Nested while read loops with fd

问题

I'm trying to read from two different inputs in nested loops without success. I've followed the best answer on this question and also took a look at the file descriptors page of the Advanced Bash-Scripting Guide.

Sample script I made to test my problem.

#!/bin/bash
while read line <&3 ; do
    echo $line
    while read _line <&4 ; do
        echo $_line
    done 4< "sample-2.txt"
done 3< "sample-1.txt"

Content of sample-1.txt

Foo
Foo

Content of sample-2.txt

Bar
Bar

Expected output

Foo
Bar
Bar
Foo
Bar
Bar

The output I get

Foo
Bar

回答1:

Your text files do not end with newlines:

$ printf 'Foo\nFoo' > sample-1.txt
$ printf 'Bar\nBar' > sample-2.txt
$ bash tmp.sh
Foo
Bar
$ printf '\n' >> sample-1.txt
$ printf '\n' >> sample-2.txt
$ bash tmp.sh
Foo
Bar
Bar
Foo
Bar
Bar

read has a non-zero exit status if it reaches the end of the file without seeing a newline character. There's a hack to work around that, but it's better to ensure that your text files correctly end with a newline character.

# While either read is successful or line is set anyway
while read line <&3 || [[ $line ]]; do

来源：https://stackoverflow.com/questions/42559013/nested-while-read-loops-with-fd