Process

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.

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.

Returns the name of the script being executed. The value is not affected by assigning a new value to $0.

This method first appeared in Ruby 2.1 to serve as a global variable free means to get the script name.

Returns a clock resolution as determined by POSIX function clock_getres():

Process.clock_getres(:CLOCK_REALTIME) # => 1.0e-09

See Process.clock_gettime for the values of clock_id and unit.

Examples:

Process.clock_getres(:CLOCK_PROCESS_CPUTIME_ID, :float_microsecond) # => 0.001
Process.clock_getres(:CLOCK_PROCESS_CPUTIME_ID, :float_millisecond) # => 1.0e-06
Process.clock_getres(:CLOCK_PROCESS_CPUTIME_ID, :float_second)      # => 1.0e-09
Process.clock_getres(:CLOCK_PROCESS_CPUTIME_ID, :microsecond)       # => 0
Process.clock_getres(:CLOCK_PROCESS_CPUTIME_ID, :millisecond)       # => 0
Process.clock_getres(:CLOCK_PROCESS_CPUTIME_ID, :nanosecond)        # => 1
Process.clock_getres(:CLOCK_PROCESS_CPUTIME_ID, :second)            # => 0

In addition to the values for unit supported in Process.clock_gettime, this method supports :hertz, the integer number of clock ticks per second (which is the reciprocal of :float_second):

Process.clock_getres(:TIMES_BASED_CLOCK_PROCESS_CPUTIME_ID, :hertz)        # => 100.0
Process.clock_getres(:TIMES_BASED_CLOCK_PROCESS_CPUTIME_ID, :float_second) # => 0.01

Accuracy: Note that the returned resolution may be inaccurate on some platforms due to underlying bugs. Inaccurate resolutions have been reported for various clocks including :CLOCK_MONOTONIC and :CLOCK_MONOTONIC_RAW on Linux, macOS, BSD or AIX platforms, when using ARM processors, or when using virtualization.

Returns a clock time as determined by POSIX function clock_gettime():

Process.clock_gettime(:CLOCK_PROCESS_CPUTIME_ID) # => 198.650379677

Argument clock_id should be a symbol or a constant that specifies the clock whose time is to be returned; see below.

Optional argument unit should be a symbol that specifies the unit to be used in the returned clock time; see below.

Argument clock_id

Argument clock_id specifies the clock whose time is to be returned; it may be a constant such as Process::CLOCK_REALTIME, or a symbol shorthand such as :CLOCK_REALTIME.

The supported clocks depend on the underlying operating system; this method supports the following clocks on the indicated platforms (raises Errno::EINVAL if called with an unsupported clock):

• :CLOCK_BOOTTIME: Linux 2.6.39.

• :CLOCK_BOOTTIME_ALARM: Linux 3.0.

• :CLOCK_MONOTONIC: SUSv3 to 4, Linux 2.5.63, FreeBSD 3.0, NetBSD 2.0, OpenBSD 3.4, macOS 10.12, Windows-2000.

• :CLOCK_MONOTONIC_COARSE: Linux 2.6.32.

• :CLOCK_MONOTONIC_FAST: FreeBSD 8.1.

• :CLOCK_MONOTONIC_PRECISE: FreeBSD 8.1.

• :CLOCK_MONOTONIC_RAW: Linux 2.6.28, macOS 10.12.

• :CLOCK_MONOTONIC_RAW_APPROX: macOS 10.12.

• :CLOCK_PROCESS_CPUTIME_ID: SUSv3 to 4, Linux 2.5.63, FreeBSD 9.3, OpenBSD 5.4, macOS 10.12.

• :CLOCK_PROF: FreeBSD 3.0, OpenBSD 2.1.

• :CLOCK_REALTIME: SUSv2 to 4, Linux 2.5.63, FreeBSD 3.0, NetBSD 2.0, OpenBSD 2.1, macOS 10.12, Windows-8/Server-2012. Time.now is recommended over +:CLOCK_REALTIME:.

• :CLOCK_REALTIME_ALARM: Linux 3.0.

• :CLOCK_REALTIME_COARSE: Linux 2.6.32.

• :CLOCK_REALTIME_FAST: FreeBSD 8.1.

• :CLOCK_REALTIME_PRECISE: FreeBSD 8.1.

• :CLOCK_SECOND: FreeBSD 8.1.

• :CLOCK_TAI: Linux 3.10.

• :CLOCK_THREAD_CPUTIME_ID: SUSv3 to 4, Linux 2.5.63, FreeBSD 7.1, OpenBSD 5.4, macOS 10.12.

• :CLOCK_UPTIME: FreeBSD 7.0, OpenBSD 5.5.

