How can I change and reload a running shell script?

Question

For learning purposes, I wish to change a non-stop script like this:

#!/bin/env bash

while true; do
        sleep 3

        echo 1 >> /tmp/log
done

I know that Linux will load the recent file into memory so I'm guessing that is the reason why I can't simply change the echo 1 >> /tmp/log to echo 2 >> /tmp/log and save the script to see the result.

So I change the script :

#!/bin/env bash

CMD="$0 $@"
handleSigHup() {
        echo "Receive single SIGHUP, reloading..."
        exec $CMD
}

trap 'handleSigHup' SIGHUP

while true; do
        sleep 3

        echo 2 >> /tmp/log
done

Now I can use kill -HUP to see the /tmp/log output:

1
1
1
2
2
2
2...

Here are my questions:

1. Is my guessing right? If so, can I change some sort of system vars to turn off the buffering and make It work?

2. My code above will only work once, when I change the script to echo 3 >> /tmp/log and kill -HUP again, It won't output the 3 as expect, WHY ?

Thanks all!

Answer 1

You can't change nor reload a running script (unless you exec $0). What I believe you should do is use two scripts (or binaries):

• One script which actually does the work, which may get rewritten
• Another script which runs every fixed period of time, and executes the second script

This way, the outer, launcher script will exeucte the newest version of the script, not something it has cached.

H-O-W-E-V-E-R...

You can do something else entirely. On most systems there's a daemon/service named cron, which executes commands by a fixed schedule set in configuration files. You could probably drop the second script, and get cron to run the first (potentially changing) script once every so many seconds.

This is not entirely trivial since cron's resolution is typically minutes, but have a look at this question:

How to get a unix script to run every 15 seconds?