Specs
Don’t think this should make a difference but I’ll include it anyway
GNU bash, version 3.2.51
Premise
So say i have a pipe with multiple parts, how can I prevent part of the pipe running before the previous part is finished.
Example
In the below examples I will try to display the issue
$ echo hello | tee /dev/tty | (echo "Next";sed 's/h/m/' )
Output
Next hello mello
With sleep to show that it is the timing that is off
$ echo hello | tee /dev/tty | (sleep 2;echo "Next";sed 's/h/m/' )
Output
hello Next mello
Expected Output
Is as above
hello Next mello
but obviously this depends on the sleep being longer than it takes for the previous command to finish which is not what i would want.
Notes
I know there are better ways to do this but I just think it would be educational for me to understand exactly how the pipe is working.
Tried
Tried variations of wait and sleep and things but nothing works consistently.
This still prints Next first though
$ echo hello | tee /dev/tty | sort |(echo "Next";sed 's/h/m/' ) Next hello mello $ echo hello | tee /dev/tty | tac | tac |(echo "Next";sed 's/h/m/' ) Next hello mello
If any more information is needed then please let me know.
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Answer
There is no language construct in bash to modify the behavior of a pipeline like you want. However, you can use a named pipe to act as a type of binary semaphore:
mkfifo block
echo hello |
{ tee /dev/tty; echo go > block; } |
(read < block; echo "Next"; sed 's/h/m/' )
The read command blocks until something writes to named pipe, which does’t happen until tee completes.
(Note that this may not completely solve your problem, as in addition to the process synchronization you may need to contend with the fact that you have multiple processes writing to the same output file, and you don’t have complete control over how the various writes are multiplexed (due to buffering, etc.)