• :CLOCK_UPTIME_FAST: FreeBSD 8.1.

• :CLOCK_UPTIME_PRECISE: FreeBSD 8.1.

• :CLOCK_UPTIME_RAW: macOS 10.12.

• :CLOCK_UPTIME_RAW_APPROX: macOS 10.12.

• :CLOCK_VIRTUAL: FreeBSD 3.0, OpenBSD 2.1.

Note that SUS stands for Single Unix Specification. SUS contains POSIX and clock_gettime is defined in the POSIX part. SUS defines :CLOCK_REALTIME as mandatory but :CLOCK_MONOTONIC, :CLOCK_PROCESS_CPUTIME_ID, and :CLOCK_THREAD_CPUTIME_ID are optional.

Certain emulations are used when the given clock_id is not supported directly:

• Emulations for :CLOCK_REALTIME:

  • :GETTIMEOFDAY_BASED_CLOCK_REALTIME: Use gettimeofday() defined by SUS (deprecated in SUSv4). The resolution is 1 microsecond.

  • :TIME_BASED_CLOCK_REALTIME: Use time() defined by ISO C. The resolution is 1 second.

• Emulations for :CLOCK_MONOTONIC:

  • :MACH_ABSOLUTE_TIME_BASED_CLOCK_MONOTONIC: Use mach_absolute_time(), available on Darwin. The resolution is CPU dependent.

  • :TIMES_BASED_CLOCK_MONOTONIC: Use the result value of times() defined by POSIX, thus:

    Upon successful completion, times() shall return the elapsed real time, in clock ticks, since an arbitrary point in the past (for example, system start-up time).

    For example, GNU/Linux returns a value based on jiffies and it is monotonic. However, 4.4BSD uses gettimeofday() and it is not monotonic. (FreeBSD uses :CLOCK_MONOTONIC instead, though.)

    The resolution is the clock tick. “getconf CLK_TCK” command shows the clock ticks per second. (The clock ticks-per-second is defined by HZ macro in older systems.) If it is 100 and clock_t is 32 bits integer type, the resolution is 10 millisecond and cannot represent over 497 days.

• Emulations for :CLOCK_PROCESS_CPUTIME_ID:

  • :GETRUSAGE_BASED_CLOCK_PROCESS_CPUTIME_ID: Use getrusage() defined by SUS. getrusage() is used with RUSAGE_SELF to obtain the time only for the calling process (excluding the time for child processes). The result is addition of user time (ru_utime) and system time (ru_stime). The resolution is 1 microsecond.

  • :TIMES_BASED_CLOCK_PROCESS_CPUTIME_ID: Use times() defined by POSIX. The result is addition of user time (tms_utime) and system time (tms_stime). tms_cutime and tms_cstime are ignored to exclude the time for child processes. The resolution is the clock tick. “getconf CLK_TCK” command shows the clock ticks per second. (The clock ticks per second is defined by HZ macro in older systems.) If it is 100, the resolution is 10 millisecond.

  • :CLOCK_BASED_CLOCK_PROCESS_CPUTIME_ID: Use clock() defined by ISO C. The resolution is 1/CLOCKS_PER_SEC. CLOCKS_PER_SEC is the C-level macro defined by time.h. SUS defines CLOCKS_PER_SEC as 1000000; other systems may define it differently. If CLOCKS_PER_SEC is 1000000 (as in SUS), the resolution is 1 microsecond. If CLOCKS_PER_SEC is 1000000 and clock_t is a 32-bit integer type, it cannot represent over 72 minutes.

Argument unit

Optional argument unit (default :float_second) specifies the unit for the returned value.

• :float_microsecond: Number of microseconds as a float.

• :float_millisecond: Number of milliseconds as a float.

• :float_second: Number of seconds as a float.

• :microsecond: Number of microseconds as an integer.

• :millisecond: Number of milliseconds as an integer.

• :nanosecond: Number of nanoseconds as an integer.

• ::second: Number of seconds as an integer.

Examples:

Process.clock_gettime(:CLOCK_PROCESS_CPUTIME_ID, :float_microsecond)
# => 203605054.825
Process.clock_gettime(:CLOCK_PROCESS_CPUTIME_ID, :float_millisecond)
# => 203643.696848
Process.clock_gettime(:CLOCK_PROCESS_CPUTIME_ID, :float_second)
# => 203.762181929
Process.clock_gettime(:CLOCK_PROCESS_CPUTIME_ID, :microsecond)
# => 204123212
Process.clock_gettime(:CLOCK_PROCESS_CPUTIME_ID, :millisecond)
# => 204298
Process.clock_gettime(:CLOCK_PROCESS_CPUTIME_ID, :nanosecond)
# => 204602286036
Process.clock_gettime(:CLOCK_PROCESS_CPUTIME_ID, :second)
# => 204

The underlying function, clock_gettime(), returns a number of nanoseconds. Float object (IEEE 754 double) is not enough to represent the return value for :CLOCK_REALTIME. If the exact nanoseconds value is required, use :nanosecond as the unit.

