Bash – How to read user input when using script in pipe

bashfile-descriptorsinteractivepiperead

General problem

I want to write a script that interacts with the user even though it is in the middle of a chain of pipes.

Concrete example

Concretely, it takes a file or stdin, displays lines (with line numbers), asks the user to input a selection or line numbers, and then prints the corresponding lines to stdout. Let's call this script selector. Then basically, I want to be able to do

grep abc foo | selector > myfile.tmp

If foo contains

blabcbla
foo abc bar
quux
xyzzy abc

then selector presents me (on the terminal, not in myfile.tmp!) with options

1) blabcbla
2) foo abc bar
3) xyzzy abc
Select options:

after which I type in

2-3

and end up with

foo abc bar
xyzzy abc

as contents of myfile.tmp.

I've got a selector script up and running, and basically it is working perfectly if I don't redirect input and output. So

selector foo

behaves like I want. However, when piping things together as in the above example, selector prints the presented options to myfile.tmp and tries to read a selection from the grepped input.

My approach

I've tried to use the -u flag of read, as in

exec 4< /proc/$PPID/fd/0
exec 4> /proc/$PPID/fd/1
nl $INPUT >4
read -u4 -p"Select options: "

but this doesn't do what I hoped it would.

Q: How do I get actual user interaction?

Best Answer

Using /proc/$PPID/fd/0 is unreliable: the parent of the selector process may not have the terminal as its input.

There is a standard path that always refers to the current process's terminal: /dev/tty.

nl "$INPUT" >/dev/tty
read -p"Select options: " </dev/tty

exec </dev/tty >/dev/tty
nl "$INPUT"
read -p"Select options: "

Related Solutions

Shell Script – How to Read User Input from a Pipe

As others have said, this is because the stdin of sh has been redirected to read from the pipe, it is not connected to the terminal as it would normally be. One thing you can do to get around this is to use /dev/tty to force the script to read from the terminal. Eg:

#!/bin/sh

read -p "Are you sure [Y/n]?" line </dev/tty
case "$line" in
  y|Y) echo "confirmed"
    ;;
  *) echo "not confirmed"
    ;;
esac

Normally you would only do this if you specifically want to prevent people from scripting the input, eg:

echo Y | sh confirmation.sh

This would still read from the terminal even though the user might expect this to automatically input Y at the prompt. It is common for programs that expect a password to do this.

BASH function to read user input OR interrupt on timeout

read has a parameter for timeout, you can use:

read -t 3 answer

If you want read to wait for a single character (whole line + Enter is default), you can limit input to 1 char:

read -t 3 -n 1 answer

After proper input, return value will be 0, so you can check for it like this:

if [ $? == 0 ]; then
    echo "Your answer is: $answer"
else
    echo "Can't wait anymore!"
fi

I guess there is no need to implement background jobs in your situation, but if you want to, here is an example:

#!/bin/bash

function ProcessA() {
    sleep 1  # do some thing
    echo 'A is done'
}

function ProcessB() {
    sleep 2  # do some other thing
    echo 'B is done'
}

echo "Starting background jobs..."

ProcessA &  # spawn process "A"
pid_a=$!    # get its PID

ProcessB &  # spawn process "B"
pid_b=$!    # get its PID too

echo "Waiting... ($pid_a, $pid_b)"
wait  # wait for all children to finish

echo 'All done.'