How to read from file and stdin in bash

Question

Here is my task: read some data from file line by line. For each line, if it satisfies some condition, then ask user to input something and proceed based on the user's input.

I know how to read content line-by-line from a shell script:

while read line; do
   echo $line
done < file.txt

However, what if I want to interact with the user inside the loop body. Conceptually, here is what I want:

while read line; do
    echo "Is this what you want: $line [Y]es/[n]o"
    # Here is the problem:
    # I want to read something from standard input here.
    # However, inside the loop body, the standard input is redirected to file.txt
    read INPUT
    if [[ $INPUT == "Y" ]]; then
       echo $line
    fi
done < file.txt

Should I use another way to read file? or another way to read stdin?

possible duplicate of [read stdin bash within a read loop](http://stackoverflow.com/questions/8886683/read-stdin-bash-within-a-read-loop) — Reinstate Monica Please, Jun 16 '14 at 01:14

ooga · Accepted Answer · 2020-02-07T07:41:22.167

9

You can open the file on a file descriptor other than standard input. For example:

while read -u 3 line; do     # read from fd 3
  read -p "Y or N: " INPUT   # read from standard input
  if [[ $INPUT == "Y" ]]; then
    echo $line
  fi
done 3< file.txt             # open file on fd 3 for input

edited Feb 07 '20 at 07:41

answered Apr 24 '14 at 05:48

ooga

15,423
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@monnand Great! I've added closing fd 3 at the end and changed the opening command from input/output to just input. – ooga Apr 24 '14 at 06:25
3

You can also just redirect the loop's standard input, rather than use a pair of `exec`s: `while read -u 3 line; do ...; done 3< file.txt`. – chepner Apr 24 '14 at 12:33
1

`done <3 file.txt` is incorrect, it should be `done 3< file.txt` – David Šmíd Feb 07 '20 at 07:04
@DavidŠmíd Thanks. – ooga Feb 07 '20 at 07:42

How to read from file *and* stdin in bash

1 Answers1

How to read from file and stdin in bash