To run a command (COMMAND) when another command (like sleep 5) starts on Debian (or any Linux system), you can use a combination of shell features. Here are two approaches:

1. Using & and wait in a Shell Script

If you want COMMAND to run immediately when sleep 5 starts, you can use backgrounding (&) and a subshell:

(sleep 5 & COMMAND)

• sleep 5 & runs sleep 5 in the background.
• COMMAND runs immediately after sleep 5 starts.

Example:

(sleep 5 & echo "Sleep started, running COMMAND now")

This will print "Sleep started, running COMMAND now" as soon as sleep 5 begins.

2. Using pidof or pgrep to Detect Process Start

If you want COMMAND to run only after confirming sleep 5 has started, you can use a loop to check for the process:

sleep 5 &
while ! pidof sleep > /dev/null; do
    sleep 0.1
done
COMMAND

• sleep 5 & starts sleep 5 in the background.
• The while loop checks if sleep is running using pidof.
• Once sleep is detected, COMMAND runs.

Example:

sleep 5 &
while ! pidof sleep > /dev/null; do
    sleep 0.1
done
echo "Sleep started, running COMMAND now"

3. Using inotifywait (Advanced)

If you want to trigger COMMAND when a specific process file (e.g., /proc/PID) appears, you can use inotifywait (from the inotify-tools package):

sudo apt install inotify-tools
sleep 5 &
inotifywait -e create /proc/$(pgrep -f "sleep 5") && COMMAND

• This waits for the /proc/PID directory to be created for the sleep 5 process.
• Once detected, COMMAND runs.

Which One to Use?

• For immediate execution, use Approach 1 ((sleep 5 & COMMAND)).
• For process confirmation, use Approach 2 (pidof loop).
• For file-based triggers, use Approach 3 (inotifywait).