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I have the following script, which should read the output from another command and then wait for changes, syncing these to a target directory. I replaced the actual commands with sleep and cat to make it easier to reproduce

cat .dirs.txt | while read dir
do
    echo "Watching files in $dir"
    while true
    do
      sleep 10
      echo "Detected change in $dir"
    done &
done
jobs
wait

I see the expected "Watching files in XY" output, but the jobs command shows no jobs, and wait immediately continues. I do see the "Detected change in XY" every 10 seconds, so the jobs are actually executing in the background. I suspect it has something to do with the read command and the pipe, because it does work with a redirect instead of a pipe. So why is this not working as I expected?

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2 Answers 2

Reset to default
3

Here's a simpler reproduction of the same behavior:

( sleep 5 & )
jobs      # prints nothing
wait      # waits for nothing

This happens because there are three separate processes:

  1. The process running the script.
  2. The subshell created by ( ... ).
    • In your case this subshell is created by ... | .... Bash runs each command in a pipeline in its own subshell.
  3. The sleep command in the background (&).
    • In your case there's a much more complicated command in the background, but that doesn't make a difference.

Process #2 is a child of process #1, and process #3 starts out as a child of process #2.

The important observation is that process #3 is not a child of process #1. So process #1 can't wait for it.

In your case, as you've observed, you can fix this by using ... <.dirs.txt instead of cat .dirs.txt | ..., so that ... isn't running in a subshell, so the background process is directly a child of the process running the script.

In a more complicated case where the input isn't just straight from a file, you can typically achieve the same thing via process substitution, e.g. ... < <(cat .dirs.txt).

Alternatively, you can make process #2 wait for process #3. Since process #2 isn't running in the background, process #1 automatically waits for it, which is basically the same as if process #1 were directly waiting for process #3:

cat .dirs.txt | {
    while read dir
    do
        echo "Watching files in $dir"
        while true
        do
          sleep 10
          echo "Detected change in $dir"
        done &
    done
    wait
}
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1

Another way to represent the problem and solution -

$: cat file | while read l; do date +"$l %c"; sleep 5 & done; wait; date +"Finished %c"
1 Fri Nov 28 13:40:06 2025
2 Fri Nov 28 13:40:06 2025
3 Fri Nov 28 13:40:06 2025
Finished Fri Nov 28 13:40:07 2025

Notice all the timestamps are the same. It didn't wait because of the pipe, which is a UUoC.

Remove the pointless cat pipe and it works.

$: while read l; do date +"$l %c"; sleep 5 & done < file; wait; date +"Finished %c"
1 Fri Nov 28 13:39:38 2025
[1] 1606
2 Fri Nov 28 13:39:38 2025
[2] 1608
3 Fri Nov 28 13:39:38 2025
[3] 1610
[1]   Done                    sleep 5
[2]-  Done                    sleep 5
[3]+  Done                    sleep 5
Finished Fri Nov 28 13:39:43 2025
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1 Comment

Sorry if I'm missing something, but this answer doesn't seem to provide any information or explanation beyond what's already in the question itself?

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