Why does this loop clear variables?

novaraz · 10-24-2008, 02:03 PM

Sorry if this is something obvious, but I can't get this script to work:

Code:

#!/bin/bash

iniconf='c150.dat'
cat $iniconf |while read x1 x2 x3 x4; do
  ((++i))
  echo  $i $x1 $x2 $x3 $x4
  if [ $i -eq 2  ]; then
    npore=$x3
    ngau=$x4
    break
  fi
done 

echo $npore $ngau

If I echo npore and ngau in the loop, it works fine. Once it breaks the loop, the variables return blanks. What's the deal?

The input file contains values separated by spaces, and I need to extract the 3rd and 4rd on the 2nd line.

Thanks!

jan61 · 10-24-2008, 02:32 PM

Moin,

your loop starts after a pipe and that's why is executed in a subshell. Variables defined in a subshell are not visible in the parent shell.

To avoid it, use input redirection:

Code:

#!/bin/bash

iniconf='c150.dat'
while read x1 x2 x3 x4; do
  ((++i))
  echo  $i $x1 $x2 $x3 $x4
  if [ $i -eq 2  ]; then
    npore=$x3
    ngau=$x4
    break
  fi
done <$iniconf

echo $npore $ngau

Jan

novaraz · 10-24-2008, 03:00 PM

Thanks Jan,
The code posted didn't work for me, but the mention of subshell led me to a solution. I don't know whats going on, but this works for now:

Code:

#!/bin/bash

exec 3<>c150.dat
while read x1 x2 x3 x4 x5 x6<&3
do {
  ((++i))
  if [ $i -eq 2  ]; then
    npore=$x3
    ngau=$x4
    break
  fi }
done 
exec 3>&-

echo $npore $ngau

:-)

jcookeman · 10-24-2008, 03:21 PM

That's a bit excessive for this situation:

Code:

iniconf='./c150.dat'
while read x1 x2 x3 x4 < $iniconf
do
  ((++i))
  echo  $i $x1 $x2 $x3 $x4
  if [ $i -eq 2  ]
  then
    npore=$x3
    ngau=$x4
    break
  fi
done

echo $npore $ngau

jcookeman · 10-24-2008, 03:32 PM

or

Code:

$ awk 'NR == 2 {print$3,$4}' c150.dat

jan61 · 10-24-2008, 03:44 PM

Moin,

Quote:

Originally Posted by jcookeman

or

Code:

$ awk 'NR == 2 {print$3,$4}' c150.dat

and then assign the output to variables:

Code:

arr=($(awk 'NR == 2 {print$3,$4}' c150.dat))
npore=${arr[0]}
ngau=${arr[1]}

Jan

novaraz · 10-25-2008, 08:40 AM

Thanks guys, you rock!