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up vote 4 down vote favorite

I have the following test script.

#!/bin/bash

function foo {
  printf "Test line break: $1\nafter line break\n\n"
}

for VARIABLE in {1..30}
do
  foo $VARIABLE &
done
wait

Now I executed the script several times, and some times the output looks like that:

Test line break: 15
Test line break: 14
after line break: 14
after line break: 15


Test line break: 16
after line break: 16

instead of the expected:

Test line break: 15
after line break: 15

Test line break: 14
after line break: 14

Test line break: 16
after line break: 16

how can I prevent that printf "interrupts" another printf, but still running it in the background?

For me it seems, that the problem is the "\n" inside printf.

share|improve this question
    
If you're going to use printf remember that its first string is actually a format string and so understands % as a format character. You would be better with printf "%s" ""Test line break: $1\nafter line break\n\n" or even printf "Test line break: %s\nafter line break\n\n" "$1" –  roaima Sep 7 at 19:19

1 Answer 1

up vote 5 down vote accepted

In fact you need a mutex :

Each sub-shell will access concurrently to the same /dev/stdout of the parent shell, so you can not ensure the order even inside the same function. To ensure it you need a lock which enforce the mutual exclusion i.e. : all other processes won't start to write in /dev/stdout until the lock is released. 

#!/bin/bash

function foo {
lockdir=/tmp/myscript.lock
 mkdir "$lockdir" 2>/dev/null
while [ $? -ne 0 ]; do mkdir "$lockdir" 2>/dev/null; done
printf "Test line break: $1\nafter line break: $1\n\n"
rm -rf $lockdir
}

for VARIABLE in {1..30}
do
  foo $VARIABLE &
done
wait

This will give this as a result : 

$ bash plop1 2>/dev/null
Test line break: 5
after line break: 5

Test line break: 3
after line break: 3

Test line break: 11
after line break: 11

Test line break: 23
after line break: 23

Test line break: 14
after line break: 14

Test line break: 17
after line break: 17

Test line break: 24
after line break: 24

Test line break: 21
after line break: 21

Test line break: 27
after line break: 27

Test line break: 6
after line break: 6

Test line break: 2
after line break: 2

Test line break: 9
after line break: 9

Test line break: 26
after line break: 26

Test line break: 29
after line break: 29

Test line break: 20
after line break: 20

Test line break: 1
after line break: 1

Test line break: 12
after line break: 12

Test line break: 4
after line break: 4

Test line break: 13
after line break: 13

Test line break: 10
after line break: 10

Test line break: 15
after line break: 15

Test line break: 28
after line break: 28

Test line break: 25
after line break: 25

Test line break: 19
after line break: 19

Test line break: 18
after line break: 18

Test line break: 8
after line break: 8

Test line break: 7
after line break: 7

Test line break: 16
after line break: 16

Test line break: 22
after line break: 22

Test line break: 30
after line break: 30
share|improve this answer
2  
you can also have a look at this excellent wiki : wiki.bash-hackers.org/howto/mutex –  netmonk Sep 7 at 13:03
    
thank you! But is it possible without having to type 2>/dev/null? –  sam Sep 7 at 13:24
    
yes, you just have to add it inside the function ! i just edited the script –  netmonk Sep 7 at 13:25
    
very cool, thanks –  sam Sep 7 at 13:28

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