Bakery Oven Times

Naming a block of commands

A function is a named block of commands you can run as if it were a command of its own:

label() {
  printf "tray-%02d\n" "$1"
}

Inside the function, $1 is the first word given after its name, $2 the second, and so on — the same way a command receives its arguments. local makes a variable belong to the function alone, so it cannot clash with a variable of the same name outside.

Two rules trip people up. First, the shell reads a script top to bottom, so a function must be defined above the first line that calls it; called too early, it is just command not found. Second, a function hands its result back by printing it. To catch that printed text in a variable you wrap the call in $(...): first=$(label 3). Write first=label 3 instead and the shell reads that as "set first to the word label, then run a command called 3".

The %02d in the format string means "a whole number, at least two digits wide, padded with zeros" — which is how 7 becomes 07 on a clock.

A worked example

label() {
  local n=$1
  printf "tray-%02d\n" "$n"
}
first=$(label 3)
echo "First tray is $first"
label 12

First tray is tray-03
tray-12

The first call is captured with $(...) and used inside a sentence. The second call is not captured, so its output goes straight to the screen.

Your turn

The editor holds a bakery oven timetable. A function done_at takes an hour, a minute and a bake time in minutes, works out when the batch comes out, and prints it as HH:MM. Three batches are listed.

Three lines are marked. The first call sits above the function definition, so at that point the function does not exist yet — move the call (and its echo) below the definition. The printf uses %d instead of %02d, so 9:10 loses its leading zero. And one capture is written without $(...), so out is set to the wrong thing and the shell tries to run 7 as a command.

Fix all three so every batch prints a proper HH:MM time.

If something goes wrong

done_at: command not found means the function is being called before it is defined. Definition first, calls after.

7: command not found means a call is missing its $(...). The whole call — name and arguments — goes inside the parentheses.

If a time prints as 9:10 rather than 09:10, the format string is still %d. Use %02d for both the hour and the minute.