Do as the shell does.

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system

Use system

Demo

  • Write a program hello that outputs something like “hello world”. Run it.

File 02_system_and_exit_values/hello.rb:

#!/usr/bin/env ruby

$stderr.puts "Hello, World. I am process #{Process::pid}."

Output is:

Hello, World. I am process 1714.

Programm ran successfully.

  • Write another programm that calls your hello program via system.

File 02_system_and_exit_values/system_hello.rb:

#!/usr/bin/env ruby

$stderr.puts "#{Process::pid} calls ./hello.rb."

system('./hello.rb')

$stderr.puts "Back in #{Process::pid}."

Output is:

1718 calls ./hello.rb.
Hello, World. I am process 1722.
Back in 1718.

Programm ran successfully.

Demo: To shell or not to shell

File 02_system_and_exit_values/call_ps.rb:

#!/usr/bin/env ruby

puts "First call from process #{Process::pid}"

system 'ps', 'f', 'T'

puts "\nSecond call from process #{Process::pid}"

system "ps f T"

puts "\nThird call from process #{Process::pid}"

system "ps f 'T'"

Output is:

First call from process 1724
  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                                                              
 1724 pts/2    Sl+    0:00      \_ ruby ./call_ps.rb
 1728 pts/2    R+     0:00          \_ ps f T

Second call from process 1724
  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                                                              
 1724 pts/2    Sl+    0:00      \_ ruby ./call_ps.rb
 1729 pts/2    R+     0:00          \_ ps f T

Third call from process 1724
  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                                                              
 1724 pts/2    Sl+    0:00      \_ ruby ./call_ps.rb
 1730 pts/2    S+     0:00          \_ sh -c ps f 'T'
 1731 pts/2    R+     0:00              \_ ps f T

Programm ran successfully.

Exit values

Happy families are all alike; every unhappy family is unhappy in its own way.

Leo Tolstoy, first sentence of “Anna Karenina”.

Demo ignoring exit values (don’t do that!)

  • Write a programm that fails (e.g., via exception).

File 02_system_and_exit_values/fail_via_exception.rb:

#!/usr/bin/env ruby

raise "#{Process::pid} fails."

Output is:

./fail_via_exception.rb:3:in `<main>': 1732 fails. (RuntimeError)

Programm failed with status 1.

  • Call it similar as above.

File 02_system_and_exit_values/system_fail_ignore.rb:

#!/usr/bin/env ruby

$stderr.puts "About to call, says #{Process::pid}."

system('./fail_via_exception.rb')

$stderr.puts "Done calling."

Output is:

About to call, says 1736.
./fail_via_exception.rb:3:in `<main>': 1740 fails. (RuntimeError)
Done calling.

Programm ran successfully.

Notice: The caller never notices anything is wrong.

Demo handling exit values

File 02_system_and_exit_values/system_fail_handle.rb:

#!/usr/bin/env ruby

$stderr.puts "About to call from #{Process::pid}."

system('./fail_via_exception.rb') or \
  raise "ERROR in system"

$stderr.puts "Done calling."

Output is:

About to call from 1742.
./fail_via_exception.rb:3:in `<main>': 1746 fails. (RuntimeError)
./system_fail_handle.rb:6:in `<main>': ERROR in system (RuntimeError)

Programm failed with status 1.

Demo analyzing exit values

You can set exit values as desired, in the range 0 .. 255:

File 02_system_and_exit_values/fail_via_exit.rb:

#!/usr/bin/env ruby

$stderr.puts "#{Process::pid} fails with 200."

exit(200)

Output is:

1751 fails with 200.

Programm failed with status 200.

File 02_system_and_exit_values/system_fail_handle_detailed.rb:

#!/usr/bin/env ruby

$stderr.puts "About to call, says upper #{Process::pid}."

system('./fail_via_exit.rb')

$stderr.puts "Done calling, $? is now \"#{$?.inspect}\", says upper #{Process::pid}."

# See http://ruby-doc.org/core-2.2.3/Process/Status.html ...

if $?.success?
  $stderr.puts "Down #{$?.pid} was happy, exit status is (of course) #{$?.exitstatus}."
elsif $?.signaled?
  $stderr.puts "Down #{$?.pid} received signal \"#{$?.termsig}\"."
else
  $stderr.puts "Down #{$?.pid} has exit status #{$?.exitstatus}."
end

Output is:

About to call, says upper 1755.
1760 fails with 200.
Done calling, $? is now "#<Process::Status: pid 1760 exit 200>", says upper 1755.
Down 1760 has exit status 200.

Programm ran successfully.

Exercise

Experiment with exit values. Suggestions:

  • Try calling /bin/true and /bin/false, if available on your system.

  • What happens if you try to give an exit value outside the 0 .. 255 range?

  • What happens if you call a malformed script? A non-existing script? A directory instead of a script?

  • Try calling :.

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