[bash] Waiting for background processes to finish before exiting script

How do I make sure that all my background processes have finished execution before I exit my script (TCL/Bash).

I was thinking of writing all my background process pids to a pidfile. And then at the end pgrep the pidfile to see if any processes are still running before I exit.

Is there some simpler way to do this? And is there a TCL specific way to do this?

GNU parallel and xargs

These two tools that can make scripts simpler, and also control the maximum number of threads (thread pool). E.g.:

seq 10 | xargs -P4 -I'{}' echo '{}'

or:

seq 10 | parallel -j4  echo '{}'

See also: how to write a process-pool bash shell

You can use kill -0 for checking whether a particular pid is running or not.

Assuming, you have list of pid numbers in a file called pid in pwd

while true;
do 
    if [ -s pid ] ; then
        for pid in `cat pid`
        do  
            echo "Checking the $pid"
            kill -0 "$pid" 2>/dev/null || sed -i "/^$pid$/d" pid
        done
    else
        echo "All your process completed" ## Do what you want here... here all your pids are in finished stated
        break
    fi
done

Even if you do not have the pid, you can trigger 'wait;' after triggering all background processes. For. eg. in commandfile.sh-

bteq < input_file1.sql > output_file1.sql &
bteq < input_file2.sql > output_file2.sql &
bteq < input_file3.sql > output_file3.sql &
wait

Then when this is triggered, as -

subprocess.call(['sh', 'commandfile.sh'])
print('all background processes done.')

This will be printed only after all the background processes are done.

WARNING: Long script ahead.

A while ago, I faced a similar problem: from a Tcl script, launch a number of processes, then wait for all of them to finish. Here is a demo script I wrote to solve this problem.

main.tcl

#!/usr/bin/env tclsh

# Launches many processes and wait for them to finish.
# This script will works on systems that has the ps command such as
# BSD, Linux, and OS X

package require Tclx; # For process-management utilities

proc updatePidList {stat} {
    global pidList
    global allFinished

    # Parse the process ID of the just-finished process
    lassign $stat processId howProcessEnded exitCode

    # Remove this process ID from the list of process IDs
    set pidList [lindex [intersect3 $pidList $processId] 0]
    set processCount [llength $pidList]

    # Occasionally, a child process quits but the signal was lost. This
    # block of code will go through the list of remaining process IDs
    # and remove those that has finished
    set updatedPidList {}
    foreach pid $pidList {
        if {![catch {exec ps $pid} errmsg]} {
            lappend updatedPidList $pid
        }
    }

    set pidList $updatedPidList

    # Show the remaining processes
    if {$processCount > 0} {
        puts "Waiting for [llength $pidList] processes"
    } else {
        set allFinished 1
        puts "All finished"
    }
}

# A signal handler that gets called when a child process finished.
# This handler needs to exit quickly, so it delegates the real works to
# the proc updatePidList
proc childTerminated {} {
    # Restart the handler
    signal -restart trap SIGCHLD childTerminated

    # Update the list of process IDs
    while {![catch {wait -nohang} stat] && $stat ne {}} {
        after idle [list updatePidList $stat]
    }
}

#
# Main starts here
#

puts "Main begins"
set NUMBER_OF_PROCESSES_TO_LAUNCH 10
set pidList {}
set allFinished 0

# When a child process exits, call proc childTerminated
signal -restart trap SIGCHLD childTerminated

# Spawn many processes
for {set i 0} {$i < $NUMBER_OF_PROCESSES_TO_LAUNCH} {incr i} {
    set childId [exec tclsh child.tcl $i &]
    puts "child #$i, pid=$childId"
    lappend pidList $childId
    after 1000
}

# Do some processing
puts "list of processes: $pidList"
puts "Waiting for child processes to finish"
# Do some more processing if required

# After all done, wait for all to finish before exiting
vwait allFinished

puts "Main ends"

child.tcl

#!/usr/bin/env tclsh
# child script: simulate some lengthy operations

proc randomInteger {min max} {
    return [expr int(rand() * ($max - $min + 1) * 1000 + $min)]
}

set duration [randomInteger 10 30]
puts "  child #$argv runs for $duration miliseconds"
after $duration
puts "  child #$argv ends"

Sample output for running main.tcl

Main begins
child #0, pid=64525
  child #0 runs for 17466 miliseconds
child #1, pid=64526
  child #1 runs for 14181 miliseconds
child #2, pid=64527
  child #2 runs for 10856 miliseconds
child #3, pid=64528
  child #3 runs for 7464 miliseconds
child #4, pid=64529
  child #4 runs for 4034 miliseconds
child #5, pid=64531
  child #5 runs for 1068 miliseconds
child #6, pid=64532
  child #6 runs for 18571 miliseconds
  child #5 ends
child #7, pid=64534
  child #7 runs for 15374 miliseconds
child #8, pid=64535
  child #8 runs for 11996 miliseconds
  child #4 ends
child #9, pid=64536
  child #9 runs for 8694 miliseconds
list of processes: 64525 64526 64527 64528 64529 64531 64532 64534 64535 64536
Waiting for child processes to finish
Waiting for 8 processes
Waiting for 8 processes
  child #3 ends
Waiting for 7 processes
  child #2 ends
Waiting for 6 processes
  child #1 ends
Waiting for 5 processes
  child #0 ends
Waiting for 4 processes
  child #9 ends
Waiting for 3 processes
  child #8 ends
Waiting for 2 processes
  child #7 ends
Waiting for 1 processes
  child #6 ends
All finished
Main ends