Methodd 1 : use shell built-in “wait” to run command in parellel

j@ubuntu2004:~$ sleep 10 & time wait
[1] 4564
[1]+  Done                    sleep 10

real	0m10.002s
user	0m0.002s
sys	0m0.000s
j@ubuntu2004:~$

We can see command wait will be waiting for “sleep 10”, so we can run particular numbers of cmds in background ,then wait then run another particular number of command, like below

cmd1 &
cmd2 &
cmd3 &
wait
cmd4 &
cmd5 &
wait
cmd6 &

A simple example below :

j@ubuntu2004:~/bin$ cat test.sh 
sleep 2 &
sleep 2 &
sleep 2 &
wait
sleep 2 &
sleep 2 &
sleep 2 &
wait
sleep 2 &
sleep 2 &
sleep 2 &
wait
j@ubuntu2004:~/bin$ time ./test.sh 

real	0m6.025s
user	0m0.011s
sys	0m0.029s

Method 2: use xargs to run cmds in parellel

-P max-procs, --max-procs=max-procs
          Run up to max-procs processes at a time; the default is 1.

Above is from manpage of command xargs

So we can run command in parellel using below format:

somecontent | xargs -n1 -P N command

Eg: suppose we want to ping multiple hosts in parellel, we can:

j@ubuntu2004:~$ cat hosts.txt 
host01
host02
host03
host04
host05
host06
host07
host08
host09
host10
j@ubuntu2004:~$ cat hosts.txt |xargs -n 1 -P 5 ping -c 1

ssh to multiple hosts in parellel (5 processes) to get “uptime”

j@ubuntu2004:~$ cat hosts.txt |xargs -n 1 -P 5 -I '{}' ssh  '{}' uptime 
 09:57:46 up  2:09,  1 user,  load average: 0.15, 0.36, 0.22
 09:57:46 up  2:09,  1 user,  load average: 0.15, 0.36, 0.22
 09:57:46 up  2:09,  1 user,  load average: 0.15, 0.36, 0.22
 09:57:46 up  2:09,  1 user,  load average: 0.15, 0.36, 0.22
 09:57:46 up  2:09,  1 user,  load average: 0.15, 0.36, 0.22
 09:57:47 up  2:09,  1 user,  load average: 2.06, 0.76, 0.35
 09:57:47 up  2:09,  1 user,  load average: 2.06, 0.76, 0.35
 09:57:47 up  2:09,  1 user,  load average: 2.06, 0.76, 0.35
 09:57:47 up  2:09,  1 user,  load average: 2.06, 0.76, 0.35
 09:57:47 up  2:09,  1 user,  load average: 2.06, 0.76, 0.35

-I replace-str
              Replace occurrences of replace-str in the initial-arguments with
              names read from standard input.
To discard “exited with status 255; aborting” error ,add “sh -c” and “   true” 
cat hosts.txt |xargs -n 1 -P 5 -I '{}' sh -c "ssh  '{}' uptime||true"