Multiple exec in a shell script

The use of & implie a sub-process.

So exec have no effect.

Demo:

export LANG=Cecho $$17259exec sh -c 'echo $$;read foo' &[1] 1753817538[1]+  Stopped                 exec sh -c 'echo $$;read foo'   fgexec sh -c 'echo $$;read foo'  17259

I run the script: echo $$;read foo in order to prevent exit before having quietly read previous output.

In this sample, the current process ID is 17259.

When run with ampersand (&), the output is another pid (bigger). when run without ampersand, the new shell replace the command and is not forked.

Replacing the command by:

sh -c 'echo $$;set >/tmp/fork_test-$$.env;read'

re-running the whole test will generate two files in /tmp.

On my desk, I could read:

197721999419772

So I found two files in /tmp:

-rw-r--r-- 1 user0 user0 2677 jan 22 00:26 /tmp/fork_test-19772.env-rw-r--r-- 1 user0 user0 2689 jan 22 00:27 /tmp/fork_test-19994.env

If I run: diff /tmp/fork_test-19*env, I read:

29c29< SHLVL='0'---> SHLVL='1'46a47> _='/bin/sh'

So the first run, with ampersand is in a sublevel.

Nota: This was tested under many different shell.

The shell forks to run the background process, but that means the new shell still needs to fork to run yes. Using exec eliminates the fork in the subshell.