blog: hot reloading with trap and kill
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title: "Hot-Reload Long Running Shell Scripts (feat. trap / kill)"
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date: 2023-01-16 00:48:08 +0300
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tags: trap kill linux
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---
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## `trap` them and `kill` them!
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There is a beautiful command in Linux called [`trap`](https://man7.org/linux/man-pages/man1/trap.1p.html) which *trap*s signals and let you run specific commands when they invoked. There is also good ol' [`kill`](https://man7.org/linux/man-pages/man1/kill.1.html) 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!
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### Basic Example
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Let's start by creating something very simple and build up from there. Create a script with the following contents:
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```bash
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#!/bin/bash
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echo "My pid is $$. Send me SIGUSR1!"
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func() {
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echo "Got SIGUSR1"
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}
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# here we are telling that run 'func' when USR1 signal is
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# received # you can run anything. Combine commands with ; etc.
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trap "func" USR1
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# The while loop is important here otherwise our script will exit
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# before we manage to get a chance to send a signal.
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i=1
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while true ; do
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echo "waiting SIGUSR1"
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sleep 1
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done
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```
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Now make it executable and run it:
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```bash
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❯ chmod +x trap_example
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❯ ./trap_example
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My pid is 2811137. Send me SIGUSR1!
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waiting SIGUSR1
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waiting SIGUSR1
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waiting SIGUSR1
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waiting SIGUSR1
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waiting SIGUSR1
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```
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Open another terminal and send your signal with `kill` to the specified pid.
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```bash
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❯ kill -s USR1 2811137
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```
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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!
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Let's move the while loop into the `func` and add some variables so you can see how powerful this is.
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```bash
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#!/bin/bash
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echo "My pid is $$. Send me SIGUSR1!"
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func() {
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i=1
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while true ; do
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echo "i: $i"
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i=$(( i + 1 ))
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sleep 1
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done
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}
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trap "echo 'Got SIGUSR1!'; func" USR1
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# we need to call the function once, otherwise script
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# will exit before we manage to send a signal
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func
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```
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Now run the script and send `SIGUSR1`. Here is the result:
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```bash
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❯ ./trap_example
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My pid is 2880704. Send me SIGUSR1!
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i: 1
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i: 2
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i: 3
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i: 4
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i: 5
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i: 6
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i: 7
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Got SIGUSR1!
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i: 1
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i: 2
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i: 3
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i: 4
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i: 5
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Got SIGUSR1!
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i: 1
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i: 2
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^C
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```
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Isn't this neat?
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### More useful example
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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! <sup>[*](## "Yeah, I know you can run a systemd service if you want but I think it is an overkill for this situation. Plus, I don't like dealing with them.")</sup> This command is awesome.
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Without further ado, let's get started. Create a script called `script1` with the following contents:
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```bash
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#!/bin/bash
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# file: script1
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i=1
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while true ; do
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echo "Hello from $0. i is $i"
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i=$(( i + 1 ))
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sleep 1
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done
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```
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And symlink it to another name just for fun:
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```bash
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❯ chmod +x script1
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❯ ln -s script1 script2
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```
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Now we can pretend they are two different scripts as their outputs differ:
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```bash
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❯ ./script1
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Hello from ./script1. i is 1
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Hello from ./script1. i is 2
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Hello from ./script1. i is 3
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Hello from ./script1. i is 4
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^C
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❯ ./script2
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Hello from ./script2. i is 1
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Hello from ./script2. i is 2
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Hello from ./script2. i is 3
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^C
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```
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Finally, create the main script which will start child scripts and restart them on our signals:
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```bash
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#!/bin/bash
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echo "My pid is $$. You know what to do ( ͡° ͜ʖ ͡°)"
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echo "You can also send me signal with 'killall `basename $0` ...'"
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pids=() # we will store the pid's of child scripts here
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scripts_to_be_executed=("./script1" "./script2")
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kill_childs(){ # wow, this sounded wild
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for pid in "${pids[@]}"
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do
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echo killing "$pid"
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kill "$pid"
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done
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pids=()
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}
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# kill childs and restart all the scripts
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restart_scripts(){
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kill_childs
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# for each script in the list
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for script in "${scripts_to_be_executed[@]}"
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do
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# run the script and store its pid.
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# '&' at the end of command is important otherwise
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# the script will block until its execution is finished.
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$script &
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pid=$!
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pids+=("$pid")
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done
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}
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# we will restart_scripts with SIGUSR1 signal
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trap 'echo "restarting scripts"; restart_scripts' USR1
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# we will kill all the childs and exit the main script with SIGINT
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# which is same signal as when you press <Control-C> on your terminal
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trap 'echo exiting; kill_childs; exit' INT
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# run the function once
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restart_scripts
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# infinite loop, otherwise main script will exit before we send signal.
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# remember, we started child processes with '&' so they won't block this script
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while true; do
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sleep 1
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done
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```
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Now, you can run your main script and reload your child scripts any time with `killall main_script -USR1`
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Here is an example run:
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```
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❯ ./trap_multiple
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My pid is 3124123. You know what to do ( ͡° ͜ʖ ͡°)
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You can also send me signal with 'killall trap_multiple ...'
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Hello from ./script1. i is 1
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Hello from ./script2. i is 1
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Hello from ./script2. i is 2
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Hello from ./script1. i is 2
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Hello from ./script2. i is 3
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Hello from ./script1. i is 3
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restarting scripts
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killing 3124125
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killing 3124126
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Hello from ./script1. i is 1
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Hello from ./script2. i is 1
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Hello from ./script2. i is 2
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Hello from ./script1. i is 2
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Hello from ./script2. i is 3
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Hello from ./script1. i is 3
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Hello from ./script2. i is 4
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Hello from ./script1. i is 4
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restarting scripts
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killing 3124304
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killing 3124305
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Hello from ./script1. i is 1
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Hello from ./script2. i is 1
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Hello from ./script1. i is 2
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Hello from ./script2. i is 2
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^Cexiting
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killing 3124875
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killing 3124876
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```
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### Final words
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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:
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> \- How can you see the contents of a file? <br>
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\+ You *`cat`* it. <br>
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\- What if you want to see them in reverse order? <br>
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\+ You *`tac`* it. <br>
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No, it is not just a joke. Try it... Man I love Gnoo slash Linux.
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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` <nobr>(° ͜ʖ ͡°)</nobr>. <sup>[*](## "jk :D")</sup>
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