The issue you're encountering is because adb logcat is a continuous stream of logs, and head -n 1 only extracts the first line but does not stop the adb logcat process from running in the background. The adb logcat command keeps running and producing output, even though head stops reading after the first line.

Solution

To stop adb logcat after outputting only the first line, you need to terminate the adb logcat process once head has extracted the first line. You can achieve this using a pipe to head and then terminating the process.

Use head with && and kill

You can use the following approach to ensure adb logcat stops after the first line:

adb logcat | head -n 1 && pkill -f "adb logcat"

Explanation:

1. adb logcat | head -n 1: This pipes the output of adb logcat to head, which extracts the first line.
2. && pkill -f "adb logcat": Once head completes, this command kills the adb logcat process.

Alternative: Use timeout

If you want to ensure the command stops after a very short duration (e.g., 1 second), you can use timeout:

timeout 1 adb logcat | head -n 1

This will force adb logcat to stop after 1 second, ensuring only the first line is captured.

Recommendation

Use the first method (pkill) if you want to ensure the process stops immediately after the first line. Use timeout if you want a more controlled approach.