The origin (time zero) of the returned value is system-dependent, and may be, for example, system start up time, process start up time, the Epoch, etc.

The origin in :CLOCK_REALTIME is defined as the Epoch: 1970-01-01 00:00:00 UTC; some systems count leap seconds and others don’t, so the result may vary across systems.

Detaches the current process from its controlling terminal and runs it in the background as system daemon; returns zero.

By default:

• Changes the current working directory to the root directory.

• Redirects $stdin, $stdout, and $stderr to the null device.

If optional argument nochdir is true, does not change the current working directory.

If optional argument noclose is true, does not redirect $stdin, $stdout, or $stderr.

Avoids the potential for a child process to become a zombie process. Process.detach prevents this by setting up a separate Ruby thread whose sole job is to reap the status of the process pid when it terminates.

This method is needed only when the parent process will never wait for the child process.

This example does not reap the second child process; that process appears as a zombie in the process status (ps) output:

pid = Process.spawn('ruby', '-e', 'exit 13') # => 312691
sleep(1)
# Find zombies.
system("ps -ho pid,state -p #{pid}")

Output:

312716 Z

This example also does not reap the second child process, but it does detach the process so that it does not become a zombie:

pid = Process.spawn('ruby', '-e', 'exit 13') # => 313213
thread = Process.detach(pid)
sleep(1)
# => #<Process::Waiter:0x00007f038f48b838 run>
system("ps -ho pid,state -p #{pid}")        # Finds no zombies.

The waiting thread can return the pid of the detached child process:

thread.join.pid                       # => 313262

Returns the effective group ID for the current process:

Process.egid # => 500

Not available on all platforms.

Sets the effective group ID for the current process.

Not available on all platforms.

Returns the effective user ID for the current process.

Process.euid # => 501

Sets the effective user ID for the current process.

Not available on all platforms.

Replaces the current process by doing one of the following:

• 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.

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.

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.

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.

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.

Returns the process group ID for the given process ID +pid+:

  Process.getpgid(Process.ppid) # => 25527

Not available on all platforms.

Returns the process group ID for the current process:

Process.getpgid(0) # => 25527
Process.getpgrp    # => 25527

Returns the scheduling priority for specified process, process group, or user.

Argument kind is one of:

• Process::PRIO_PROCESS: return priority for process.

• Process::PRIO_PGRP: return priority for process group.

• Process::PRIO_USER: return priority for user.

Argument id is the ID for the process, process group, or user; zero specified the current ID for kind.

Examples:

Process.getpriority(Process::PRIO_USER, 0)    # => 19
Process.getpriority(Process::PRIO_PROCESS, 0) # => 19

Not available on all platforms.

Returns a 2-element array of the current (soft) limit and maximum (hard) limit for the given resource.

Argument resource specifies the resource whose limits are to be returned; see Process.setrlimit.

Each of the returned values cur_limit and max_limit is an integer; see Process.setrlimit.

Example:

Process.getrlimit(:CORE) # => [0, 18446744073709551615]

See Process.setrlimit.

Not available on all platforms.

Returns the session ID of the given process ID pid, or of the current process if not given:

Process.getsid                # => 27422
Process.getsid(0)             # => 27422
Process.getsid(Process.pid()) # => 27422

Not available on all platforms.

Returns the (real) group ID for the current process:

Process.gid # => 1000

Sets the group ID for the current process to new_gid:

Process.gid = 1000 # => 1000

Returns an array of the group IDs in the supplemental group access list for the current process:

Process.groups # => [4, 24, 27, 30, 46, 122, 135, 136, 1000]

These properties of the returned array are system-dependent:

• Whether (and how) the array is sorted.

• Whether the array includes effective group IDs.

• Whether the array includes duplicate group IDs.

• Whether the array size exceeds the value of Process.maxgroups.

Use this call to get a sorted and unique array:

Process.groups.uniq.sort

Sets the supplemental group access list to the given array of group IDs.

Process.groups                     # => [0, 1, 2, 3, 4, 6, 10, 11, 20, 26, 27]
Process.groups = [27, 6, 10, 11]   # => [27, 6, 10, 11]
Process.groups                     # => [27, 6, 10, 11]

Sets the supplemental group access list; the new list includes:

• The group IDs of those groups to which the user given by username belongs.

• The group ID gid.

