Cat command with pipe working in bash but not in script

Question

Pretty simple script, but I am stuck.

It connects to a battery balancer, spits out the info into a json formatted file. I then have a pipe the output into jq to obtain the info I need. It works in the bash shell, but not in the script:

Here is the script:

echo "Checking battery voltages"
jkbms -p 3C:A5:19:7B:28:09 -o json > /home/bms/batt.log
echo cat /home/bms/batt.log | jq -r '.highest_cell_voltage'
echo "done"

The cat line shows this in the script output:

Checking battery voltages
parse error: Invalid numeric literal at line 1, column 4
done

From the shell it works as expected:

cat /home/bms/batt.log | jq -r '.highest_cell_voltage'
4.152044773101807

I have tried enclosing the whole cat command in quotes etc, but I am at a loss.

This, however, works:

echo "Checking battery voltages"
jkbms -p 3C:A5:19:7B:28:09 -o json > /home/bms/batt.log
batt=$(cat /home/bms/batt.log)
echo $batt | jq -r '.highest_cell_voltage'
#echo /usr/bin/cat /home/bms/batt.log
echo "done"

Armali · Accepted Answer · 2021-04-04 19:25:54Z

2

jkbms -p 3C:A5:19:7B:28:09 -o json > /home/bms/batt.log
echo cat /home/bms/batt.log | jq -r '.highest_cell_voltage'

The echo here is wrong. By the way, you can simplify the above to:

jkbms -p 3C:A5:19:7B:28:09 -o json|tee /home/bms/batt.log|jq -r '.highest_cell_voltage'

If I need to print the output of the comand on the screen, how do i do it without using echo?

If you want the saved output in /home/bms/batt.log, you can cat /home/bms/batt.log anytime.
If you want to print the output of the comand on the screen only at the time of execution, you can tee /dev/tty instead of tee /home/bms/batt.log.
If at the time of execution you want the output on screen as well as in the log file, you can tee /home/bms/batt.log /dev/tty at once.

edited Apr 4, 2021 at 19:25

answered Apr 4, 2021 at 8:58

Armali

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2 Comments

A H Over a year ago

Thanks for the simplication, but I need to understand how to do what I need above, as will need to be logging various things into variables to compare values. If I need to print the output of the comand on the screen, how do i do it without using echo?

Armali Over a year ago

I amended the answer.

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