wait bash built-in command

gistfile1.txt

execute something/service in the background
and based on its exist status, execute something else

example
i have installed MySQL NDB cluster. It needs around 2-3 minutes to synchronise the database in between cluster nodes. Once the synchronisation finished only, I can start another application called heartbeat. Therefore in my shell script, I want to check the availability of the ndbd service all the times. Once this service become available, I need to start the heartbeat application 



# send to background
./ndb_startup_script & 
# 
# wait on pid of ndb_startup_script
wait $!

if [ $? -eq 0 ] # assuming 0 is ret code for successful run
then 
    ./heartbeat_startup_script
else 
    # extra stuff to handle error
fi



wget -q http://cloud.centos.org/centos/7/images/CentOS-7-x86_64-GenericCloud-1503.qcow2.xz
wait $!

gistfile2.txt

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wait makes the shell wait for the given subprocess. e.g.:

workhard &
[1] 27408
workharder &
[2] 27409
wait %1 %2
delays the shell until both of the subprocesses have finished


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The wait command in bash has a -n option:

If the  -n option is supplied, wait waits for any job to terminate and returns its exit status.

This means that you may do

command1 &
command2 &
command3 &

wait -n
printf 'One command exited with exit code %d\n' "$?"


-

wget -q http://isoredirect.centos.org/altarch/7/isos/aarch64/CentOS-7-aarch64-NetInstall-1810.iso &

wget -q http://cloud.centos.org/centos/7/images/CentOS-7-x86_64-GenericCloud.qcow2.xz &

  
wait -n
printf 'One command exited with exit code %d\n' "$?"
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