Example:

Process.groups                # => [0, 1, 2, 3, 4, 6, 10, 11, 20, 26, 27]
Process.initgroups('me', 30)  # => [30, 6, 10, 11]
Process.groups                # => [30, 6, 10, 11]

Not available on all platforms.

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.

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>

Returns the maximum number of group IDs allowed in the supplemental group access list:

Process.maxgroups # => 32

Sets the maximum number of group IDs allowed in the supplemental group access list.

Returns the process ID of the current process:

Process.pid # => 15668

Returns the process ID of the parent of the current process:

puts "Pid is #{Process.pid}."
fork { puts "Parent pid is #{Process.ppid}." }

Output:

Pid is 271290.
Parent pid is 271290

May not return a trustworthy value on certain platforms.

Sets the process group ID for the process given by process ID pid to pgid.

Not available on all platforms.

Equivalent to setpgid(0, 0).

Not available on all platforms.

See Process.getpriority.

Examples:

Process.setpriority(Process::PRIO_USER, 0, 19)    # => 0
Process.setpriority(Process::PRIO_PROCESS, 0, 19) # => 0
Process.getpriority(Process::PRIO_USER, 0)        # => 19
Process.getpriority(Process::PRIO_PROCESS, 0)     # => 19

Not available on all platforms.

Sets the process title that appears on the ps(1) command. Not necessarily effective on all platforms. No exception will be raised regardless of the result, nor will NotImplementedError be raised even if the platform does not support the feature.

Calling this method does not affect the value of $0.

Process.setproctitle('myapp: worker #%d' % worker_id)

This method first appeared in Ruby 2.1 to serve as a global variable free means to change the process title.

Sets limits for the current process for the given resource to cur_limit (soft limit) and max_limit (hard limit); returns nil.

Argument resource specifies the resource whose limits are to be set; the argument may be given as a symbol, as a string, or as a constant beginning with Process::RLIMIT_ (e.g., :CORE, 'CORE', or Process::RLIMIT_CORE.

The resources available and supported are system-dependent, and may include (here expressed as symbols):

• :AS: Total available memory (bytes) (SUSv3, NetBSD, FreeBSD, OpenBSD except 4.4BSD-Lite).

• :CORE: Core size (bytes) (SUSv3).

• :CPU: CPU time (seconds) (SUSv3).

• :DATA: Data segment (bytes) (SUSv3).

• :FSIZE: File size (bytes) (SUSv3).

• :MEMLOCK: Total size for mlock(2) (bytes) (4.4BSD, GNU/Linux).

• :MSGQUEUE: Allocation for POSIX message queues (bytes) (GNU/Linux).

• :NICE: Ceiling on process’s nice(2) value (number) (GNU/Linux).

• :NOFILE: File descriptors (number) (SUSv3).

• :NPROC: Number of processes for the user (number) (4.4BSD, GNU/Linux).

• :NPTS: Number of pseudo terminals (number) (FreeBSD).

• :RSS: Resident memory size (bytes) (4.2BSD, GNU/Linux).

• :RTPRIO: Ceiling on the process’s real-time priority (number) (GNU/Linux).

• :RTTIME: CPU time for real-time process (us) (GNU/Linux).

• :SBSIZE: All socket buffers (bytes) (NetBSD, FreeBSD).

• :SIGPENDING: Number of queued signals allowed (signals) (GNU/Linux).

• :STACK: Stack size (bytes) (SUSv3).

Arguments cur_limit and max_limit may be:

• Integers (max_limit should not be smaller than cur_limit).

• Symbol :SAVED_MAX, string 'SAVED_MAX', or constant Process::RLIM_SAVED_MAX: saved maximum limit.

• Symbol :SAVED_CUR, string 'SAVED_CUR', or constant Process::RLIM_SAVED_CUR: saved current limit.

• Symbol :INFINITY, string 'INFINITY', or constant Process::RLIM_INFINITY: no limit on resource.

This example raises the soft limit of core size to the hard limit to try to make core dump possible:

Process.setrlimit(:CORE, Process.getrlimit(:CORE)[1])

Not available on all platforms.

Establishes the current process as a new session and process group leader, with no controlling tty; returns the session ID:

Process.setsid # => 27422

Not available on all platforms.

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.

Returns a Process::Tms structure that contains user and system CPU times for the current process, and for its children processes:

Process.times
# => #<struct Process::Tms utime=55.122118, stime=35.533068, cutime=0.0, cstime=0.002846>

The precision is platform-defined.

Returns the (real) user ID of the current process.

Process.uid # => 1000

Sets the (user) user ID for the current process to new_uid:

Process.uid = 1000 # => 1000

Not available on all platforms.

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.