#get line number of occurrence in text file and then print out x lines from that point
#get the line number of the first occurrence of our search text
linenumber=`grep -n "someuniqueitem" inventory.txt|head -n1|cut -d ":" -f 1`
#linestop represents how many lines after the occurring line we want to print out in this case "20"
linestop=$((linenumber+20))
# sed prints out the line range we want f........
First find the line number:
awk '/what you are searching for/{ print NR; exit }' input-file
86
Now use sed to replace it:
sed -i 86s/.*/"your replacement text"/ $file
Here is a full sample script to automate it:
file=some/f........