TTY::Command

Run external commands with pretty output logging and capture stdout, stderr and exit status. Redirect stdin, stdout and stderr of each command to a file or a string.

TTY::Command provides independent command execution component for TTY toolkit.

Motivation

Complex software projects aren't just a single app. These projects usually spawn dozens or hundreds of supplementary standalone scripts which are just as important as the app itself. Examples include - data validation, deployment, monitoring, database maintenance, backup & restore, configuration management, crawling, ETL, analytics, log file processing, custom reports, etc. One of the contributors to TTY::Command counted 222 scripts in the bin directory for his startup.

Why should we be handcuffed to sh or bash for these scripts when we could be using Ruby? Ruby is easier to write and more fun, and we gain a lot by using a better language. It's nice for everyone to just use Ruby everywhere.

TTY::Command tries to add value in other ways. It'll halt automatically if a command fails. It's easy to get verbose or quiet output as appropriate, or even capture output and parse it with Ruby. Escaping arguments is a breeze. These are all areas where traditional shell scripts tend to fall flat.

Installation

Add this line to your application's Gemfile:

gem "tty-command"

And then execute:

$ bundle

Or install it yourself as:

$ gem install tty-command

Contents

• 1. Usage
• 2. Interface
  • 2.1. Run
  • 2.2. Run!
  • 2.3. Logging
    • 2.3.1. Color
    • 2.3.2. UUID
    • 2.3.3. Only output on error
    • 2.3.4. Verbose
  • 2.4. Dry run
  • 2.5. Wait
  • 2.6. Test
  • 2.7. Ruby interpreter
• 3. Advanced Interface
  • 3.1. Environment variables
  • 3.2. Options
    • 3.2.1. Redirection
    • 3.2.2. Handling input
    • 3.2.3. Timeout
    • 3.2.4. Binary mode
    • 3.2.5. Signal
    • 3.2.6. PTY(pseudo-terminal)
    • 3.2.7. Current directory
    • 3.2.8. User
    • 3.2.9. Group
    • 3.2.10. Umask
  • 3.3. Result
    • 3.3.1. success?
    • 3.3.2. failure?
    • 3.3.3. exited?
    • 3.3.4. each
  • 3.4. Custom printer
• 4. Example

1. Usage

Create a command instance and then run some commands:

require "tty-command"

cmd = TTY::Command.new
cmd.run("ls -la")
cmd.run("echo Hello!")

Note that run will throw an exception if the command fails. This is already an improvement over ordinary shell scripts, which just keep on going when things go bad. That usually makes things worse.

You can use the return value to capture stdout and stderr:

out, err = cmd.run("cat ~/.bashrc | grep alias")

Instead of using a plain old string, you can break up the arguments and they'll get escaped if necessary:

path = "hello world"
FileUtils.touch(path)
cmd.run("sum #{path}")  # this will fail due to bad escaping
cmd.run("sum", path)    # this gets escaped automatically

2. Interface

2.1 Run

Run starts the specified command and waits for it to complete.

The argument signature of run is as follows:

run([env], command, [argv1, ...], [options])

The env, command and options arguments are described in the following sections.

For example, to display file contents:

cmd.run("cat file.txt")

If the command succeeds, a TTY::Command::Result is returned that records stdout and stderr:

out, err = cmd.run("date")
puts "The date is #{out}"
# => "The date is Tue 10 May 2016 22:30:15 BST\n"

You can also pass a block that gets invoked anytime stdout and/or stderr receive output:

cmd.run("long running script") do |out, err|
  output << out if out
  errors << err if err
end

If the command fails (with a non-zero exit code), a TTY::Command::ExitError is raised. The ExitError message will include:

• the name of command executed
• the exit status
• stdout bytes
• stderr bytes

If the error output is very long, the stderr may contain only a prefix, number of omitted bytes and suffix.

2.2 Run!

If you expect a command to fail occasionally, use run! instead. Then you can detect failures and respond appropriately. For example:

if cmd.run!("which xyzzy").failure?
  cmd.run("brew install xyzzy")
end

2.3 Logging

By default, when a command is run, the command and the output are printed to stdout using the :pretty printer. If you wish to change printer you can do so by passing a :printer option:

• :null - no output
• :pretty - colorful output
• :progress - minimal output with green dot for success and F for failure
• :quiet - only output actual command stdout and stderr

like so:

cmd = TTY::Command.new(printer: :progress)

By default the printers log to stdout but this can be changed by passing an object that responds to << message:

logger = Logger.new("dev.log")
cmd = TTY::Command.new(output: logger)

You can force the printer to always in print in color by passing the :color option:

cmd = TTY::Command.new(color: true)

If the default printers don't meet your needs you can always create a custom printer

2.3.1 Color

When using printers you can switch off coloring by using :color option set to false.

