In a shell script, how can I (1) start a command in the background (2) wait x seconds (3) run a second command while that command is running?

Question

This is what I need to happen:

1. start process A in the background
2. wait for x seconds
3. start process B in the foreground

How can I make the wait happen?

I'm seeing that 'sleep' seems to halt everything and I don't actually want to 'wait' for process A to finish entirely. I've seen some time based loops but I'm wondering if there's something cleaner.

Answer 1

Unless I'm misunderstanding your question, it can simply be achieved with this short script:

#!/bin/bash

process_a &
sleep x
process_b

(and add an extra wait at the end if you want your script to wait for process_a to finish before exiting).

You can even do this as an one-liner, without the need for a script (as suggested by @BaardKopperud):

process_a & sleep x ; process_b

Answer 2

You can use the background control operator (&) to run a process in the background and the sleep command to wait before running a second process, i.e.:

#!/usr/bin/env bash
# script.sh

command1 &
sleep x
command2

Here is an example of two commands that print out some time-stamped messages:

#!/usr/bin/env bash

# Execute a process in the background
echo "$(date) - Running first process in the background..."
for i in {1..1000}; do
    echo "$(date) - I am running in the background";
    sleep 1;
done &> background-process-output.txt &

# Wait for 5 seconds
echo "$(date) - Sleeping..."
sleep 5 

# Execute a second process in the foreground
echo "$(date) - Running second process in the foreground..."
for i in {1..1000}; do
    echo "$(date) - I am running in the foreground";
    sleep 1;
done

Run it to verify that it exhibits the desired behavior:

user@host:~$ bash script.sh

Fri Dec  1 13:41:10 CST 2017 - Running first process in the background...
Fri Dec  1 13:41:10 CST 2017 - Sleeping...
Fri Dec  1 13:41:15 CST 2017 - Running second process in the foreground...
Fri Dec  1 13:41:15 CST 2017 - I am running in the foreground
Fri Dec  1 13:41:16 CST 2017 - I am running in the foreground
Fri Dec  1 13:41:17 CST 2017 - I am running in the foreground
Fri Dec  1 13:41:18 CST 2017 - I am running in the foreground
Fri Dec  1 13:41:19 CST 2017 - I am running in the foreground
Fri Dec  1 13:41:20 CST 2017 - I am running in the foreground
...
...
...

Answer 3

I like @dr01's answer but he doesn't check the exit code and so you don't know if you were successful or not.

Here's a solution that checks the exitcodes.

#!/bin/bash

# run processes
process_a &
PID1=$!
sleep x
process_b &
PID2=$!
exitcode=0

# check the exitcode for process A
wait $PID1    
if (($? != 0)); then
    echo "ERROR: process_a exited with non-zero exitcode" >&2
    exitcode=$((exitcode+1))
fi

# check the exitcode for process B
wait $PID2
if (($? != 0)); then
    echo "ERROR: process_b exited with non-zero exitcode" >&2
    exitcode=$((exitcode+1))
fi
exit ${exitcode}

usually i store the PIDs in a bash array and then the pid checking is a for loop.