回答:
変数がURLの末尾になく、変数がアンダースコアの間にある場合:
example.com/imageID_1_2018.gif
example.com/imageID_2_2018.gif
example.com/imageID_3_2018.gif
。
。
。
example.com/imageID_100_2018.gif
wget example.com/imageID_{1..100}_2018.gif
同様に:
for ((i=1;i<=100;i++)); do wget example.com/imageID_${i}_2018.gif; done
$i
、引用符で:wget "example.com/imageID=$i"