bash: variable loses value at end of while read loop

Solution 1:

This is kind of a 'common' mistake. Pipes create SubShells, so the while read is running on a different shell than your script, that makes your CNT variable never changes (only the one inside the pipe subshell).

Group the last echo with the subshell while to fix it (there are many other way to fix it, this is one. Iain and Ignacio's answers have others.)

CNT=0

 cat input.data | ( while read 
do
  let CNT++;
  echo "Counting to $CNT"
done 
echo "Count is $CNT" )

Long explanation:

1. You declare CNT on your script to be value 0;
2. A SubShell is started on the | to while read;
3. Your $CNT variable is exported to the SubShell with value 0;
4. The SubShell counts and increase the CNT value to 5;
5. SubShell ends, variables and values are destroyed (they don't get back to the calling process/script).
6. You echo your original CNT value of 0.

Solution 2:

See argument @ Bash FAQ entry #24: "I set variables in a loop. Why do they suddenly disappear after the loop terminates? Or, why can't I pipe data to read?" (most recently archived here).

Summary: This is only supported from bash 4.2 and up. You need to use different ways like command substitutions instead of a pipe if you are using bash.

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Solution 3:

This works

CNT=0 

while read ;
do
  let CNT++;
  echo "Counting to $CNT"
done <input.data
echo "Count is $CNT"

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Solution 4:

Try passing the data in a sub-shell instead, like it's a file before the while loop. This is similar to lain's solution, but assumes you don't want some intermittent file:

total=0
while read var
do
  echo "variable: $var"
  ((total+=var))
done < <(echo 45) #output from a command, script, or function
echo "total: $total"