Load single color channel to OpenGL texture

[lechoo]

To load all channels I'm doing this:

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im.read(filename);
im.write(&blob, "RGB", 8);
glTexImage2D(GL_TEXTURE_2D, 0, 3, im.columns(), im.rows(), 0, GL_RGB, GL_UNSIGNED_BYTE, blob.data());

Loading single channel should look like this:

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im.read(filename);
im.write(&blob, "R", 8);
glTexImage2D(GL_TEXTURE_2D, 0, 1, im.columns(), im.rows(), 0, GL_RED, GL_UNSIGNED_BYTE, blob.data());

Bu it works only for red channel, if I try same with green or blue channel I'm getting black image.

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im.read(filename);
im.write(&blob, "G", 8);
glTexImage2D(GL_TEXTURE_2D, 0, 1, im.columns(), im.rows(), 0, GL_GREEN, GL_UNSIGNED_BYTE, blob.data());

Is this the right way to do it?