#!/bin/bash
input_str='abc_123_def_20181217T2345'
echo $input_str | sed 's/\(.*\)_\([0-9]\{8\}\).*/\1/'
# abc_123_def
#Case 2:
input_str='abc_123_def_20181217T2345_20181218T2345'
sed 's/_[0-9]\{8\}.*//' <<< "$input_str"
#abc_123_def_20181217T2345
IyEvYmluL2Jhc2gKaW5wdXRfc3RyPSdhYmNfMTIzX2RlZl8yMDE4MTIxN1QyMzQ1JwplY2hvICRpbnB1dF9zdHIgfCBzZWQgJ3MvXCguKlwpX1woWzAtOV1cezhcfVwpLiovXDEvJwojIGFiY18xMjNfZGVmCiNDYXNlIDI6CmlucHV0X3N0cj0nYWJjXzEyM19kZWZfMjAxODEyMTdUMjM0NV8yMDE4MTIxOFQyMzQ1JwpzZWQgJ3MvX1swLTldXHs4XH0uKi8vJyA8PDwgIiRpbnB1dF9zdHIiCiNhYmNfMTIzX2RlZl8yMDE4MTIxN1QyMzQ1