awk '{ print gensub(/^(.{0,100}).*/, "\\1", "g", $0);}'

# or

cut -c -100

# or

awk -v len=100 '{ if (length($0) > len) print substr($0, 1, len-3) "..."; else print; }'

text processing – Shorten lines, adding “…” ellipsis – Ask Ubuntu
cut – How to truncate text lines to N characters maximum? – Unix & Linux Stack Exchange