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About bash : How-do-you-run-multiple-programs-in-parallel-from-a-bash-script

Question Detail

I am trying to write a .sh file that runs many programs simultaneously

I tried this


But that runs prog1 then waits until prog1 ends and then starts prog2…

So how can I run them in parallel?

Question Answer

How about:

prog1 & prog2 && fg

This will:

  1. Start prog1.
  2. Send it to background, but keep printing its output.
  3. Start prog2, and keep it in foreground, so you can close it with ctrl-c.
  4. When you close prog2, you’ll return to prog1‘s foreground, so you can also close it with ctrl-c.

To run multiple programs in parallel:

prog1 &
prog2 &

If you need your script to wait for the programs to finish, you can add:


at the point where you want the script to wait for them.

If you want to be able to easily run and kill multiple process with ctrl-c, this is my favorite method: spawn multiple background processes in a (…) subshell, and trap SIGINT to execute kill 0, which will kill everything spawned in the subshell group:

(trap 'kill 0' SIGINT; prog1 & prog2 & prog3)

You can have complex process execution structures, and everything will close with a single ctrl-c (just make sure the last process is run in the foreground, i.e., don’t include a & after prog1.3):

(trap 'kill 0' SIGINT; prog1.1 && prog1.2 & (prog2.1 | prog2.2 || prog2.3) & prog1.3)

You can use wait:

some_command &
other_command &
wait $P1 $P2

It assigns the background program PIDs to variables ($! is the last launched process’ PID), then the wait command waits for them. It is nice because if you kill the script, it kills the processes too!

With GNU Parallel http://www.gnu.org/software/parallel/ it is as easy as:

(echo prog1; echo prog2) | parallel

Or if you prefer:

parallel ::: prog1 prog2

Learn more:

  • Watch the intro video for a quick introduction:
  • Walk through the tutorial (man parallel_tutorial). Your command line
    will love you for it.
  • Read: Ole Tange, GNU Parallel 2018 (Ole Tange, 2018).

xargs -P <n> allows you to run <n> commands in parallel.

While -P is a nonstandard option, both the GNU (Linux) and macOS/BSD implementations support it.

The following example:

  • runs at most 3 commands in parallel at a time,
  • with additional commands starting only when a previously launched process terminates.
time xargs -P 3 -I {} sh -c 'eval "$1"' - {} <<'EOF'
sleep 1; echo 1
sleep 2; echo 2
sleep 3; echo 3
echo 4

The output looks something like:

1   # output from 1st command 
4   # output from *last* command, which started as soon as the count dropped below 3
2   # output from 2nd command
3   # output from 3rd command

real    0m3.012s
user    0m0.011s
sys 0m0.008s

The timing shows that the commands were run in parallel (the last command was launched only after the first of the original 3 terminated, but executed very quickly).

The xargs command itself won’t return until all commands have finished, but you can execute it in the background by terminating it with control operator & and then using the wait builtin to wait for the entire xargs command to finish.

  xargs -P 3 -I {} sh -c 'eval "$1"' - {} <<'EOF'
sleep 1; echo 1
sleep 2; echo 2
sleep 3; echo 3
echo 4
} &

# Script execution continues here while `xargs` is running 
# in the background.
echo "Waiting for commands to finish..."

# Wait for `xargs` to finish, via special variable $!, which contains
# the PID of the most recently started background process.
wait $!


  • BSD/macOS xargs requires you to specify the count of commands to run in parallel explicitly, whereas GNU xargs allows you to specify -P 0 to run as many as possible in parallel.

  • Output from the processes run in parallel arrives as it is being generated, so it will be unpredictably interleaved.

    • GNU parallel, as mentioned in Ole’s answer (does not come standard with most platforms), conveniently serializes (groups) the output on a per-process basis and offers many more advanced features.

Here is a function I use in order to run at max n process in parallel (n=4 in the example):


function parallel {
  local time1=$(date +"%H:%M:%S")
  local time2=""

  # for the sake of the example, I'm using $2 as a description, you may be interested in other description
  echo "starting $2 ($time1)..."
  "[email protected]" && time2=$(date +"%H:%M:%S") && echo "finishing $2 ($time1 -- $time2)..." &

  local my_pid=$$
  local children=$(ps -eo ppid | grep -w $my_pid | wc -w)
  if [[ $children -ge $max_children ]]; then
    wait -n

parallel sleep 5
parallel sleep 6
parallel sleep 7
parallel sleep 8
parallel sleep 9

If max_children is set to the number of cores, this function will try to avoid idle cores.

prog1 & 2> .errorprog1.log; prog2 & 2> .errorprog2.log

Redirect errors to separate logs.

There is a very useful program that calls nohup.

     nohup - run a command immune to hangups, with output to a non-tty

This works beautifully for me (found here):

sh -c 'command1 & command2 & command3 & wait'

It outputs all the logs of each command intermingled (which is what I wanted), and all are killed with ctrl+c.

