Library Overdue Report

Gluing it all together

This last exercise is a small report script, and every piece of it is something you have already used: a here-document to build a data file, two functions, a case, a while read loop, arithmetic, printf, and a couple of pipelines at the end. The one new idea is how to read a line that has several fields in it.

read splits each line into words using the characters in IFS — by default, spaces and tabs. If your file uses commas, tell read to split on commas instead, just for that one command, by putting IFS=, in front of it:

while IFS=, read -r who kg; do

Because the assignment is written on the same line as read, it applies only to read; the rest of the script keeps its normal word splitting. Without it, Dune,3 lands in the first variable as one word and the second variable is empty — and a title with a space in it, like The Hobbit,0, gets split in the wrong place entirely.

Two other idioms to notice: sort -t, -k2 -rn sorts on the second comma-separated field as a number, largest first; and cut -d, -f1 keeps only the first comma-separated field.

A worked example

cat > parcels.txt <<'END'
Ana,2
Bo,5
END
cost() {
  echo $(( $1 * 3 ))
}
while IFS=, read -r who kg; do
  c=$(cost "$kg")
  case "$kg" in
    [0-3]) size=small ;;
    *)     size=large ;;
  esac
  printf "%-4s %2d kg %3dp %s\n" "$who" "$kg" "$c" "$size"
done < parcels.txt
echo "Heaviest: $(sort -t, -k2 -rn parcels.txt | head -1 | cut -d, -f1)"

Ana   2 kg   6p small
Bo    5 kg  15p large
Heaviest: Bo

Each line is split on the comma, the function's printed result is captured with $(...), the case picks a size, and printf lines the columns up. The final pipeline sorts by weight, keeps the top line, and trims it to the name.

Your turn

The editor holds a library overdue report. loans.txt has four books with the number of days each is overdue. For each book the script should print the title, days, fine at 25p a day, and the action: "on time" for 0 days, "reminder" for 1 to 6, "final notice" for more. Then it prints the total owed, how many were on time, and the most overdue title.

Two lines are marked. The loop's read has no IFS=,, so titles and days are not being separated. The fine is captured without $(...), so fine is never a number.

Fix both, and check every column of your output against the expected output — the widths in the printf are part of the answer.

If something goes wrong

If the titles print with ,3 still attached and the days column is all zeros, IFS=, is missing from the read line.

printf: fine_for: invalid number means the fine is not being captured. Wrap the function call in $(...).

If a title with a space in it comes out as just its first word, IFS=, is missing too — with default splitting, the space inside the title is what read cuts on.