Re: exec

["Mark J. Reed" <markjreed@gmail.com>]

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On Mon, Jun 3, 2024 at 10:54 AM Mark J. Reed <markjreed@gmail.com> wrote:

> Eahat's the terminal supposed to do with nothing running in it anymore?
>

That was a rather creative typo for "What" at the start of that question. :)


>  Exec has the same impact on the shell as *exit* - shell go bye bye - it
> just leaves another program in its place.
>

In your tests, that other program is *echo*. Since that's a builtin, things
are a little different. In bash or ksh, *exec* only works on external
programs, so if you type *exec echo*, you'll be running */bin/echo* or
*/usr/bin/echo* (or wherever the binary lives) instead of the shell
builtin. But in Zsh, *exec*'ed builtins are still builtins; the shell
simulates the effect of *exec* by exiting after it executes the builtin
command. So *exec echo whatever* is just a shorter way of writing *echo
whatever; exit*.

But if you *exec* an external program that takes a while to run, say *exec
sleep 300*, then you can run *ps *in another window and see that the shell
from which you launched the *sleep* is no longer there; the *sleep* has
replaced it. In fact, you can see that the process ID formerly belonging to
it has been taken over:

Window 1:

*zsh% echo $$*
*42566*
*zsh% exec sleep 300*


Window 2:

*zsh% ps -fp42566*

*UID   PID  PPID   C STIME   TTY           TIME CMD*
*  501 42566 42383   0 11:05AM ttys003    0:00.11 sleep 300*



> Mark J. Reed <markjreed@gmail.com>
>
>
> On Mon, Jun 3, 2024 at 10:42 Ray Andrews <rayandrews@eastlink.ca> wrote:
>
>>
>>
>> On 2024-06-03 07:17, Mark J. Reed wrote:
>>
>> The *exec* builtin replaces the running shell with whatever program you
>> run.  The point is to avoid clogging the process table with shells that are
>> just hanging out waiting to do nothing but exit as soon as their child
>> process finishes.
>>
>> I get that.
>>
>>
>> In your case, the script exists to set things up in the environment and
>> then run xfce4-session; there's nothing for it to do after xfce4-session
>> completes, so it uses *exec* to tidy up.
>>
>>
>> Sure.  But then what? I understand that if a script or function has
>> nothing more to do, it may as well pre-kill itself. But the difference is
>> that 'exec' kills the entire terminal, it doesn't just return to the prompt
>> in a more efficient way -- which would be easy to understand, as above.
>> exec seems to pull the rug out from under itself, not just end a script
>> more efficiently.  In my case, from what I've heard control seems to pass
>> to dbus.  Mind, if dbus called the script then that's what one might
>> expect.
>>
>>
>>

-- 
Mark J. Reed <markjreed@gmail.com>

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