I had a similar situation recently where I needed to run multiple programs at the same time, redirect their outputs to separated log files and wait for them to finish and I ended up with something like that:


# Add the full path processes to run to the array
PROCESSES_TO_RUN=("/home/joao/Code/test/prog_1/prog1" \
# You can keep adding processes to the array...

for i in ${PROCESSES_TO_RUN[@]}; do
    ${i%/*}/./${i##*/} > ${i}.log 2>&1 &
    # ${i%/*} -> Get folder name until the /
    # ${i##*/} -> Get the filename after the /

# Wait for the processes to finish

Source: http://joaoperibeiro.com/execute-multiple-programs-and-redirect-their-outputs-linux/

You can try ppss (abandoned). ppss is rather powerful – you can even create a mini-cluster.
xargs -P can also be useful if you’ve got a batch of embarrassingly parallel processing to do.

Process Spawning Manager

Sure, technically these are processes, and this program should really be called a process spawning manager, but this is only due to the way that BASH works when it forks using the ampersand, it uses the fork() or perhaps clone() system call which clones into a separate memory space, rather than something like pthread_create() which would share memory. If BASH supported the latter, each “sequence of execution” would operate just the same and could be termed to be traditional threads whilst gaining a more efficient memory footprint. Functionally however it works the same, though a bit more difficult since GLOBAL variables are not available in each worker clone hence the use of the inter-process communication file and the rudimentary flock semaphore to manage critical sections. Forking from BASH of course is the basic answer here but I feel as if people know that but are really looking to manage what is spawned rather than just fork it and forget it. This demonstrates a way to manage up to 200 instances of forked processes all accessing a single resource. Clearly this is overkill but I enjoyed writing it so I kept on. Increase the size of your terminal accordingly. I hope you find this useful.

ME=$(basename $0)
IPC="/tmp/$ME.ipc"      #interprocess communication file (global thread accounting stats)
echo 0 > $IPC           #initalize counter
SPAWN=1000              #number of jobs to process
SPEEDFACTOR=1           #dynamically compensates for execution time
THREADLIMIT=50          #maximum concurrent threads
TPS=1                   #threads per second delay
THREADCOUNT=0           #number of running threads
SCALE="scale=5"         #controls bc's precision
START=$(date +%s)       #whence we began
MAXTHREADDUR=6         #maximum thread life span - demo mode

LOWER=$[$THREADLIMIT*100*90/10000]   #90% worker utilization threshold
UPPER=$[$THREADLIMIT*100*95/10000]   #95% worker utilization threshold
DELTA=10                             #initial percent speed change

threadspeed()        #dynamically adjust spawn rate based on worker utilization
   #vaguely assumes thread execution average will be consistent
   if [ $THREADCOUNT -ge $LOWER ] && [ $THREADCOUNT -le $UPPER ] ;then
      echo SPEED HOLD >> $DBG
   elif [ $THREADCOUNT -lt $LOWER ] ;then
      #if maxthread is free speed up
      SPEEDFACTOR=$(echo "$SCALE;$SPEEDFACTOR*(1-($DELTA/100))"|bc)
      echo SPEED UP $DELTA%>> $DBG
   elif [ $THREADCOUNT -gt $UPPER ];then
      #if maxthread is active then slow down
      SPEEDFACTOR=$(echo "$SCALE;$SPEEDFACTOR*(1+($DELTA/100))"|bc)
      DELTA=1                            #begin fine grain control
      echo SLOW DOWN $DELTA%>> $DBG


   #average thread duration   (total elapsed time / number of threads completed)
   #if threads completed is zero (less than 100), default to maxdelay/2  maxthreads

   COMPLETE=$(cat $IPC)

   if [ -z $COMPLETE ];then
      echo BAD IPC READ ============================================== >> $DBG

   #echo Threads COMPLETE $COMPLETE >> $DBG
   if [ $COMPLETE -lt 100 ];then
      ELAPSED=$[$(date +%s)-$START]
      #echo Elapsed Time $ELAPSED >> $DBG
   echo AVGTHREAD Duration is $AVGTHREAD >> $DBG

   #calculate timing to achieve spawning each workers fast enough
   # to utilize threadlimit - average time it takes to complete one thread / max number of threads
   #TPS=$(echo "$SCALE;$AVGTHREAD/$THREADLIMIT"|bc)  # maintains pretty good
   #echo TPS $TPS >> $DBG

function plot()
   echo -en \\033[${2}\;${1}H

   if [ -n "$3" ];then
         if [[ $4 = "good" ]];then
            echo -en "\\033[1;32m"
         elif [[ $4 = "warn" ]];then
            echo -en "\\033[1;33m"
         elif [[ $4 = "fail" ]];then
            echo -en "\\033[1;31m"
         elif [[ $4 = "crit" ]];then
            echo -en "\\033[1;31;4m"
      echo -n "$3"
      echo -en "\\033[0;39m"

trackthread()   #displays thread status
   ACTION=$3    #setactive | setfree | update

