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Top Forums UNIX for Dummies Questions & Answers How do you wait for command substitution processes to complete? Post 302397633 by mrvwman on Monday 22nd of February 2010 07:39:34 PM
Old 02-22-2010
How do you wait for command substitution processes to complete?

When running a command using the >(cmd) syntax in bash how do you wait for the command to complete before moving on in your script?

Here is a simple example:
Code:
zcat largefile.gz | tee >(wc && echo “HELLO”) > /dev/null
# I tried wait, here but it doesn't wait for the process in the subshell.
echo "WORLD"

This produces the output:
Code:
WORLD
22245  175374 3598423
HELLO

My objective is to guarantee echo "WORLD" gets printed after all of the >() processes are done.

The bigger picture is that I have a lot of large log files to process through a variety of log parsers and using this syntax does have the positive effect of reading the files from disk once to process them in parallel in many sub processes taking advantage of multi-core CPUs. Everything works fine except for the fact that I can't tell when those processes are done.

I can think of one solution where I could have my log processing scripts write an "I'm Done" message to a file or pipe somewhere for the parent to loop listening for.

Is there a better way?
 

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FORK(2) 							System Calls Manual							   FORK(2)

NAME
fork - spawn new process SYNOPSIS
fork( ) DESCRIPTION
Fork is the only way new processes are created. The new process's core image is a copy of that of the caller of fork. The only distinc- tion is the fact that the value returned in the old (parent) process contains the process ID of the new (child) process, while the value returned in the child is 0. Process ID's range from 1 to 30,000. This process ID is used by wait(2). Files open before the fork are shared, and have a common read-write pointer. In particular, this is the way that standard input and output files are passed and also how pipes are set up. SEE ALSO
wait(2), exec(2) DIAGNOSTICS
Returns -1 and fails to create a process if: there is inadequate swap space, the user is not super-user and has too many processes, or the system's process table is full. Only the super-user can take the last process-table slot. ASSEMBLER
(fork = 2.) sys fork (new process return) (old process return, new process ID in r0) The return locations in the old and new process differ by one word. The C-bit is set in the old process if a new process could not be cre- ated. FORK(2)
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