Silently start task in background

Here's how to do it in both {ba,z}sh.

Using a function helps not needing to save and restore the state of zsh's monitor variable by use of setopt local_options.

# Run the command given by "$@" in the background
silent_background() {
  if [[ -n $ZSH_VERSION ]]; then  # zsh:  https://superuser.com/a/1285272/365890
    setopt local_options no_notify no_monitor
    "$@" &
  elif [[ -n $BASH_VERSION ]]; then  # bash: https://stackoverflow.com/a/27340076/5353461
    { 2>&3 "$@"& } 3>&2 2>/dev/null
  else  # Unknownness - just background it
    "$@" &
  fi
}

Using this as follows:

trap 'echo child done' CHLD
silent_background sleep 1

Will produce the expected child done message.

The redirection of the command output seems to be irrelevant because the notification is send by the shell when a job is started asynchronously. More precisely, it is a shell feature (fonctionality) related to job control.

Here, a quote that comes from the "Bash Reference Manual", chapter "Job Control", section one.

The shell associates a JOB with each pipeline. It keeps a table of currently executing jobs, which may be listed with the jobs command. When Bash starts a job asynchronously, it prints a line that looks like:

[1] 25647

indicating that this job is job number 1 and that the process ID of the last process in the pipeline associated with this job is 25647. All of the processes in a single pipeline are members of the same job. Bash uses the JOB abstraction as the basis for job control.

Note that a shell script does not display this notification.

$ cat test
#!/bin/bash

true & echo true
$ ./test
true

In facts

Zsh

The Zsh documentation provides similar indications about these notifications, see man 1 zshmisc, section "JOBS". These notifications are not displayed when job control is disabled.

MONITOR (-m, ksh: -m)

    Allow job control. Set by default in interactive shell.

zsh_prompt % setopt no_monitor
zsh_prompt % true & echo true
true
zsh_prompt %
zsh_prompt % 

Bash

It seems that Bash always displays e.g. [1] 25647. The "final notification" e.g. [1]+ Done true is not displayed when job control is disabled.

bash_prompt $ true & echo true
[1] 25647
true
bash_prompt $ 
[1]+  Done                    true

Job control disabled

bash_prompt $ set +m       # disable job control
bash_prompt $ true & echo true
[1] 25685
bash_prompt $
bash_prompt $

Conclusion

I do not know if disable job control to hide the notifications is a good thing?

Resources

• man 1 zshmisc, section JOBS.
• Zsh, MONITOR: man 1 zshoptions.
• Bash Reference Manual - Job Control
• Bash Reference Manual - The Set Builtin

You may invoke zsh as a non-interactive shell to disable job notifications.

zsh -c '
   trap "echo trapped child_done" CHLD
   sleep 5 &
   #wait
   sleep 10 # simulate other running cmds
   echo script done
'

See the following example :

trap 'echo "DONE"' 0
{ find /usr & } &> /dev/null
wait

That works well. No need ( ) : that makes a not needed subshell. { } are just grouping commands.

This is an old question, but a possible answer (in bash, not tested with zsh) is to use the job control in the subshell: (command & wait)

Note: bash suppresses the start/end background job messages in the subshell, and there no need for redirection.

2 Possible Answers:

- Subshell with redirects.

     or

- Push to a Screen ( a multiplexer / virtual terminal ) session

• The 'Screen' option maybe a bit convoluted, but it also gives you the ability to attach the 'Named' screen session and see what's happening.

1. You can run in a subshell, and redirect all output to /dev/null.

$( { sleep 10; touch a_file } >/dev/null 2>&1 & ) >/dev/null 2>&1;

2. A detached screen session.

• Create Screen with a name Detached.

screen -dmS "${SCREEN_NAME}";

• Run (stuff) the command(s) into the named screen session.

screen -S "${SCREEN_NAME}" -X stuff "${CMD_LINES}\nsleep ${SECS}\ntouch a_file\nexit\n";

I like adding a touched file to watch and wait for the backgrounded process to complete before Exiting the main script