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?

Answer 1

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.

Answer 2

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