How to call shell commands from Ruby

Question

How do I call shell commands from inside of a Ruby program  How do I then get output from these commands back into Ruby

User · Answer

Using the answers here and linked in Mihai's answer, I put together a function that meets these requirements:

Neatly captures STDOUT and STDERR so they don't "leak" when my script is run from the console.
Allows arguments to be passed to the shell as an array, so there's no need to worry about escaping.
Captures the exit status of the command so it is clear when an error has occurred.

As a bonus, this one will also return STDOUT in cases where the shell command exits successfully (0) and puts anything on STDOUT. In this manner, it differs from system, which simply returns true in such cases.

Code follows. The specific function is system_quietly:

require 'open3'

class ShellError < StandardError; end

#actual function:
def system_quietly(*cmd)
  exit_status=nil
  err=nil
  out=nil
  Open3.popen3(*cmd) do |stdin, stdout, stderr, wait_thread|
    err = stderr.gets(nil)
    out = stdout.gets(nil)
    [stdin, stdout, stderr].each{|stream| stream.send('close')}
    exit_status = wait_thread.value
  end
  if exit_status.to_i > 0
    err = err.chomp if err
    raise ShellError, err
  elsif out
    return out.chomp
  else
    return true
  end
end

#calling it:
begin
  puts system_quietly('which', 'ruby')
rescue ShellError
  abort "Looks like you don't have the `ruby` command. Odd."
end

#output: => "/Users/me/.rvm/rubies/ruby-1.9.2-p136/bin/ruby"

User · Answer

Given a command like attrib   require  open3   a  attrib  Open3 popen3 a  do  stdin  stdout  stderr    puts stdout read end   I ve found that while this method isn t as memorable as   system  thecommand     or     thecommand    in backticks   a good thing about this method compared to other methods is   backticks don t seem to let me puts the command I run store the command I want to run in a variable  and system  thecommand   doesn t seem to let me get the output  whereas this method lets me do both of those things  and it lets me access stdin  stdout and stderr independently   See  Executing commands in ruby  and Ruby s Open3 documentation

User · Answer

Some things to think about when choosing between these mechanisms are    Do you just want stdout or do you need stderr as well  Or even separated out  How big is your output   Do you want to hold the entire result in memory  Do you want to read some of your output while the subprocess is still running  Do you need result codes  Do you need a Ruby object that represents the process and lets you kill it on demand    You may need anything from simple backticks       system    and IO popen to full-blown Kernel fork Kernel exec with IO pipe and IO select   You may also want to throw timeouts into the mix if a sub-process takes too long to execute   Unfortunately  it very much depends

User · Answer

One more option   When you    need stderr as well as stdout  can t won t use Open3 Open4  they throw exceptions in NetBeans on my Mac  no idea why    You can use shell redirection   puts  x cat bogus txt  inspect     gt      puts  x cat bogus txt 2 gt  amp 1  inspect     gt   cat  bogus txt  No such file or directory n    The 2 gt  amp 1 syntax works across Linux  Mac and Windows since the early days of MS-DOS

User · Answer

Here s a cool one that I use in a ruby script on OS X  so that I can start a script and get an update even after toggling away from the window    cmd    Q osascript -e  display notification  Server was reset  with title  Posted Update    system   cmd

User · Answer

If you have a more complex case than the common case that can not be handled with     then check out  Kernel spawn    This seems to be the most generic full-featured provided by stock Ruby to execute external commands   You can use it to    create process groups  Windows   redirect in  out  error to files each-other  set env vars  umask  change the directory before executing a command  set resource limits for CPU data etc  Do everything that can be done with other options in other answers  but with more code    The Ruby documentation has good enough examples   env  hash   name   gt  val   set the environment variable   name   gt  nil   unset the environment variable command       commandline                   command line string which is passed to the standard shell   cmdname  arg1                 command name and one or more arguments  no shell     cmdname  argv0   arg1        command name  argv 0  and zero or more arguments  no shell  options  hash   clearing environment variables       unsetenv others   gt  true     clear environment variables except specified by env      unsetenv others   gt  false    dont clear  default    process group       pgroup   gt  true or 0   make a new process group      pgroup   gt  pgid        join to specified process group      pgroup   gt  nil         dont change the process group  default    create new process group  Windows only      new pgroup   gt  true    the new process is the root process of a new process group      new pgroup   gt  false   dont create a new process group  default    resource limit  resourcename is core  cpu  data  etc   See Process setrlimit       rlimit resourcename   gt  limit      rlimit resourcename   gt   cur limit  max limit    current directory       chdir   gt  str   umask       umask   gt  int   redirection      key        FD                single file descriptor in child process        FD  FD           multiple file descriptor in child process     value        FD                          redirect to the file descriptor in parent process       string                      redirect to file with open string   r  or  w          string                     redirect to file with open string  File  RDONLY         string  open mode          redirect to file with open string  open mode  0644         string  open mode  perm    redirect to file with open string  open mode  perm          child  FD                 redirect to the redirected file descriptor        close                      close the file descriptor in child process     FD is one of follows        in       the file descriptor 0 which is the standard input        out      the file descriptor 1 which is the standard output        err      the file descriptor 2 which is the standard error       integer   the file descriptor of specified the integer       io        the file descriptor specified as io fileno   file descriptor inheritance  close non-redirected non-standard fds  3  4  5       or not      close others   gt  false   inherit fds  default for system and exec       close others   gt  true    dont inherit  default for spawn and IO popen

