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My bash skills are near to nil. I came up with a script to monitor the data transfer speed of my wifi interface.

#!/bin/bash 
rx_old=0
tx_old=0
speed() {
    read rx_bytes < /sys/class/net/wlp9s0f0/statistics/rx_bytes
    read tx_bytes < /sys/class/net/wlp9s0f0/statistics/tx_bytes
    drx=$(((rx_bytes - rx_old) / 1024))
    dtx=$(((tx_bytes - tx_old) / 1024))
   rx_old=$rx_bytes
    tx_old=$tx_bytes
    echo "$drx KB/s $dtx KB/s"  
}
while true;
do
    echo $(speed)
    sleep 1
done

This is not working, I mean the variable rx_old and tx_old does not seem to update. I think I am not able to change those variables from inside the function. Can anybody give me some insight on this?

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  • How do you know they are not updated? A few suggestions for improvement: rx_bytes=$(cat /sys/class...), and instead of echo $(speed), just type speed. Commented Apr 21, 2021 at 10:45
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    Oops, the problem is that you are running speed in a subshell, which does not update the old variables. Commented Apr 21, 2021 at 10:50
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    @berndbausch, you should write an answer :-) Commented Apr 21, 2021 at 10:55
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    Replying to the above comment. A workaround is to let speed manipulate variables in the current shell (not in a subshell), including a variable named (e.g.) outcome. The function itself may be silent. Then you simply call speed, this updates the variables. You retrieve the result with xsetroot -name "$outcome". Commented Apr 21, 2021 at 11:35
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    @berndbausch I pass. Go ahead. There's a good general answer to the linked SU question. The answer is 97% mine and I don't feel like repeating myself here. :) Commented Apr 21, 2021 at 12:35

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The problem is echo $(speed). Commands inside $(...) are executed in a subshell, in other words in a child process of the shell that executes the script. A subshell can't change variables in its parent.

As a consequence, rx_old and tx_old in the main shell are indeed not updated. Variables with the same names are updated in the subshell, but they disappear when the subshell exits.

There is no need for $(...), since all it does is echoing text that was already echoed by the speed function. It can be replaced by a "naked" speed, which runs in the main shell and therefore updates those variables.

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