Bash – How to Run an Infinite Loop in the Background

If you can guarantee that some character will never occur in the first file then you can use paste.

For example you know for sure that @ will never occur:

paste -d@ file1 file2 | while IFS="@" read -r f1 f2
do
  printf 'f1: %s\n' "$f1"
  printf 'f2: %s\n' "$f2"
done

Note that it is enough if the character is guaranteed to not occur in the first file. This is because read will ignore IFS when filling the last variable. So even if @ occurs in the second file it will not be split.

Example using some bash features for arguably cleaner code and paste using default delimiter tab:

while IFS=$'\t' read -r f1 f2
do
  printf 'f1: %s\n' "$f1"
  printf 'f2: %s\n' "$f2"
done < <(paste file1 file2)

Bash features used: ansi c string ($'\t') and process substitution (<(...)) to avoid the while loop in a subshell problem.

If you cannot be certain that any character will never occur in both files then you can use two file descriptors.

while true
do
  read -r f1 <&3 || break
  read -r f2 <&4 || break
  printf 'f1: %s\n' "$f1"
  printf 'f2: %s\n' "$f2"
done 3<file1 4<file2

Not tested much. Might break on empty lines.

File descriptors number 0, 1, and 2 are already used for stdin, stdout, and stderr, respectively. File descriptors from 3 and up are (usually) free. The bash manual warns from using file descriptors greater than 9, because they are "used internally".

Note that open file descriptors are inherited to shell functions and external programs. Functions and programs inheriting an open file descriptor can read from (and write to) the file descriptor. You should take care to close all file descriptors which are not required before calling a function or external program.

Here is the same program as above with the actual work (the printing) separated from the meta-work (reading line by line from two files in parallel).

work() {
  printf 'f1: %s\n' "$1"
  printf 'f2: %s\n' "$2"
}

while true
do
  read -r f1 <&3 || break
  read -r f2 <&4 || break
  work "$f1" "$f2"
done 3<file1 4<file2

Now we pretend that we have no control over the work code and that code, for whatever reason, tries to read from file descriptor 3.

unknowncode() {
  printf 'f1: %s\n' "$1"
  printf 'f2: %s\n' "$2"
  read -r yoink <&3 && printf 'yoink: %s\n' "$yoink"
}

while true
do
  read -r f1 <&3 || break
  read -r f2 <&4 || break
  unknowncode "$f1" "$f2"
done 3<file1 4<file2

Here is an example output. Note that the second line from the first file is "stolen" from the loop.

f1: file1 line1
f2: file2 line1
yoink: file1 line2
f1: file1 line3
f2: file2 line2

Here is how you should close the file descriptors before calling external code (or any code for that matter).

while true
do
  read -r f1 <&3 || break
  read -r f2 <&4 || break
  # this will close fd3 and fd4 before executing anycode
  anycode "$f1" "$f2" 3<&- 4<&-
  # note that fd3 and fd4 are still open in the loop
done 3<file1 4<file2

Just a guess (this is bash specific): the documentation for the huponexit shell option says

If set, Bash will send SIGHUP to all jobs when an interactive login shell exits

On my system, it does not seem to be set by default. You can check with

shopt -p huponexit

If the output includes -u, it is unset.