[Linux 常見問題] Shell script wait for background command

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Question
I am writing a script, but there is something I need that I can't find a way to do it...

I need to make a command in the background "command1 &" and then somewhere in the script I need to wait for it to finish before I do command2. Basically, I need this:

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1. a=1  
2.   
3. while [$a -lt 4 ]  
4.   
5.      . command1 &  
6.    #Generates 1 Process    
7.   
8.      a= `export $a +1`  
9. done  
10.   
11.    #Wait until the 4 process end and then run the command2   
12.     . command2  

I've seen something about a wait command with the pid process number, but that didn't work also.

HowTo
You can use the command wait PID to wait for a process to end; You can also retrieve the PID of the last command with $!

In your case, something like this would work:
- test.sh

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1. #!/bin/sh  
2. L=1  
3. PID_LIST=""  
4. while [ $L -lt 4 ]  
5. do  
6.     sleep 10 && echo "Loop-$L" &  
7.     PID=$!  
8.     PID_LIST="$PID_LIST $PID"  
9.     L=$((L+1))  
10. done  
11.   
12. echo "Wait for all command1 to be completed..."  
13. wait $PID_LIST  
14. echo "All done!"  

Execution result:

# ./test.sh
Wait for all command1 to be completed...
Loop-1
Loop-2
Loop-3
All done!

Supplement
* Bash shell script – while 迴圈
* How to Increment and Decrement Variable in Bash (Counter)