Difference between system() versus exec Kernel methods

[Alex DeCaria]

I'm trying to understand the difference between the Kernel.system and
Kernel.exec methods. It was explained in an earlier post that .system
started a new process while keeping the old one running, while .exec
replaced the current process with the new one (if I read it correctly).

I created a simple program called test.rb as follows

put "hello"
gets
puts "bye"

and then called it from another program that was simply

system("ruby test.rb")

or

exec "ruby test.rb"

I then looked at the processes in Task Manager while running the second
program which calls the first. I expected that with system("ruby
test.rb") I would see two ruby processes being run, which indeed
occured.

When I ran the second version with exec "ruby test.rb" there were still
two active Ruby processes being run (I expected only one, since one
process would be replaced by the other).

So, what am I missing? What is the difference between .system and
exec?

--Alex

 

[Michael Guterl]

I'm trying to understand the difference between the Kernel.system and
Kernel.exec methods. =C2=A0It was explained in an earlier post that .syst= em
started a new process while keeping the old one running, while .exec
replaced the current process with the new one (if I read it correctly).

I created a simple program called test.rb as follows

put "hello"
gets
puts "bye"

and then called it from another program that was simply

system("ruby test.rb")

or

exec "ruby test.rb"

I then looked at the processes in Task Manager while running the second
program which calls the first. =C2=A0I expected that with system("ruby
test.rb") I would see two ruby processes being run, which indeed
occured.

When I ran the second version with exec "ruby test.rb" there were still
two active Ruby processes being run (I expected only one, since one
process would be replaced by the other).

So, what am I missing? =C2=A0What is the difference between .system and
.exec?

ri Kernel#exec

------------------------------------------------------------ Kernel#exec
exec(command [, arg, ...])
------------------------------------------------------------------------
Replaces the current process by running the given external
_command_. If +exec+ is given a single argument, that argument is
taken as a line that is subject to shell expansion before being
executed. If multiple arguments are given, the second and
subsequent arguments are passed as parameters to _command_ with no
shell expansion. If the first argument is a two-element array, the
first element is the command to be executed, and the second
argument is used as the +argv[0]+ value, which may show up in
process listings. In MSDOS environments, the command is executed in
a subshell; otherwise, one of the +exec(2)+ system calls is used,
so the running command may inherit some of the environment of the
original program (including open file descriptors).

exec "echo *" # echoes list of files in current directory
# never get here

exec "echo", "*" # echoes an asterisk
# never get here


ri Kernel#system

---------------------------------------------------------- Kernel#system
system(cmd [, arg, ...]) =3D> true or false
------------------------------------------------------------------------
Executes _cmd_ in a subshell, returning +true+ if the command was
found and ran successfully, +false+ otherwise. An error status is
available in +$?+. The arguments are processed in the same way as
for +Kernel::exec+.

system("echo *")
system("echo", "*")

_produces:_

config.h main.rb
*

I can't explain it any better than the docs.

Best regards,
Michael Guterl

 

[Alex DeCaria]

That's well and good. I've already read the documentation. But, based
on the documentation I would expect to see only a single process running
after calling the exec method. So, why are there two?

I'm obviously not reading the documentation correctly, and that's why I
need someone who can explain it.

--Alex

 

[Walton Hoops]

That's well and good. I've already read the documentation. But, based
on the documentation I would expect to see only a single process running
after calling the exec method. So, why are there two?

I'm obviously not reading the documentation correctly, and that's why I
need someone who can explain it.

--Alex

This line from the docs explains it:

In MSDOS environments, the command is executed in
a subshell; otherwise, one of the +exec(2)+ system calls is used,
so the running command may inherit some of the environment of the
original program (including open file descriptors).

The trick is, exec uses the underlying OSes exec. In Linux, that does as you
expect and simply replaces the running process in memory. In Windows however,
there is no equivalent to the Linux exec, so we pretend. Ruby execs the
specified command (via the Windows version of exec which starts a second process)
and exits once it's done, thus there are two processes.

To demonstrate this try running the following:

exec "echo boo"
puts "I hope I don't get here"


Notice that the puts statement never gets run.

 

[Eric Christopherson]

This line from the docs explains it:

=A0 =A0 In MSDOS environments, the command is executed in
=A0 =A0 a subshell; otherwise, one of the +exec(2)+ system calls is used,
=A0 =A0 so the running command may inherit some of the environment of the
=A0 =A0 original program (including open file descriptors).

The trick is, exec uses the underlying OSes exec. =A0In Linux, that does = as
you
expect and simply replaces the running process in memory. =A0In Windows
however,
there is no equivalent to the Linux exec, so we pretend. Ruby execs the
specified command (via the Windows version of exec which starts a second
process)
and exits once it's done, thus there are two processes.

Sounds like the doc should be updated to mention Windows.

 

[Alex DeCaria]

Thanks! That makes it clearer. --Alex