec2(1, 2.0); <-- only supported in GLSL 120

• You can use f to define a float: float f = 2.5f;

GLSL 130 Additions

• int and uint support (and bitwise operations with them)
• switch statement support
• New built-ins: trunc(), round(), roundEven(), isnan(), isinf(), modf()
• Fragment output can be user-defined
• Input and output is declared with in and out syntax instead of attribute andvarying

GLSL 150 Additions

• texture() should now be used instead of texture2D()

GLSL 330 Additions

• Layout qualifiers can declare the location of vertex shader inputs and fragment shader outputs, eg:

layout(location = 2) in vec3 values[4];

Formally this was only possible with ARB_explicit_attrib_location extension