2.3.2 UUID

By default, when logging is enabled and pretty printer is used, each log entry is prefixed by specific command run uuid number. This number can be switched off using the :uuid option at initialization:

cmd = TTY::Command.new(uuid: false)
cmd.run("rm -R all_my_files")
# =>
#  Running rm -r all_my_files
#  ...
#  Finished in 6 seconds with exit status 0 (successful)

or individually per command run:

cmd = TTY::Command.new
cmd.run("echo hello", uuid: false)
# =>
#  Running echo hello
#      hello
#  Finished in 0.003 seconds with exit status 0 (successful)

2.3.3 Only output on error

When using a command that can fail, setting :only_output_on_error option to true hides the output if the command succeeds:

cmd = TTY::Command.new
cmd.run("non_failing_command", only_output_on_error: true)

This will only print the Running and Finished lines, while:

cmd.run("non_failing_command")

will also print any output that the non_failing_command might generate.

Running either:

cmd.run("failing_command", only_output_on_error: true)

either:

cmd.run("failing_command")

will also print the output.

Setting this option will cause the output to show at once, at the end of the command.

2.3.4 Verbose

By default commands will produce warnings when, for example pty option is not supported on a given platform. You can switch off such warnings with :verbose option set to false.

cmd.run("echo '\e[32mColors!\e[0m'", pty: true, verbose: false)

2.4 Dry run

Sometimes it can be useful to put your script into a "dry run" mode that prints commands without actually running them. To simulate execution of the command use the :dry_run option:

cmd = TTY::Command.new(dry_run: true)
cmd.run(:rm, "all_my_files")
# => [123abc] (dry run) rm all_my_files

To check what mode the command is in use the dry_run? query helper:

cmd.dry_run? # => true

2.5 Wait

If you need to wait for a long running script and stop it when a given pattern has been matched use wait like so:

cmd.wait "tail -f /var/log/production.log", /something happened/

2.6 Test

To simulate classic bash test command you case use test method with expression to check as a first argument:

if cmd.test "-e /etc/passwd"
  puts "Sweet..."
else
  puts "Ohh no! Where is it?"
  exit 1
end

2.7 Ruby interpreter

In order to run a command with Ruby interpreter do:

cmd.ruby %q{-e "puts 'Hello world'"}

3. Advanced Interface

3.1 Environment variables

The environment variables need to be provided as hash entries, that can be set directly as a first argument:

cmd.run({"RAILS_ENV" => "PRODUCTION"}, :rails, "server")

or as an option with :env key:

cmd.run(:rails, "server", env: {rails_env: :production})

When a value in env is nil, the variable is unset in the child process:

cmd.run(:echo, "hello", env: {foo: "bar", baz: nil})

3.2 Options

When a hash is given in the last argument (options), it allows to specify a current directory, umask, user, group and zero or more fd redirects for the child process.

3.2.1 Redirection

There are few ways you can redirect commands output.

You can directly use shell redirection like so:

out, err = cmd.run("ls 1&>2")
puts err
# =>
# CHANGELOG.md
# CODE_OF_CONDUCT.md
# Gemfile
# ...

You can provide redirection as additional hash options where the key is one of :in, :out, :err, an integer (a file descriptor for the child process), an IO or array. For example, stderr can be merged into stdout as follows:

cmd.run(:ls, :err => :out)
cmd.run(:ls, :stderr => :stdout)
cmd.run(:ls, 2 => 1)
cmd.run(:ls, STDERR => :out)
cmd.run(:ls, STDERR => STDOUT)

The hash key and value specify a file descriptor in the child process (stderr & stdout in the examples).

You can also redirect to a file:

cmd.run(:cat, :in => "file")
cmd.run(:cat, :in => open("/etc/passwd"))
cmd.run(:ls, :out => "log")
cmd.run(:ls, :out => "/dev/null")
cmd.run(:ls, :out => "out.log", :err => "err.log")
cmd.run(:ls, [:out, :err] => "log")
cmd.run("ls 1>&2", :err => "log")

It is possible to specify flags and permissions of file creation explicitly by passing an array value:

cmd.run(:ls, :out => ["log", "w"]) # 0664 assumed
cmd.run(:ls, :out => ["log", "w", 0600])
cmd.run(:ls, :out => ["log", File::WRONLY|File::EXCL|File::CREAT, 0600])

You can, for example, read data from one source and output to another:

cmd.run("cat", :in => "Gemfile", :out => "gemfile.log")

3.2.2 Handling Input

You can provide input to stdin stream using the :input key. For instance, given the following executable called cli that expects name from stdin:

name = $stdin.gets
puts "Your name: #{name}"

In order to execute cli with name input do:

cmd.run("cli", input: "Piotr\n")
# => Your name: Piotr

Alternatively, you can pass input via the :in option, by passing a StringIO Object. This object might have more than one line, if the executed command reads more than once from STDIN.

Assume you have run a program, that first asks for your email address and then for a password:

in_stream = StringIO.new
in_stream.puts "[email protected]"
in_stream.puts "password"
in_stream.rewind

cmd.run("my_cli_program", "login", in: in_stream).out

3.2.3 Timeout

You can timeout command execution by providing the :timeout option in seconds:

cmd.run("while test 1; sleep 1; done", timeout: 5)

And to set it for all commands do:

cmd = TTY::Command.new(timeout: 5)

Please run examples/timeout.rb to see timeout in action.

