exec

Cease control

exit cease to exist, either contentedly or grumbling.
system cease control while some other process runs, get back control when that other process is done.
exec cease to exist, leaving a successor

File 04_exec/plain_exec.rb:

#!/usr/bin/env ruby

$stderr.puts "I am #{Process::pid} about to call exec."

# Replace this script with an invocation of ps:
exec 'ps', 'f', 'T'
# Use 'ps', '-j', '-T' on Mac

# This should never get executed:
raise "Exec did not work out."

Output is:

I am 1901 about to call exec.
  PID TTY      STAT   TIME COMMAND
 3240 pts/2    Ss     0:00 -bash
 1023 pts/2    Sl     0:03  \_ emacs 08_dup/
 1712 pts/2    Sl+    0:00  \_ ruby /home/andreask/.rvm/gems/ruby-2.3.0/bin/rake                                                              
 1901 pts/2    R+     0:00      \_ ps f T

Programm ran successfully.

Often combined

File 04_exec/fork_exec_wait.rb:

#!/usr/bin/env ruby

$stderr.puts "#{Process::pid} about to fork and exec."

child_pid = fork

if child_pid.nil?
  exec 'ps', 'f', 'T'
  # Use 'ps', '-j', '-T' on Mac
  
  # This should never get executed:
  raise "Exec did not work out."
else
  Process::wait(child_pid)
  if not $?.success?
    raise "ERROR ps has #{$?.exitstatus}."
  end
end

Output is:

1905 about to fork and exec.
  PID TTY      STAT   TIME COMMAND
 3240 pts/2    Ss     0:00 -bash
 1023 pts/2    Sl     0:03  \_ emacs 08_dup/
 1712 pts/2    Sl+    0:00  \_ ruby /home/andreask/.rvm/gems/ruby-2.3.0/bin/rake                                                              
 1905 pts/2    Sl+    0:00      \_ ruby ./fork_exec_wait.rb
 1909 pts/2    R+     0:00          \_ ps f T

Programm ran successfully.

Abbreviations

fork + exec = spawn

fork + exec + wait = system

That’s true on UNIX (Linux, Mac). But spawn and system may be coded differently on other systems (Windows). So those two are generally more portable, they may exist on platforms that do not provide fork.

Exercise

Run the fork_exec_wait - Example on your laptop.
Port it to using spawn.