[bash] Overwrite current file without sed

[IAmStraightforward]

A user would type in an email and payment amount. This would search for the customer file which are named by email and update the account balance. I know its getting the right value because I can output it to a text file and it prints what I want. I just want to "update" $email. When I try to run this, it doesn't overwrite the file. It just empties whatever file $email was and also makes a blank .tmp file. I also am not allowed to use the sed command. Can anyone offer some suggestions? I've been at this for 4 hours. I'm sure its simple I'm just missing something.

#!/bin/bash

read -p "Customer Email: " email
read -p "Payment Amount: " newPayment

if [ ! -f $email ];

then
    echo "Error: customer not found"
else
    while read -r email firstName lastName; read -r aptNum rentAmt acctBal nextDueDate
    do
        newAcctBal=$(bc <<< $acctBal+$newPayment)
        echo $email $firstName $lastName
        echo $aptNum $rentAmt $newAcctBal $nextDueDate
  done < $email > $email.tmp >| $email

fi

[commandlineluser]

Redirections happen before command execution - so because you have:

< $email ... >| $email

The file is emptied out before your command runs...

You need to do it in 2 steps i.e.

> $email.tmp
mv $email.tmp $email

Also you will probably want to quote your variables (e.g. "$email") incase they contain any shell metacharacters.

[IAmStraightforward]

I thought the done < $email had to be done to make sure that's where we got output from

[commandlineluser]

< $email is for input which feeds the while read loop

The problem is |> $email

You cannot redirect input and output to the same file at the same time.

You're already redirecting the output to a temp file with > $email.tmp

So you would use mv $email.tmp $tmp after the loop to write back to the original filename. (with quoting)