bash - Prepend timestamp to every output line of a command

Question

I have this script that should prepend the timestamp to every output line of a command:

  $command 2>&1 | while IFS= read -r line; do printf '[%s] %s
' "$(date '+%Y-%m-%d %H:%M:%S')" "$line"; done 

It's working fine for most cases, but I have a certain case in which I have an issue.

Let's take for example this:

read -rep "Your age: " age | while IFS= read -r line; do printf '[%s] %s
' "$(date '+%Y-%m-%d %H:%M:%S')" "$line"; done

In this case the timestamp is not prepended.

I understand the problem here is that the newline is not reached so the newline is not printed. I was wondering if is possible to print all the chars as they are received and then print the timestamp once we get a newline.

question from:https://stackoverflow.com/questions/65922422/prepend-timestamp-to-every-output-line-of-a-command

Answer 1

The issue here is that the read command is not writing anything to stout and so the while loop has no output to process and loop on. By adding an echo of the age, you get the required result.

read -rep "Your age: " age;
echo $age | while IFS= read -r line; 
do 
  printf '[%s] %s
' "$(date '+%Y-%m-%d %H:%M:%S')"; 
done