3.2.4 Binary mode

By default the standard input, output and error are non-binary. However, you can change to read and write in binary mode by using the :binmode option like so:

cmd.run("echo 'hello'", binmode: true)

To set all commands to be run in binary mode do:

cmd = TTY::Command.new(binmode: true)

3.2.5 Signal

You can specify process termination signal other than the default SIGTERM:

cmd.run("whilte test1; sleep1; done", timeout: 5, signal: :KILL)

3.2.6 PTY(pseudo terminal)

The :pty configuration option causes the command to be executed in subprocess where each stream is a pseudo terminal. By default this options is set to false.

If you require to interface with interactive subprocess then setting this option to true will enable a pty terminal device. For example, a command may emit colored output only if it is running via terminal device. You may also wish to run a program that waits for user input, and simulates typing in commands and reading responses.

This option will only work on systems that support BSD pty devices such as Linux or OS X, and it will gracefully fallback to non-pty device on all the other.

In order to run command in pseudo terminal, either set the flag globally for all commands:

cmd = TTY::Command.new(pty: true)

or individually for each executed command:

cmd.run("echo 'hello'", pty: true)

Please note that setting :pty to true may change how the command behaves. It's important to understand the difference between interactive and non-interactive modes. For example, executing git log to view the commit history in default non-interactive mode:

cmd.run("git log") # => finishes and produces full output

However, in interactive mode with pty flag on:

cmd.run("git log", pty: true) # => uses pager and waits for user input (never returns)

In addition, when pty device is used, any input to command may be echoed to the standard output, as well as some redirects may not work.

3.2.7 Current directory

To change directory in which the command is run pass the :chdir option:

cmd.run(:echo, "hello", chdir: "/var/tmp")

3.2.8 User

To run command as a given user do:

cmd.run(:echo, "hello", user: "piotr")

3.2.9 Group

To run command as part of group do:

cmd.run(:echo, "hello", group: "devs")

3.2.10 Umask

To run command with umask do:

cmd.run(:echo, "hello", umask: "007")

3.3 Result

Each time you run command the stdout and stderr are captured and return as result. The result can be examined directly by casting it to tuple:

out, err = cmd.run(:echo, "Hello")

However, if you want to you can defer reading:

result = cmd.run(:echo, "Hello")
result.out
result.err

3.3.1 success?

To check if command exited successfully use success?:

result = cmd.run(:echo, "Hello")
result.success? # => true

3.3.2 failure?

To check if command exited unsuccessfully use failure? or failed?:

result = cmd.run(:echo, "Hello")
result.failure?  # => false
result.failed?   # => false

3.3.3 exited?

To check if command ran to completion use exited? or complete?:

result = cmd.run(:echo, "Hello")
result.exited?    # => true
result.complete?  # => true

3.3.4 each

The result itself is an enumerable and allows you to iterate over the stdout output:

result = cmd.run(:ls, "-1")
result.each { |line| puts line }
# =>
#  CHANGELOG.md
#  CODE_OF_CONDUCT.md
#  Gemfile
#  Gemfile.lock
#  ...
#  lib
#  pkg
#  spec
#  tasks

By default the linefeed character \n is used as a delimiter but this can be changed either globally by calling record_separator:

TTY::Command.record_separator = "\n\r"

or configured per each call by passing delimiter as an argument:

cmd.run(:ls, "-1").each("\t") { ... }

3.4 Custom printer

If the built-in printers do not meet your requirements you can create your own. A printer is a regular Ruby class that can be registered through :printer option to receive notifications about received command data.

As the command runs the custom printer will be notified when the command starts, when data is printed to stdout, when data is printed to stderr and when the command exits.

Please see lib/tty/command/printers/abstract.rb for a full set of methods that you can override.

At the very minimum you need to specify the write method that will be called during the lifecycle of command execution. The write accepts two arguments, first the currently run command instance and second the message to be printed:

CustomPrinter < TTY::Command::Printers::Abstract
  def write(cmd, message)
    puts cmd.to_command + message
  end
end

cmd = TTY::Command.new(printer: CustomPrinter)

4. Example

Here's a slightly more elaborate example to illustrate how tty-command can improve on plain old shell scripts. This example installs a new version of Ruby on an Ubuntu machine.

cmd = TTY::Command.new

# dependencies
cmd.run "apt-get -y install build-essential checkinstall"

# fetch ruby if necessary
if !File.exists?("ruby-2.3.0.tar.gz")
  puts "Downloading..."
  cmd.run "wget http://ftp.ruby-lang.org/pub/ruby/2.3/ruby-2.3.0.tar.gz"
  cmd.run "tar xvzf ruby-2.3.0.tar.gz"
end

# now install
Dir.chdir("ruby-2.3.0") do
  puts "Building..."
  cmd.run "./configure --prefix=/usr/local"
  cmd.run "make"
end

Development

After checking out the repo, run bin/setup to install dependencies. Then, run rake spec to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

Contributing

Bug reports and pull requests are welcome on GitHub at https://github.com/piotrmurach/tty-command. This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the Contributor Covenant code of conduct.

License

The gem is available as open source under the terms of the MIT License.

Copyright

Copyright (c) 2016 Piotr Murach. See LICENSE for further details.