Run shell script file once after reboot

Question

I am stucked on how to execute a shell script only once after the machine reboots, the file that I have has the following content where I am writing a test to get only the first element from the file:

#!/bin/bash

echo "A" >> /home/vtr/Documents/test.txt
echo "B" >> /home/vtr/Documents/test.txt 
echo "C" >> /home/vtr/Documents/test.txt

sleep 10  #this sleep I put only to the lines below be executed when the machine came back to the desktop
test=$(sort /home/vtr/Documents/test.txt | uniq | head -n 1)
echo $test >> /home/vtr/Documents/finaltest.txt

I already went through cron and by systemd but on both of them instead have only A as output I am getting a file which was ran several times:

A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A

And this is my /etc/systemd/system/testing.service file:

[Unit]
Description=One test script
After=network.target

[Service]
Type=oneshot
ExecStart=/home/vtr/Documents/testing.sh

[Install]
WantedBy=multi-user.target

I also tried to perform a test only writing "A","B" and "C" without the sleep, test and echo lines above echo "C" >> /home/vtr/Documents/test.txt but I am having the same thing where the output file should be:

A
B
C

But in fact it's printing and running several times also:

A
B
C
A
B
C
A
B
C
A
B
C
A
B
C
A
B
C
A
B
C
A
B
C
A
B
C

Is there anyway to run a script after reboot once?

Answer 1

First you can simplify your script:

#!/bin/bash

printf "A\nB\nC\n" >> /home/vtr/Documents/test.txt

sleep 10
sort -u /home/vtr/Documents/test.txt |tee /home/vtr/Documents/test.txt | head -1 >> /home/vtr/Documents/finaltest.txt

And if you add in cron records like:

@reboot /home/vtr/Documents/testing.sh

you will have script execured on boot

For testing you ca make the script like this:

printf "A\nB\nC\n" >> /home/vtr/Documents/1.txt
    
    sleep 10
    sort -u /home/vtr/Documents/test.txt >/home/vtr/Documents/2.txt
sort -u /home/vtr/Documents/test.txt|tee /home/vtr/Documents/test.txt >/home/vtr/Documents/3.txt
sort -u /home/vtr/Documents/test.txt |tee /home/vtr/Documents/test.txt | head -1 >> /home/vtr/Documents/4.txt

and check the content of these files. also you can add after first line and make it

#!/bin/bash 
set -x

Answer 2

Have you tried the crontab with @reboot parameter? Perhaps you should add @reboot /path/to/bashscript.sh to the end of your crontab file with crontab -e command. Also change the >> operators in your bash script to > operator and try again.