Bash capture output of running script

Question

I may simply not know how to phrase what I am asking but I haven't been able to get the answers that I am looking for.

So what I am trying to to is run a command or script within a bash script capture the output line by line and check it against some value.

I have tried things to the effect of

#!/bin/bash
./runningscript.sh | while read line; do
echo "output $line"
done

and

#!/bin/bash

./runningscript.sh | {read line echo "output $line"}

both just seem to execute the script giving me the normal output. What I want is to handle each line of output from runningscript.sh from within this bash script as it is output I don't want it to wait until runningscript.sh is finished running to handle it.

Sorry I am a very occasional and simple bash user so this may be the stupidest question to ask but any help would be greatly appreciated

Answer 1

Your scripts work. but the output doesn't appear until late, maybe even after the commands are done? This is typically because of buffering: runningscript.sh has to fill up a buffer, typically 4 kB, before its output is transferred through the pipe to the next process in the pipeline.

A fix is to use the unbuffer command which is part of the expect package:

unbuffer runningscript.sh | something_else

This tricks runningscript.sh into thinking it is writing to an interactive terminal. As a result, it doesn't buffer.

For more, see https://unix.stackexchange.com/questions/25372/turn-off-buffering-in-pipe

Alternatively, if you have GNU coreutils 7.5 or better, you can disable output buffering with the stdbuf command:

stdbuf -oO runningscript.sh | something_else

Answer 2

The form I see most often is:

while read line; do
    stuff to $line
done < $(somescript.sh)