Kernel#system

system([env,] command [, args...,]) -> true, false, or nil

The system method executes an external command as a subprocess and returns its exit status. It runs the command directly—without spawning a shell interpreter—making it more secure than using backticks or %x{} for commands with untrusted input. The method blocks the current thread until the command completes, then returns a value indicating success or failure.

Unlike backticks, system does not capture the command’s stdout or stderr. The output goes directly to the parent’s stdout/stderr, which is useful when you want the subprocess output to appear in the terminal without interception.

Syntax

system(command)
system(command, arg1, arg2, ...)
system(env_var: value, command, arg1, ...)

Parameters

Parameter	Type	Default	Description
env	Hash	nil	Optional environment variables to set for the subprocess
command	String	—	The command to execute. Can include spaces without shell interpretation
args	Array	nil	Additional arguments passed as separate parameters to the command

Examples

Basic usage

# Returns true if command succeeds (exit code 0)
system("ls")
# => true

# Returns false if command fails (non-zero exit code)
system("ls /nonexistent")
# => false

# Returns nil if command was not found
system("nonexistent_command")
# => nil

Passing arguments safely

# Arguments are passed directly to the executable, not interpreted by a shell
# This avoids shell injection vulnerabilities
system("echo", "Hello", "World")
# => true
# Terminal output: Hello World

# Compare with shell interpretation (dangerous with user input)
system("echo $HOME")    # Shell expands $HOME
system("echo", "$HOME") # Prints literal "$HOME"

Setting environment variables

# Set custom environment variables for just this subprocess
system({ "RAILS_ENV" => "test" }, "rake", "test")
# => true
# The parent process's RAILS_ENV is unchanged

Common Patterns

Running a script and checking success

if system("ruby script.rb")
  puts "Script succeeded"
else
  puts "Script failed with exit code: #{$?.exitstatus}"
end

Checking the exit status

system("rake", "db:migrate")
case $?.exitstatus
when 0 then puts "Success"
when 1 then puts "Failed"
when 2 then puts "Runtime error"
end

Differences from similar methods

Method	Captures Output	Shell Interpretation	Returns
system	No	No	true/false/nil
` `	Yes	Yes	String
%x{}	Yes	Yes	String
exec	No	No	Does not return

Errors

• Errno::ENOENT: Raised if the command executable is not found or not executable.
• ArgumentError: Raised if no command is provided.

See Also

• Kernel#abort
• Kernel#exit
• Kernel#exit!