XiaoBing Jiang wrote: > Greg Wooledge wrote: > > If you want to ENSURE that the child shell process is replaced by the > > external sleep 20, use an explicit exec. > > yes, I want to know why bash not optimize this. or any strategy ?
Because it wouldn't save anything significant. Since the parent shell is bash the extra bash in memory is already in memory. A forked process uses almost the same OS memory as the original. The OS will share the code pages between the two processes at the OS memory page level. Data pages will split when they are individually modified. Even if you use 'exec' to replace the forked and backgrounded copy another one is still in memory as the script runs. Only unique data pages will go through the copy-on-write process as the two bash processes diverge. Therefore even if you *think* you are saving something by optimizing it out the actual savings is insignificantly small. Plus this is really only a problem if the main part of the script exits leaving the forked subshell running in the background. However that is not a good thing to do. If you want to leave the process running that is a daemon process. Launching a daemon process needs more setup than this to avoid other problems. Since leaving processes behind like that is a bad thing to do it again doesn't make much sense to try to optimize the doing of it. It is possible to contrive some examples where it makes sense to do this optimization. But mostly they are very unusual cases. Not typical cases. Bob
