How to spawn a background process in a bash script
Posted on In QAFor example, I want to spawn many ssh background processes in one bash script:
for i in `cat ./all-hosts`
do
ssh $i "ifconfig | grep Link"
done
Simply adding a & to the ssh commands does not work.
Here is the script that works:
for i in `cat ./all-hosts`
do
ssh $i "ifconfig | grep Link" &
done
Or in one command
for i in `cat ./all-hosts`; do ssh $i "ifconfig | grep Link" & done
What is needed is to create a subshell.
The makes many processes run in parallel.
To wait for all processes to exit to continue next step:
wait
For the above script:
for i in `cat ./all-hosts`
do
ssh $i "ifconfig | grep Link" &
done
wait
# continue for next steps
From ABS guide:
Running a shell script launches a new process, a subshell.
Definition: A subshell is a child process launched by a shell (or shell script).
A subshell is a separate instance of the command processor — the shell that gives you the prompt at the console or in an xterm window. Just as your commands are interpreted at the command-line prompt, similarly does a script batch-process a list of commands. Each shell script running is, in effect, a subprocess (child process) of the parent shell.
A shell script can itself launch subprocesses. These subshells let the script do parallel processing, in effect executing multiple subtasks simultaneously.