The new process is created using the exec system call; it may inherit some of its environment from the calling program (possibly including open file descriptors).

Argument env, if given, is a hash that affects ENV for the new process; see Execution Environment.

Argument options is a hash of options for the new process; see Execution Options.

The first required argument is one of the following:

command_line if it is a string, and if it begins with a shell reserved word or special built-in, or if it contains one or more meta characters.
exe_path otherwise.

Argument command_line

String argument command_line is a command line to be passed to a shell; it must begin with a shell reserved word, begin with a special built-in, or contain meta characters:

exec('if true; then echo "Foo"; fi') # Shell reserved word.
exec('exit')                         # Built-in.
exec('date > date.tmp')              # Contains meta character.

The command line may also contain arguments and options for the command:

exec('echo "Foo"')

Output:

Foo

See Execution Shell for details about the shell.

Raises an exception if the new process could not execute.

Argument exe_path

Argument exe_path is one of the following:

The string path to an executable to be called.
A 2-element array containing the path to an executable and the string to be used as the name of the executing process.

Example:

exec('/usr/bin/date')

Output:

Sat Aug 26 09:38:00 AM CDT 2023

Ruby invokes the executable directly. This form does not use the shell; see Arguments args for caveats.

exec('doesnt_exist') # Raises Errno::ENOENT

If one or more args is given, each is an argument or option to be passed to the executable:

exec('echo', 'C*')
exec('echo', 'hello', 'world')

Output:

C*
hello world

Raises an exception if the new process could not execute.

Process::fork

Creates a child process.

With a block given, runs the block in the child process; on block exit, the child terminates with a status of zero:

puts "Before the fork: #{Process.pid}"
fork do
  puts "In the child process: #{Process.pid}"
end                   # => 382141
puts "After the fork: #{Process.pid}"

Output:

Before the fork: 420496
After the fork: 420496
In the child process: 420520

With no block given, the fork call returns twice:

Once in the parent process, returning the pid of the child process.
Once in the child process, returning nil.

Example:

puts "This is the first line before the fork (pid #{Process.pid})"
puts fork
puts "This is the second line after the fork (pid #{Process.pid})"

Output:

This is the first line before the fork (pid 420199)
420223
This is the second line after the fork (pid 420199)

This is the second line after the fork (pid 420223)

In either case, the child process may exit using Kernel.exit! to avoid the call to Kernel#at_exit.

To avoid zombie processes, the parent process should call either:

Process.wait, to collect the termination statuses of its children.
Process.detach, to register disinterest in their status.

The thread calling fork is the only thread in the created child process; fork doesn’t copy other threads.

Note that method fork is available on some platforms, but not on others:

Process.respond_to?(:fork) # => true # Would be false on some.

If not, you may use ::spawn instead of fork.

Process::spawn

Creates a new child process by doing one of the following in that process:

Passing string command_line to the shell.
Invoking the executable at exe_path.

This method has potential security vulnerabilities if called with untrusted input; see Command Injection.

Returns the process ID (pid) of the new process, without waiting for it to complete.

To avoid zombie processes, the parent process should call either:

Process.wait, to collect the termination statuses of its children.
Process.detach, to register disinterest in their status.

The new process is created using the exec system call; it may inherit some of its environment from the calling program (possibly including open file descriptors).

Argument env, if given, is a hash that affects ENV for the new process; see Execution Environment.

Argument options is a hash of options for the new process; see Execution Options.

The first required argument is one of the following:

command_line if it is a string, and if it begins with a shell reserved word or special built-in, or if it contains one or more meta characters.
exe_path otherwise.

Argument command_line

String argument command_line is a command line to be passed to a shell; it must begin with a shell reserved word, begin with a special built-in, or contain meta characters:

spawn('if true; then echo "Foo"; fi') # => 798847 # Shell reserved word.
Process.wait                          # => 798847
spawn('exit')                         # => 798848 # Built-in.
Process.wait                          # => 798848
spawn('date > /tmp/date.tmp')         # => 798879 # Contains meta character.
Process.wait                          # => 798849
spawn('date > /nop/date.tmp')         # => 798882 # Issues error message.
Process.wait                          # => 798882

The command line may also contain arguments and options for the command:

spawn('echo "Foo"') # => 799031
Process.wait        # => 799031

Output:

Foo

See Execution Shell for details about the shell.

Raises an exception if the new process could not execute.

Argument exe_path

Argument exe_path is one of the following:

The string path to an executable to be called.
A 2-element array containing the path to an executable to be called, and the string to be used as the name of the executing process.
```
spawn('/usr/bin/date') # Path to date on Unix-style system.
Process.wait
```
Output:
```
Mon Aug 28 11:43:10 AM CDT 2023
```

Ruby invokes the executable directly. This form does not use the shell; see Arguments args for caveats.

