Match and exit

This StackExchange question provides a snippet which runs a subprocess, saves its PID, and then ends the process when a certain output string is encountered in its output.

sh -c 'echo "$$"; exec stdbuf -oL "$0" "$@"' my-program with its args | (
  IFS= read -r pid &&
    sed '/Was exported to:/q' &&
    kill -s PIPE "$pid"
)

Match and exit, with short-circuit

I encountered a situation in which I wanted to exit early when it was determined that the matching condition would never be met.

The output of my-program is a list of newline-separated JSON objects, so each line needs to be processed individually, checking these two conditions:

1. Success, which should output the entire line (matching JSON object)
2. Short-circuit failure, which should not output any text (no match)

sh -c 'echo "$$"; exec stdbuf -oL "$0" "$@"' my-program with its args | (
    IFS=
    read -r pid
    while read -r line; do
        # Break when the condition occurs, and output the matching JSON object
        if echo "$line" | jq -e "select(.key == \"value\")" > /dev/null; then
            echo "$line"
            break
        fi
        # Break when it is determined that the condition cannot be met
        if ! echo "$line" | jq -e "select(.key == \"value\")" > /dev/null; then
            break
        fi
    done
    kill -s PIPE "$pid" 2>/dev/null
)

Here, jq’s -e flag will set a failing exit status if the select() function fails.

To output the JSON object in the matching case, echo "$line" is run before breaking out of the loop.

Shorter conditionals with logical operators

We can also shorten the conditionals by taking advantage of the fact that select() will output the matching object:

# Break when the condition occurs, and output the matching JSON object
echo "$line" | jq -ce "select(.key == \"value\")" && break
# Break when it is determined that the condition cannot be met
echo "$line" | jq -e "select(.key == \"value\")" > /dev/null || break

jq’s -c flag ensures that the matching object is printed on one line. The matching case no longer redirects to /dev/null since we want the output, and the ! negation in the short-circuit case results in using a logical || instead of &&.