sahinakkaya.dev/_posts/2023-01-16-hot-reloading-with-trap-and-kill.md

title	date	tags
Hot-Reload Long Running Shell Scripts (feat. trap / kill)	2023-01-16 00:48:08 +0300	trap kill linux

trap them and kill them!

There is a beautiful command in Linux called trap which traps signals and let you run specific commands when they invoked. There is also good ol' kill command which not only kills processes but allows you to specify a signal to send. By combining these two, you can run specific functions from your scripts any time!

Basic Example

Let's start by creating something very simple and build up from there. Create a script with the following contents:

#!/bin/bash

echo "My pid is $$. Send me SIGUSR1!"

func() {
  echo "Got SIGUSR1"
}

# here we are telling that run 'func' when USR1 signal is
# received. You can run anything. Combine commands with ; etc.
trap "func" USR1

# The while loop is important here otherwise our script will exit
# before we manage to get a chance to send a signal.
while true ; do
    echo "waiting SIGUSR1"
    sleep 1
done

Now make it executable and run it:

❯ chmod +x trap_example
❯ ./trap_example
My pid is 2811137. Send me SIGUSR1!
waiting SIGUSR1
waiting SIGUSR1
waiting SIGUSR1
waiting SIGUSR1
waiting SIGUSR1

Open another terminal and send your signal with kill to the specified pid.

❯ kill -s USR1 2811137

You should receive "Got SIGUSR!" from the other process. That's it! Now, imagine you write whatever thing you want to execute in func and then you can simply kill -s ... anytime and as many times you want!

Let's move the while loop into the func and add some variables so you can see how powerful this is.

#!/bin/bash

echo "My pid is $$. Send me SIGUSR1!"

func() {
    i=1
    while  true ; do
        echo "i: $i"
        i=$(( i + 1 ))
        sleep 1
    done
}

trap "echo 'Got SIGUSR1!'; func" USR1

# we need to call the function once, otherwise script
# will exit before we manage to send a signal
func

Now run the script and send SIGUSR1. Here is the result:

❯ ./trap_example
My pid is 2880704. Send me SIGUSR1!
i: 1
i: 2
i: 3
i: 4
i: 5
i: 6
i: 7
Got SIGUSR1!
i: 1
i: 2
i: 3
i: 4
i: 5
Got SIGUSR1!
i: 1
i: 2
^C

Isn't this neat?

More useful example

Let's imagine you have multiple long running (infinite loops basically) scripts and you want to restart them without manually searching for their pid's and killing them. trap is for the rescue, again! ^* This command is awesome.

Without further ado, let's get started. Create a script called script1 with the following contents:

#!/bin/bash
# file: script1

i=1
while  true ; do
    echo "Hello from $0. i is $i"
    i=$(( i + 1 ))
    sleep 1
done

And symlink it to another name just for fun:

❯ chmod +x script1
❯ ln -s script1 script2

Now we can pretend they are two different scripts as their outputs differ:

❯ ./script1
Hello from ./script1. i is 1
Hello from ./script1. i is 2
Hello from ./script1. i is 3
Hello from ./script1. i is 4
^C
❯ ./script2
Hello from ./script2. i is 1
Hello from ./script2. i is 2
Hello from ./script2. i is 3
^C

Finally, create the main script which will start child scripts and restart them on our signals:

#!/bin/bash
echo "My pid is $$. You know what to do  ( ͡° ͜ʖ ͡°)"
echo "You can also kill me with 'kill -s INT -\`pgrep -f `basename $0`\`'"

pids=() # we will store the pid's of child scripts here
scripts_to_be_executed=("./script1" "./script2")

kill_childs(){ # wow, this sounded wild
    for pid in "${pids[@]}"
    do
      echo killing "$pid"
      # -P: kill all the processes whose parent process is 'pid'
      # see how we are creating processes below
      pkill -P "$pid"
    done
    pids=()
}

# kill childs and restart all the scripts
restart_scripts(){
    kill_childs
    # for each script in the list
    for script in "${scripts_to_be_executed[@]}"
    do
        # Run the script and store its pid.
        # note the '&' at the end of command. Without it the script will
        # block until its execution is finished. Also we are putting it
        # into braces because we want to create a "process group" so that
        # we can kill all its children later by specifying parent pid
        # (useful if you have pipes (|) or other &'s in your script!)
        ($script) &
        pids+=("$!")
    done
}

# we will restart_scripts with SIGUSR1 signal
trap 'echo "restarting scripts"; restart_scripts' USR1

# we will kill all the childs and exit the main script with SIGINT
# which is same signal as when you press <Control-C> on your terminal
trap 'echo exiting; kill_childs; exit' INT

# run the function once
restart_scripts

# infinite loop, otherwise main script will exit before we send signal.
# remember, we started child processes with '&' so they won't block this script
while true; do
  sleep 1
done

Now, you can run your main script and reload your child scripts any time with killall main_script -USR1

Here is an example run:

❯ ./trap_multiple
My pid is 3124123. You know what to do  ( ͡° ͜ʖ ͡°)
You can also kill me with 'kill -s INT -`pgrep -f trap_multiple`'
Hello from ./script1. i is 1
Hello from ./script2. i is 1
Hello from ./script2. i is 2
Hello from ./script1. i is 2
Hello from ./script2. i is 3
Hello from ./script1. i is 3
restarting scripts
killing 3124125
killing 3124126
Hello from ./script1. i is 1
Hello from ./script2. i is 1
Hello from ./script2. i is 2
Hello from ./script1. i is 2
Hello from ./script2. i is 3
Hello from ./script1. i is 3
Hello from ./script2. i is 4
Hello from ./script1. i is 4
restarting scripts
killing 3124304
killing 3124305
Hello from ./script1. i is 1
Hello from ./script2. i is 1
Hello from ./script1. i is 2
Hello from ./script2. i is 2
^Cexiting
killing 3124875
killing 3124876

Final words

I think I am started to getting obsessed with trap command because it has such a good name and purpose. FOSS people are really on another level when it comes to naming. Here is another good one:

- How can you see the contents of a file?
+ You cat it.
- What if you want to see them in reverse order?
+ You tac it.

No, it is not just a joke. Try it... Man I love Gnoo slash Linux.

Anyway, I hope now you know how to trap and kill. Next week I will explain how to unzip; strip; touch; finger; grep; mount; fsck; more; yes; fsck; fsck; umount; clean; sleep ( ͡° ͜ʖ ͡°). ^*