How do you read STDOUT into variables in bash?

Question

I read the bandwidth usage by ifstat, which prints the download and upload into STDOUT every second.

ifstat -i wlp7s0 wlp7s0 KB/s in KB/s out 1390.13 81.20 1039.14 74.05 1810.63 102.08 865.60 183.15 1272.91 274.19 1174.00 400.04

How can I run ifstat in a bash script and read the values into variables for an if statement to run a command if the values are smaller than an amount?

For example,

if [ $in -lt 100 ] && [ $out -lt 100 ] then echo Network is slow. else echo Network is fast. fi

How can I read the output of ifstat into $in and $out variables to check the if statement every second?

John1024 · Accepted Answer · 2020-07-21 21:11:25Z

Since the network speeds are not integers, we need to supplement with other tools such as awk to process the numbers. Try:

ifstat -ni wlp7s0 | awk 'NR>2{if ($1+0<100 && $2+0<100) print "Network is slow."; else print "Network is fast."}'

Or, for those who like their commands spread over multiple lines:

ifstat -ni wlp7s0 | awk ' NR>2{ if ($1+0<100 && $2+0<100) print "Network is slow." else print "Network is fast." }'

How it works

The -n option is added to ifstat to suppress the periodic repeat of the header lines.

NR>2{...} tells awk to process the commands in curly braces only if the line number, NR, is greater than two. This has the effect of skipping over the header lines.

if ($1+0<100 && $2+0<100) tests whether both the first field, $, and the second field, $2, are less than 100. If they are, then print "Network is slow." is executed. If not, then print "Network is fast." is executed.

glenn jackman · Accepted Answer · 2020-07-22 18:52:06Z

John1024 is right about floating point numbers, but we can just truncate the numbers. With plain bash:

n=0 LC_NUMERIC=C ifstat -i $interface \ | while read -r in out; do ((++n < 2)) && continue # skip the header if (( ${in%.*} < 100 && ${out%.*} < 100 )); then echo Network is slow. else echo Network is fast. fi done

This only works in locales that use a period as the decimal separator (which seems to be OP's case). Setting LC_NUMERIC=C would take care of other locales. — TooTea
– TooTea, Commented Jul 22, 2020 at 9:07

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