How to use sed to find the corresponding place in

问题:

Book name:author:price:Qty:Qty Sold
==================================
harry potter:james:12.99:197:101
===============================

I want to update the QTY which is the value of 197 in this case but i cant seems to be able to update the value with my program please help me as i have just started learning shell programming. Thanks

function update_qty_available
{
grep -c "$title:$author" BookDB.txt > /dev/null # Look for a line with matching values

if [ $? == 0 ]; 
then # If found then offer to change Qty
echo "Update Qty to what?"
read newQty
sed -i "s/\($title:$author\):[^:]:[^:]*:/\1:$newQty/" BookDB.txt
    echo "Book's Qty has been updated successfully!"
fi

回答1:

Refactored to use grep -q; fix the if idiom; anchor the search to start of line; use read -p instead of a separate echo; capture the price inside the parens so that it's not lost; add the missing repeat in the regex; and add back the separator colon after the new qty value; also, add an else clause so that the function doesn't fail silently.

function update_qty_available
{
    if grep -q "^$title:$author:" BookDB.txt
    then
        read -p "Update Qty to what?" newQty
        sed -i "s/^\($title:$author:[^:]*\):[^:]*:/\1:$newQty:/" BookDB.txt
        echo "Book's Qty has been updated successfully!"
    else
        echo "$0: BookDB.Txt: no '$title' by '$author'" >&2
    fi
}

回答2:

You're missing an asterisk after the first character class. And don't forget to expand the group.

回答3:

Here is how you'd do with awk:

Content of script.awk:

BEGIN {
    FS=OFS=":"
    printf "Enter the book's name: "
    getline name < "-"
    printf "Enter author's name: "
    getline author < "-"
    printf "Enter new quantity: "
    getline newQty < "-"
}

$1==name && $2==author { $4=newQty }1

Run:

$ cat BookDB.txt 
Book name:author:price:Qty:Qty Sold
==================================
harry potter:james:12.99:197:101
===============================

$ awk -f script.awk BookDB.txt 
Enter the book's name: harry potter
Enter author's name: james
Enter new quantity: 5000
Book name:author:price:Qty:Qty Sold
==================================
harry potter:james:12.99:5000:101
===============================