Unix & Linux Stack Exchange is a question and answer site for users of Linux, FreeBSD and other Un*x-like operating systems. It only takes a minute to sign up.
Sign up to join this community
I have a script that I would like to fork at one point so two copies of the same script are running.
For example, I would like the following bash script to exist:
echo $$
do_fork()
echo $$
If this bash script truly existed, the expected output would be:
<ProcessA PID>
<ProcessB PID>
<ProcessA PID>
or
<ProcessA PID>
<ProcessA PID>
<ProcessB PID>
Is there something that I can put in place of "do_fork()" to get this kind of output, or to cause the bash script to do a C-like fork?
Yes. Forking is spelled &:
echo child & echo parent
What may be confusing you is that $$ is not the PID of the shell process, it's the PID of the original shell process. The point of making it this way is that $$ is a unique identifier for a particular instance of the shell script: it doesn't change during the script's execution, and it's different from $$ in any other concurrently running script. One way to get the shell process's actual PID is sh -c 'echo $PPID'.
The control flow in the shell isn't the same as C. If in C you'd write
first(); fork(); second(); third();
then a shell equivalent is
after_fork () { second; third; }
first; after_fork & after_fork
The simple shell form first; child & parent corresponds to the usual C idiom
first(); if (fork()) parent(); else child();
& and $$ exist and behave this way in every Bourne-style shell and in (t)csh. $PPID didn't exist in the orignal Bourne shell but is in POSIX (so it's in ash, bash, ksh, zsh, …).
& is fork, there's no exec involved. Fork+exec is when you launch an external command.
Feb 18, 2011 at 0:47
& starts a subshell in which the given command is executed. Fork + exec. You can't just put & with no preceding command to execute.
echo is a builtin. Try it with /bin/echo and of course there is an exec. But I don't mean literally; I mean conceptually. You can't fork and continue from there without giving some command — it just doesn't work like that.
& on a line by itself. But you can't. It doesn't mean "fork here". It means "execute the preceding command in the background in a subshell".
Yes, it's called subshells. Shell code inside parenthesis is run as a subshell (fork). However the first shell normally waits for the child to complete. You can make it asynchronous using the & terminator. See it in in action with something like this:
#!/bin/bash
(sleep 2; echo "subsh 1")&
echo "topsh"
$ bash subsh.sh
& is fork alone. If you want to execute more than one pipeline in the child process, it's enough to use braces (which perform grouping without creating a child process): { sleep 2; echo child; } &
Feb 18, 2011 at 8:31
There's no native bash (or, to my knowledge, any other typical *nix shell) way of doing this. There's a lot of ways to spawn forked processes that do something else asynchronously, but I don't think there's anything that follows the exact semantics of the fork() system call.
The typical approach would be to have your top-level script spawn off helpers that do just the work you want split out. If you do $0 $@ & or whatever, you'll start at the beginning again and need to figure that out somehow.
I'm actually starting to think of several clever ways in which one might do just that....
But, before my brain gets too carried away with that, I think a pretty good rule is: if you're trying to write something in shell and it's getting full of clever tricks and you're wishing for more language features, it's time to switch to a programming language with those features.
