wait

grob115 · 03-08-2011, 11:08 AM

Hi, if I am writing a script and want to wait for one of the lines to finish executing before moving on, how do I do this? For example, how do I wait for the find command in the following script to be completely finished before printing out the "done"?

#!/bin/bash
find / -name 'file*log' > /home/search_results.log
<wait>
echo "done"

Snark1994 · 03-08-2011, 11:11 AM

Um... That's default behaviour, isn't it?

Code:

[joshua@joshua-desktop:~/test]$ cat test2.sh                      (08-03 17:11)
#!/bin/bash

find / -name 'file*log' > search_results.log
echo "done"
[joshua@joshua-desktop:~/test]$ sh test2.sh  

<lots of other permission messages>

find: `/root': Permission denied
find: `/usr/lib64/mozilla/extensions': Permission denied
find: `/tmp/tmux-0': Permission denied
find: `/lost+found': Permission denied
find: `/etc/openvpn/certs': Permission denied
find: `/etc/openvpn/keys': Permission denied
find: `/etc/cups/ssl': Permission denied
find: `/etc/polkit-1/localauthority': Permission denied
find: `/etc/samba/private': Permission denied
done

grob115 · 03-08-2011, 11:16 AM

Okay I must admit I didn't test that but typed it in a hurry trying to make a point but I guess that was a bad example. Basically I have come across some scripts within which some of the commands moved on before finishing. Am wondering how I can put a wait or something to halt it before moving on. Do I make sense?

theNbomr · 03-08-2011, 12:00 PM

As Snark1994 points out, that is the default behavior. If you want a process to run as a background process in a shell script, you have to explicitly launch it as such by appending the '&' character to the commandline. This is probably the case that you saw, without realizing it.

Some applications are capable of launching themselves as daemon processes, by detaching themselves from the controlling terminal. These are intended to run persistently, and you probably don't want to wait for them to complete.

--- rod.

grob115 · 03-08-2011, 12:04 PM

Ah yes correct the cases I saw were executed in the background. Okay so in this case, how do I make it such that the next command in the script will wait for the previous line to complete in background? I know I can simply change it by dropping the "&" at the end but is there a way?

theNbomr · 03-08-2011, 03:14 PM

I cannot think of as way to do that without running the risk of a race problem. The bash wait command requires a PID, and you would have to do a bit of shell gymnastics to locate the PID of the process of interest. In the meantime, the process may have already terminated.
--- rod.

Nominal Animal · 03-08-2011, 03:17 PM

Yes, see the Job Control Builtins section in the Bash Reference Manual for details.

For example, if you wanted to make sure all the background tasks the current shell has started, just use the wait Bash built-in (no arguments, that's it).

(theNbomr, wait does not need a PID. And if you wish to select one from the background tasks, use jobs and e.g. sed to pick out the interesting ones, and feed the job ID's to wait.)

theNbomr · 03-08-2011, 04:47 PM

Okay, I actually knew you could supply a job ID. But how will you get a job ID to pass to wait (assuming you don't want to wait on all running processes)? Seems to me that you still end up with the possibility of the process terminating before you've figured out wait you're waiting for.

In any case, the whole concept seems to be little more than of academic interest, unless someone can explain why you would want to do it that way, or what problem it solves that couldn't be solved the 'usual' way.

--- rod.

ntubski · 03-08-2011, 06:05 PM

Quote:

Originally Posted by theNbomr

I cannot think of as way to do that without running the risk of a race problem. The bash wait command requires a PID, and you would have to do a bit of shell gymnastics to locate the PID of the process of interest. In the meantime, the process may have already terminated.
--- rod.

You can just use $! to get the PID.

Quote:

!

Expands to the process id of the most recently executed background (asynchronous) command.

Special Parameters

theNbomr · 03-08-2011, 06:27 PM

Yes, of course. But you would still have to do something like

Code:

mycommand
#
#  Somewhere in here, 'mycommand' may terminate
#
wait $!

You could argue that waiting for a non-existant PID is harmless, except it produces the error message 'bash: wait: pid XXXXXXX is not a child of this shell'.
I just don't see the point of the exercise when there is a better way to do it.

--- rod.

ntubski · 03-08-2011, 09:10 PM

Quote:

Originally Posted by theNbomr

You could argue that waiting for a non-existant PID is harmless, except it produces the error message 'bash: wait: pid XXXXXXX is not a child of this shell'.
I just don't see the point of the exercise when there is a better way to do it.

No, the pid remains in the process table as a zombie process until the parent waits for it:

Code:

~$ false &
[1] 11881
~$ 
[1]+  Exit 1                  false
~$ wait $! ; echo $?
1