Variable in while loop being read from unexpected source


I am trying to compare the content of file2 with that of file1 and based on that I need to take some action.

But when I try to take input from user (in variable answer) whether to start or not, the program does not wait for user input and takes value assigned to variable line automatically.

#!/bin/bash while read line; do var=`grep $line file1.txt` if [ -z "$var"] then echo "$line is not running" echo "Do you want to start? (Y/N)" read answer if [ "$answer" = 'Y' ] || [ "$answer" = 'N' ] then if [ "$answer" = 'Y' ] then (some action) else (action) fi else (action) fi fi done < file2


You redirect stdin for the while loop to file2. So inside the loop, stdin is redirected and a read will read from the file, not from the terminal.

With bash, you can easily fix that by using a different file descriptor:

while read -r -u3 line; do echo "$line" read -p "Continue? " yesno if [[ $yesno != [Yy]* ]]; then break; fi done 3<file2

The -u3 command-line flag to read causes it to read from fd 3, while the 3<file2 redirection redirects fd 3 to file (opening file for reading).


Another approach to the excellent answer offered by @rici, this time not requiring bash:

while read -r line <&3; do echo "$line" printf "Continue? " >&2 read yesno case $yesno in [Yy]*) : ;; *) break ;; esac done 3<file2

Using read <&3 reads from FD 3, just as the bash extension read -u 3 would do.


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