nested bash while loop not taking user input

Question

I'm trying to write in bash (Mac OSX but with GNU shell programs installed) a "Yes or No" interactive line by line iteration reading from an input file. As an example, suppose I have a file with numbers from 1 to 5 (one by line) as an input and I want to ask wether they are prime numbers.

#!bin/bash
while read NUMBER
  do echo "Prime number (answer y or n)?"
  while read ANSWER
  do
    test "$ANSWER" == "y" && echo "Prime" && break
    test "$ANSWER" == "n" && echo "Non-prime" && break
    test "$ANSWER" != "y" -a "$ANSWER" != "n" && echo "Type y or n"
  done
done < 1_to_5_file.txt

The below output doesn't open for user input

Prime number (answer y or n)?
Type y or n
Type y or n
Type y or n

Possible duplicate of Pause Shell script until Press Enter in while loop — Benjamin W.
– Benjamin W., Commented Jan 29, 2016 at 18:04

Jonas Berlin · Accepted Answer · 2016-01-29 18:07:57Z

1

Because the outer while loop's stdin (the input to the script) is set to read from 1_to_5_file.txt, that is inherited to all commands running inside the while loop. One way to work around that is to save the original stdin in a different descriptor, which we can then use inside the loop. In my example I will store it in file descriptor 3.

#!/bin/bash
while read NUMBER
  do echo "Prime number (answer y or n)?"
  while read ANSWER
  do
    test "$ANSWER" == "y" && echo "Prime" && break
    test "$ANSWER" == "n" && echo "Non-prime" && break
    test "$ANSWER" != "y" -a "$ANSWER" != "n" && echo "Type y or n"
  done 0<&3
done 3<&0 < 1_to_5_file.txt

The last line makes a copy of file descriptor 0 in file descriptor 3. Then inside the loop file descriptor 3 points to the original stdin. On the inner while loop's last line I then copy file descriptor 3 to file descriptor 0 which thus makes the stdin inside the loop again point to the original stdin.

For the record, "<" is the same as "0<" and ">" is the same as "1>". Thus the 0<&3 could also have been written just <&3.

Another, more elagant solution proposed by @chepner would be to open the 1_to_5_file.txt on file descriptor 3 instead and just change the first read command to read from that one instead:

#!/bin/bash
while read NUMBER <&3
  do echo "Prime number (answer y or n)?"
  while read ANSWER
  do
    test "$ANSWER" == "y" && echo "Prime" && break
    test "$ANSWER" == "n" && echo "Non-prime" && break
    test "$ANSWER" != "y" -a "$ANSWER" != "n" && echo "Type y or n"
  done
done 3< 1_to_5_file.txt

Please note that compared to the code in the question, the last line has 3< instead of just 3.

edited Jan 29, 2016 at 18:07

answered Jan 29, 2016 at 18:02

Jonas Berlin

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2 Comments

chepner Over a year ago

It would be simpler to put 1_to_5_file.txt on file descriptor 3 rather than copying the inherited standard input around. while read NUMBER <&3; do ... done <3 1_to_5_file.txt.

Jonas Berlin Over a year ago

@chepner added improved solution based on your suggestion.

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