Hello beautiful individuals! 👋 As a part of a analysis I’m doing I needed to write some bash scripts which have been alleged to run each couple of minutes. I made some embarrassing errors alongside the best way. I’ll write about these errors in hopes that you just don’t make them if and if you write your individual bash scripts as cronjobs.

So with out losing any time lets start!

I’ve heard individuals say since eternally that add a shebang on the high of your scripts. This tells the terminal which program to make use of to run your scripts in case your script is an executable file. I wrote code in bash however after I requested the terminal to run it, it threw all kinds of bizarre errors. At first, I assumed my pc has gone rogue however fortunately the issue was easier. I had forgotten to jot down this one line on the high of my script:

It’s helpful and in some instances even required to learn the output generated by the background job. You’ll be able to redirect the output of the script to a log file like this:

*/15 * * * * /house/localadmin/Desktop/tcp_script.sh >> /house/localadmin/Desktop/pipe.log 2>&1

• */15 makes the command run each quarter-hour
• >> is redirecting the output to the pipe.log file
• 2>&1 is ensuring that stderr additionally will get redirected to the identical vacation spot (file descriptor) the place the usual output (&1) is being redirected

Oh my God, this one was probably the most embarrassing subject. I used to be operating a Whereas true loop in my bash script. If you end up drained issues like this could typically creep in. I needed some code to run for some time (4 minutes). The issue was that this infinite loop was additionally a cronjob so cron stored on operating a brand new occasion each quarter-hour however the outdated one by no means stopped executing.

For this explicit downside the answer was to make use of this:

#! /bin/bash
finish=$((SECONDS+240))

whereas [ $SECONDS -lt $end ];
do

# Different code .....

finished

This makes certain that your loop solely runs for 240 seconds. The SECONDS variable retains monitor of seconds elapsed for the reason that script began executing.

I used to be utilizing tcpdump to seize wi-fi packets and retailer some details about them. The issue was that tcpdump retains on operating for so long as it could actually till it receives a give up or terminate sign. For my cronjob I needed it to run for less than 4 minutes and after that self-terminate.

The answer was to make use of the timeout command:

timeout 240 tcpdump <args_for_tcpdump>

240 implies that tcpdump will mechanically give up after 240 seconds. This may turn out to be useful in case you are working with one thing like tcpdump which you need to terminate when operating from inside a script.

There are two crontabs. One is for the traditional person and one is for root. In case your script requires sudo privileges then it’s essential to put your job in root’s crontab.

Whenever you kind crontab -e within the terminal you’re enhancing present person’s crontab. Nonetheless, if you kind sudo crontab -e you’re enhancing root’s crontab. root’s crontab jobs are run with sudo privileges.

Holy moly this one had me pulling my hair out. I used to be operating some jobs through root’s crontab. The issue was that my bash script contained program names solely. For instance, I used to be utilizing ifconfig and I solely had this:

However I used to be getting errors that there isn’t any command referred to as ifconfig. Because it seems root’s crontab doesn’t get entry to the whole set of atmosphere variables and doesn’t learn about all of the instructions that are sometimes accessible in a standard terminal session.

Just be sure you both propagate your atmosphere to root’s crontab earlier than the script is run otherwise you prefix each command with the complete path to that program on disk. I used the latter technique as a result of I knew I wasn’t going to make use of this script on another machine so I might get away with hardcoding the paths.

For instance, I rewrote the ifconfig command like this:

Issues began working and I used to be as completely satisfied as a clam 😄 See you all within the subsequent put up the place we’ll end up our greenhouse monitoring system! ❤️

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