+ +
+ + + +
+ + +
+ + Şahin Akkaya + +
+ + +
+ +

Şahin Akkaya

+ + +
+

A perfectionist who likes to tinker everything until it is just right.

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+

Hot-Reload Long Running Shell Scripts (feat. trap / kill) +

+ + +

+ + + + + + + + + + + + + + + + + + + 5 minute read + + + +

+ + +
+ + +
+ +

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.
+i=1
+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 send me signal with 'killall `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 "$pid"
+      kill "$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.
+        # '&' at the end of command is important otherwise
+        # the script will block until its execution is finished.
+        $script &
+        pid=$!
+        pids+=("$pid")
+    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 send me signal with 'killall 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. FOSS folks are at something 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 ͡°). *

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Updated:

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