perl program run from crontab yields wrong values i2cget raspberry pi

Question

I've read a number of other Q&A's that seem to be related, but haven't been able to track down the issue here.

I have the following perl script that I use on my Raspberry Pi. I'm hoping to log temperature data from an I2C sensor to a sqlite3 database. The program below works when being run from the command line but not when run from cron.

I'm assuming that the value from i2cget is incorrect when run from cron, but I'm not sure how to figure out what part of my environment i2cget needs to work properly.

#!/usr/bin/perl

#printf '%d\n' `i2cget -y 1 0x48 0x0`
$temp = `i2cget -y 1 0x48 0x0`;
#$temp = sprintf("%d\n", $temp);
$temp = hex($temp);
#print $temp, "\n";

use DBI;

#/home/techplex/ece331/project2_temp_data_grapher/
$dbh = DBI->connect( "dbi:SQLite:tempdata.db" ) or die "Cannot connect: $DBI::errstr";

$dbh->do( "CREATE TABLE IF NOT EXISTS temperature (timestamp datetime, temperature float);" );
$dbh->do( "INSERT INTO temperature (timestamp, temperature) VALUES (datetime('now', 'localtime'), $temp);" );

$dbh->disconnect;

I've added this line to my crontab:

*/1 * * * * cd /home/techplex/temp_data_grapher; ./datalogger.pl

Answer 1

You need full path to i2c inside your perl script:

$temp = `/full/path/to/i2cget -y 1 0x48 0x0`;

Answer 2

Try to analyze the cron environment versus the command line:

From the command line:

set > command.lis

Temporarily add this to your crontab, remove it after you get a result:

* * * * * set > /home/techplex/crontab.lis

( assuming /home/techplex is a directory and exists. And is your home directory, change it as needed. )

Now you have two files.

diff crontab.lis command.lis

will show you where things differ in your environment.