If one or more args is given, each is an argument or option to be passed to the executable:

spawn('echo', 'C*')             # => 799392
Process.wait                    # => 799392
spawn('echo', 'hello', 'world') # => 799393
Process.wait                    # => 799393

Output:

C*
hello world

Raises an exception if the new process could not execute.

Process::exit!

Exits the process immediately; no exit handlers are called. Returns exit status status to the underlying operating system.

Process.exit!(true)

Values true and false for argument status indicate, respectively, success and failure; The meanings of integer values are system-dependent.

Process::exit

Initiates termination of the Ruby script by raising SystemExit; the exception may be caught. Returns exit status status to the underlying operating system.

Values true and false for argument status indicate, respectively, success and failure; The meanings of integer values are system-dependent.

Example:

begin
  exit
  puts 'Never get here.'
rescue SystemExit
  puts 'Rescued a SystemExit exception.'
end
puts 'After begin block.'

Output:

Rescued a SystemExit exception.
After begin block.

Just prior to final termination, Ruby executes any at-exit procedures (see Kernel::at_exit) and any object finalizers (see ObjectSpace::define_finalizer).

Example:

at_exit { puts 'In at_exit function.' }
ObjectSpace.define_finalizer('string', proc { puts 'In finalizer.' })
exit

Output:

In at_exit function.
In finalizer.

Process::abort

Terminates execution immediately, effectively by calling Kernel.exit(false).

If string argument msg is given, it is written to STDERR prior to termination; otherwise, if an exception was raised, prints its message and backtrace.

Process::last_status

Returns a Process::Status object representing the most recently exited child process in the current thread, or nil if none:

Process.spawn('ruby', '-e', 'exit 13')
Process.wait
Process.last_status # => #<Process::Status: pid 14396 exit 13>

Process.spawn('ruby', '-e', 'exit 14')
Process.wait
Process.last_status # => #<Process::Status: pid 4692 exit 14>

Process.spawn('ruby', '-e', 'exit 15')
# 'exit 15' has not been reaped by #wait.
Process.last_status # => #<Process::Status: pid 4692 exit 14>
Process.wait
Process.last_status # => #<Process::Status: pid 1380 exit 15>

Process::_fork

An internal API for fork. Do not call this method directly. Currently, this is called via Kernel#fork, Process.fork, and IO.popen with "-".

This method is not for casual code but for application monitoring libraries. You can add custom code before and after fork events by overriding this method.

Note: Process.daemon may be implemented using fork(2) BUT does not go through this method. Thus, depending on your reason to hook into this method, you may also want to hook into that one. See this issue for a more detailed discussion of this.

Process::kill

Sends a signal to each process specified by ids (which must specify at least one ID); returns the count of signals sent.

For each given id, if id is:

Positive, sends the signal to the process whose process ID is id.
Zero, send the signal to all processes in the current process group.
Negative, sends the signal to a system-dependent collection of processes.

Argument signal specifies the signal to be sent; the argument may be:

An integer signal number: e.g., -29, 0, 29.
A signal name (string), with or without leading 'SIG', and with or without a further prefixed minus sign ('-'): e.g.:
- 'SIGPOLL'.
- 'POLL',
- '-SIGPOLL'.
- '-POLL'.
A signal symbol, with or without leading 'SIG', and with or without a further prefixed minus sign ('-'): e.g.:
- :SIGPOLL.
- :POLL.
- :'-SIGPOLL'.
- :'-POLL'.

If signal is:

A non-negative integer, or a signal name or symbol without prefixed '-', each process with process ID id is signalled.
A negative integer, or a signal name or symbol with prefixed '-', each process group with group ID id is signalled.

Use method Signal.list to see which signals are supported by Ruby on the underlying platform; the method returns a hash of the string names and non-negative integer values of the supported signals. The size and content of the returned hash varies widely among platforms.

Additionally, signal 0 is useful to determine if the process exists.

Example:

pid = fork do
  Signal.trap('HUP') { puts 'Ouch!'; exit }
  # ... do some work ...
end
# ...
Process.kill('HUP', pid)
Process.wait

Output:

Ouch!

Exceptions:

Raises Errno::EINVAL or RangeError if signal is an integer but invalid.
Raises ArgumentError if signal is a string or symbol but invalid.
Raises Errno::ESRCH or RangeError if one of ids is invalid.
Raises Errno::EPERM if needed permissions are not in force.

In the last two cases, signals may have been sent to some processes.

