What does “exec some_cmd &” mean in bash

Question

I found a use of exec some_cmd & in this script:

...
exec elixir \
  -pa _build/prod/consolidated \
  --no-halt \
  --erl "+A$THREAD_COUNT" \
  --erl "+K true" \
  --erl "-smp auto" \
  --erl "+scl false" \
  --erl "+spp true" \
  --erl "+swt low" \
  --erl "+sbwt long" \
  --sname $NODE \
  --cookie $COOKIE \
  -S mix run \
    --no-compile \
$@ \
&
...

exec some_cmd replaces the shell with some_cmd. some_cmd & spawns some_cmd as a child process in background. So what happens when combining them?

I gave it a try with bash 3.2 and the result shows that it looks like it spawns a background process:

# script
echo "Shell PID: $$"
exec sh -c 'echo "Child PID: $$"' &
BG_PID=$!
echo "Background process PID: $BG_PID"

# output
Shell PID: 8852
Background process PID: 8853
Child PID: 8853

Though I'm not sure whether it is exactly the same as some_cmd &.

Answer 1

The exec command has no effect in this case. Normally, exec would prevent bash from forking before calling execve, but in this case bash forks anyway because you said &.

So exec some_cmd & is the same as some_cmd &.

You might use exec with & anyway if you need to use one of the exec flags. Type help exec to see what flags exec supports.