while read loop

Question

I have a file like one below

var     3   2014   string
var1    4   2011   string4
var2    6   1999   string2
var3    1   2016   string6

Then i have this while read loop to compare one of the columns to a number then echo something. However, instead of echoing my desired phrase, it echoes something else.

while read num
do
if [ "$num" = "0" ]; then
echo "Number is equal to zero"
else
echo "number is not equal to 0"
fi
done < home/dir/file.txt | awk '{print $2}'

instead of echoing the above, it echoes the 2nd column of the file.

I can't actually reproduce the problem when running the script as-is, but in any case you have other problems with your script. Mainly, you do read num and expect the result to be a single number (which you compare against the string "0"), but you are actually reading an input file with 4 columns so $num will be a string with 4 columns in it. — Celada
– Celada, Commented Jun 11, 2015 at 7:19
Not tested, but I think your problem is that the pipe will be done after the while loop? Anyway, why not all in awk: awk '{if ($2==0){print "equal"}else{print "not"}}' file.txt ? — fredtantini
– fredtantini, Commented Jun 11, 2015 at 7:20
As presented, that code snippet would print the second word from each line produced by the while ... done loop. I.e.: is x4 — Henk Langeveld
– Henk Langeveld, Commented Jun 12, 2015 at 11:03

Archemar · Accepted Answer · 2015-06-11 07:46:24Z

12

you should try

awk '{print $2}' home/dir/file.txt | while read num
do
  if [ "$num" = "0" ]; then
    echo "Number is equal to zero"
  else
    echo "number is not equal to 0"
  fi
 done

for a mixed awk/bash solution.

As other have pointed out, awk redirection occur later.

answered Jun 11, 2015 at 7:46

Archemar

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kos · Accepted Answer · 2015-06-11 07:48:02Z

It looks like you're trying to redirect the output of home/dir/file.txt | awk '{print $2}' to the while loop;

first I guess that the correct path should be /home/dir/file.txt (however this is just an assumption);

second /home/dir/file.txt | awk '{print $2}' doesn't redirect the content of /home/dir/file.txt to awk, while < /home/dir/file.txt awk '{print $2}' does;

third, you're redirecting the output of the command as a file, but it's a string, and you should redirect it as it: <<< "$(< /home/dir/file.txt awk '{print $2}')".

Also, alternatively, you could pipe the output of the command directly to the while loop instead: < /home/dir/file.txt awk '{print $2}' | while read num.

while read num
do
    if [ "$num" = "0" ]; then
    echo "Number is equal to zero"
else
    echo "number is not equal to 0"
fi
done <<< "$(< /home/dir/file.txt awk '{print $2}')"

or

< /home/dir/file.txt awk '{print $2}' | while read num
do
    if [ "$num" = "0" ]; then
    echo "Number is equal to zero"
else
    echo "number is not equal to 0"
fi
done

StackzOfZtuff · Accepted Answer · 2023-07-14 15:21:08Z

6

Since I can't comment, just wondering, why people have not used IFS

while IFS=" " read var num year string
do
  if [[ $num -eq 0 ]]; then
    echo "Number is equal to zero"
  else
    echo "number is not equal to 0"
  fi
done < home/dir/file.txt

edited Jul 14, 2023 at 15:21

StackzOfZtuff

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answered Jan 11, 2018 at 13:14

rebelution

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