Read stdin for user input when a file is already redirected to stdin [duplicate]

Question

So I'm trying to do something like the following:

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

So say file.txt has:

Hello? Goodbye!

Running the program would create:

Hello? James Hello? James Goodbye! Farewell Goodbye! Farewell

The issue (naturally) becomes that the userinput read reads from stdin which in our case is file.txt. Is there a way to change where it's reading from temporarily to the terminal in order to grab user input?

Note: The file I am working with is 200,000 lines long. and each line is about 500 chars long. So keep that in mind if needed

l0b0 · Accepted Answer · 2022-05-25 11:10:06Z

Instead of using redirection, you can open file.txt to a file descriptor (for example 3) and use read -u 3 to read from the file instead of from stdin:

exec 3<file.txt while read -u 3 line; do echo $line read userInput echo "$line $userInput" done

Alternatively, as suggested by Jaypal Singh, this can be written as:

while read line <&3; do echo $line read userInput echo "$line $userInput" done 3<file.txt

The advantage of this version is that it's POSIX compliant (the -u option for read does not appear in POSIX shells such as /bin/sh).

@jcollado Though I have one small suggestion, instead of doing exec on a separate line you can have it written like this - while read line <&3; do echo $line; read userInput; echo "$line $userInput"; done 3<file.txt :)
@JaypalSingh I've updated my answer. Thanks a lot for your suggestion because the code now works also in sh.

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