Process::wait

Waits for a suitable child process to exit, returns its process ID, and sets $? to a Process::Status object containing information on that process. Which child it waits for depends on the value of the given pid:

Positive integer: Waits for the child process whose process ID is pid:

pid0 = Process.spawn('ruby', '-e', 'exit 13') # => 230866
pid1 = Process.spawn('ruby', '-e', 'exit 14') # => 230891
Process.wait(pid0)                            # => 230866
$?                                            # => #<Process::Status: pid 230866 exit 13>
Process.wait(pid1)                            # => 230891
$?                                            # => #<Process::Status: pid 230891 exit 14>
Process.wait(pid0)                            # Raises Errno::ECHILD

0: Waits for any child process whose group ID is the same as that of the current process:

parent_pgpid = Process.getpgid(Process.pid)
puts "Parent process group ID is #{parent_pgpid}."
child0_pid = fork do
  puts "Child 0 pid is #{Process.pid}"
  child0_pgid = Process.getpgid(Process.pid)
  puts "Child 0 process group ID is #{child0_pgid} (same as parent's)."
end
child1_pid = fork do
  puts "Child 1 pid is #{Process.pid}"
  Process.setpgid(0, Process.pid)
  child1_pgid = Process.getpgid(Process.pid)
  puts "Child 1 process group ID is #{child1_pgid} (different from parent's)."
end
retrieved_pid = Process.wait(0)
puts "Process.wait(0) returned pid #{retrieved_pid}, which is child 0 pid."
begin
  Process.wait(0)
rescue Errno::ECHILD => x
  puts "Raised #{x.class}, because child 1 process group ID differs from parent process group ID."
end

Output:

Parent process group ID is 225764.
Child 0 pid is 225788
Child 0 process group ID is 225764 (same as parent's).
Child 1 pid is 225789
Child 1 process group ID is 225789 (different from parent's).
Process.wait(0) returned pid 225788, which is child 0 pid.
Raised Errno::ECHILD, because child 1 process group ID differs from parent process group ID.

-1 (default): Waits for any child process:

parent_pgpid = Process.getpgid(Process.pid)
puts "Parent process group ID is #{parent_pgpid}."
child0_pid = fork do
  puts "Child 0 pid is #{Process.pid}"
  child0_pgid = Process.getpgid(Process.pid)
  puts "Child 0 process group ID is #{child0_pgid} (same as parent's)."
end
child1_pid = fork do
  puts "Child 1 pid is #{Process.pid}"
  Process.setpgid(0, Process.pid)
  child1_pgid = Process.getpgid(Process.pid)
  puts "Child 1 process group ID is #{child1_pgid} (different from parent's)."
  sleep 3 # To force child 1 to exit later than child 0 exit.
end
child_pids = [child0_pid, child1_pid]
retrieved_pid = Process.wait(-1)
puts child_pids.include?(retrieved_pid)
retrieved_pid = Process.wait(-1)
puts child_pids.include?(retrieved_pid)

Output:

Parent process group ID is 228736.
Child 0 pid is 228758
Child 0 process group ID is 228736 (same as parent's).
Child 1 pid is 228759
Child 1 process group ID is 228759 (different from parent's).
true
true

Less than -1: Waits for any child whose process group ID is -pid:

parent_pgpid = Process.getpgid(Process.pid)
puts "Parent process group ID is #{parent_pgpid}."
child0_pid = fork do
  puts "Child 0 pid is #{Process.pid}"
  child0_pgid = Process.getpgid(Process.pid)
  puts "Child 0 process group ID is #{child0_pgid} (same as parent's)."
end
child1_pid = fork do
  puts "Child 1 pid is #{Process.pid}"
  Process.setpgid(0, Process.pid)
  child1_pgid = Process.getpgid(Process.pid)
  puts "Child 1 process group ID is #{child1_pgid} (different from parent's)."
end
sleep 1
retrieved_pid = Process.wait(-child1_pid)
puts "Process.wait(-child1_pid) returned pid #{retrieved_pid}, which is child 1 pid."
begin
  Process.wait(-child1_pid)
rescue Errno::ECHILD => x
  puts "Raised #{x.class}, because there's no longer a child with process group id #{child1_pid}."
end

Output:

Parent process group ID is 230083.
Child 0 pid is 230108
Child 0 process group ID is 230083 (same as parent's).
Child 1 pid is 230109
Child 1 process group ID is 230109 (different from parent's).
Process.wait(-child1_pid) returned pid 230109, which is child 1 pid.
Raised Errno::ECHILD, because there's no longer a child with process group id 230109.

Argument flags should be given as one of the following constants, or as the logical OR of both:

Process::WNOHANG: Does not block if no child process is available.
Process::WUNTRACED: May return a stopped child process, even if not yet reported.

Not all flags are available on all platforms.

Raises Errno::ECHILD if there is no suitable child process.

Not available on all platforms.

Process.waitpid is an alias for Process.wait.

Process::wait2

Like Process.waitpid, but returns an array containing the child process pid and Process::Status status:

pid = Process.spawn('ruby', '-e', 'exit 13') # => 309581
Process.wait2(pid)
# => [309581, #<Process::Status: pid 309581 exit 13>]

Process.waitpid2 is an alias for Process.wait2.