Bash - Execute background process whilst reading output

Question

I'm trying to start a process (target_executable) and have it run in the background. I know I could do that via ./target_executable & but in the bash script that's running the show, I want to read the output of the process looking for a particular output. Then once the output is . found, I want to let the the script complete whilst the target process is left running in the background.

This is what I have so far but there are a number of things wrong (its not running the process in the background, and it never hits "Finished Reading" even though the ID is found):

echo "Starting Process..."
TARGET_ID=""
./target_executable | while read -r line || [[ "$TARGET_ID" == "" ]] ; do
    TARGET_ID=$(echo "$line" | grep -oE 'Id = [0-9A-Z]+' | grep -oE '[0-9A-Z]{10,}')

    if [ "$TARGET_ID" != "" ]
    then
        echo "Processing $line '$TARGET_ID'"
    fi
done
echo "Finished Reading..."

Any thoughts?

Answer 1

This looks like a job for coproc. From the help:

coproc: coproc [NAME] command [redirections]
    Create a coprocess named NAME.

    Execute COMMAND asynchronously, with the standard output and standard
    input of the command connected via a pipe to file descriptors assigned
    to indices 0 and 1 of an array variable NAME in the executing shell.
    The default NAME is "COPROC".

So it would look something like:

echo "Starting Process..."
TARGET_ID=""
coproc (trap '' PIPE; ./target_executable < /dev/null & disown) # since it's in the bg, input won't be useful
while read -r line || [[ "$TARGET_ID" == "" ]] ; do
    TARGET_ID=$(echo "$line" | grep -oE 'Id = [0-9A-Z]+' | grep -oE '[0-9A-Z]{10,}')

    if [ "$TARGET_ID" != "" ]
    then
        echo "Processing $line '$TARGET_ID'"
        break
    fi
done <&${COPROC[0]} # redirect in from coprocess output

Note that bash sets up a pipe for the input/output of the coprocess, so the application must be able to handle a broken output pipe. Not all commands can. (That's why I trap SIGPIPE in the subshell.)