   TS=$(date +%s)


   case $ACTION in
      "setactive" )
         touch /tmp/$ME.$F1$WORKERID  #redundant - see main loop
         #echo created file $ME.$F1$WORKERID >> $DBG
         plot $COL $ROW "Worker$WORKERID: ACTIVE-TID:$THREADID INIT    " good
      "update" )
         plot $COL $ROW "Worker$WORKERID: ACTIVE-TID:$THREADID AGE:$AGE" warn
      "setfree" )
         plot $COL $ROW "Worker$WORKERID: FREE                         " fail
         rm /tmp/$ME.$F1$WORKERID
      * )


   for i in $(seq 1 $[$THREADLIMIT+1])
      if [ ! -e /tmp/$ME.$F1$i ];then
         #echo "getfreeworkerid returned $i" >> $DBG
   if [ $i -eq $[$THREADLIMIT+1] ];then
      #echo "no free threads" >> $DBG
      echo 0
      echo $i

   COMPLETE=$(cat $IPC)        #read IPC
   COMPLETE=$[$COMPLETE+1]     #increment IPC
   echo $COMPLETE > $IPC       #write back to IPC


   #accessing common terminal requires critical blocking section
   (flock -x -w 10 201
      trackthread $WORKERID $THREADID setactive


   for s in $(seq 1 $RND)               #simulate random lifespan
      sleep 1;
      (flock -x -w 10 201
         trackthread $WORKERID $THREADID update $s

   (flock -x -w 10 201
      trackthread $WORKERID $THREADID setfree

   (flock -x -w 10 201

   TC=$(ls /tmp/$ME.$F1* 2> /dev/null | wc -l)
   #echo threadcount is $TC >> $DBG
   echo $TC

   #summary status line
   COMPLETE=$(cat $IPC)
   echo -en '\033[K'                   #clear to end of line

function main()
   while [ $SPAWNED -lt $SPAWN ]
      while [ $(threadcount) -lt $THREADLIMIT ] && [ $SPAWNED -lt $SPAWN ]
         worker $WID $SPAWNED &
         touch /tmp/$ME.$F1$WID    #if this loops faster than file creation in the worker thread it steps on itself, thread tracking is best in main loop
         (flock -x -w 10 201
         sleep $TPS
        if ((! $[$SPAWNED%100]));then
           #rethink thread timing every 100 threads
      sleep $TPS

   while [ "$(threadcount)" -gt 0 ]
      (flock -x -w 10 201
      sleep 1;



Your script should look like:

prog1 &
prog2 &
progn &
progn+1 &
progn+2 &

Assuming your system can take n jobs at a time. use wait to run only n jobs at a time.

If you have a GUI terminal, you could spawn a new tabbed terminal instance for each process you want to run in parallel.

This has the benefit that each program runs in its own tab where it can be interacted with and managed independently of the other running programs.

For example, on Ubuntu 20.04:

gnome-terminal --tab -- bash -c 'prog1'
gnome-terminal --tab -- bash -c 'prog2'

To run certain programs or other commands sequentially, you can add ;

gnome-terminal --tab -- bash -c 'prog1_1; prog1_2'
gnome-terminal --tab -- bash -c 'prog2'

I’ve found that for some programs, the terminal closes before they start up. For these programs I append the terminal command with ; wait or ; sleep 1

gnome-terminal --tab -- bash -c 'prog1; wait'

For Mac OS, you would have to find an equivalent command for the terminal you are using – I haven’t tested on Mac OS since I don’t own a Mac.

If you’re:

  • On Mac and have iTerm
  • Want to start various processes that stay open long-term / until Ctrl+C
  • Want to be able to easily see the output from each process
  • Want to be able to easily stop a specific process with Ctrl+C

One option is scripting the terminal itself if your use case is more app monitoring / management.

For example I recently did the following. Granted it’s Mac specific, iTerm specific, and relies on a deprecated Apple Script API (iTerm has a newer Python option). It doesn’t win any elegance awards but gets the job done.


osascript <<THEEND
tell application "iTerm"
  set newWindow to (create window with default profile)

  tell current session of newWindow
    set name to "Auth API"
    write text "pushd $root_path && $auth_api_script"
  end tell

  tell newWindow
    set newTab to (create tab with default profile)
    tell current session of newTab
        set name to "Admin API"
        write text "dotnet run --debug -p $admin_api_proj"
    end tell
  end tell

  tell newWindow
    set newTab to (create tab with default profile)
    tell current session of newTab
        set name to "Agent"
        write text "dotnet run --debug -p $agent_proj"
    end tell
  end tell

  tell newWindow
    set newTab to (create tab with default profile)
    tell current session of newTab
        set name to "Dashboard"
        write text "pushd $dashboard_path; ng serve -o"
    end tell
  end tell

end tell

With bashj ( https://sourceforge.net/projects/bashj/ ) , you should be able to run not only multiple processes (the way others suggested) but also multiple Threads in one JVM controlled from your script. But of course this requires a java JDK. Threads consume less resource than processes.

Here is a working code:



public static int cnt=0;

private static void loop() {u.p("java says cnt= "+(cnt++));u.sleep(1.0);}

public static void startThread()
{(new Thread(() ->  {while (true) {loop();}})).start();}



while [ j.cnt -lt 4 ]
  echo "bash views cnt=" j.cnt
  sleep 0.5

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