User · Answer

You can also use the backtick operators      similar to Perl   directoryListing    ls    puts directoryListing   prints the contents of the root directory   Handy if you need something simple   Which method you want to use depends on exactly what you re trying to accomplish  check the docs for more details about the different methods

User · Answer

The easiest way is  for example   reboot    init 6  puts reboot

User · Answer

My favourite is Open3    require  open3     Open3 popen3  nroff -man      stdin  stdout  stderr

User · Answer

Don t forget the spawn command to create a background process to execute the specified command  You can even wait for its completion using the Process class and the returned pid   pid   spawn  tar xf ruby-2 0 0-p195 tar bz2   Process wait pid  pid   spawn RbConfig ruby   -eputs Hello  world     Process wait pid   The doc says  This method is similar to  system but it doesn t wait for the command to finish

User · Answer

The backticks     method is the easiest one to call shell commands from Ruby  It returns the result of the shell command        url request    http   google com       result of shell command    curl   url request

User · Answer

If you really need Bash  per the note in the  best  answer      First  note that when Ruby calls out to a shell  it typically calls  bin sh  not Bash  Some Bash syntax is not supported by  bin sh on all systems     If you need to use Bash  insert bash -c  your Bash-only command  inside of your desired calling method   quick output   system  ls -la     quick bash   system  bash -c  ls -la      To test   system  echo  SHELL   system  bash -c  echo  SHELL      Or if you are running an existing script file like  script output   system    my script sh     Ruby should honor the shebang  but you could always use   system  bash   my script sh     to make sure  though there may be a slight overhead from  bin sh running  bin bash  you probably won t notice

User · Answer

We can achieve it in multiple ways   Using Kernel exec  nothing after this command is executed   exec  ls       Using backticks or  x   ls      gt   Applications nDesktop nDocuments   x ls      gt   Applications nDesktop nDocuments    Using Kernel system command  returns true if successful  false if unsuccessful and returns nil if command execution fails   system  ls       gt  true

User · Answer

This is not really an answer but maybe someone will find it useful   When using TK GUI on Windows  and you need to call shell commands from rubyw  you will always have an annoying CMD window popping up for less then a second   To avoid this you can use   WIN32OLE new  Shell Application   ShellExecute  ipconfig  gt  log txt         open  0    or   WIN32OLE new  WScript Shell   Run  ipconfig  gt  log txt  0 0    Both will store the ipconfig output inside log txt  but no windows will come up   You will need to require  win32ole  inside your script   system    exec   and spawn   will all pop up that annoying window when using TK and rubyw

User · Answer

The answers above are already quite great  but I really want to share the following summary article   6 Ways to Run Shell Commands in Ruby   Basically  it tells us   Kernel exec   exec  echo  hello  HOSTNAME     system and      system  false   puts      Backticks       today    date    IO popen   IO popen  date      f  puts f gets     Open3 popen3 -- stdlib   require  open3  stdin  stdout  stderr   Open3 popen3  dc      Open4 popen4 -- a gem   require  open4   pid  stdin  stdout  stderr   Open4  popen4  false      gt   26327    lt IO 0x6dff24 gt     lt IO 0x6dfee8 gt     lt IO 0x6dfe84 gt

User · Answer

I m definitely not a Ruby expert  but I ll give it a shot     irb  system  echo Hi  Hi   gt  true   You should also be able to do things like   cmd    ls  system cmd

User · Answer

Here s a flowchart based on  When to use each method of launching a subprocess in Ruby    See also   Trick an application into thinking its stdout is a terminal  not a pipe

User · Answer

Here s the best article in my opinion about running shell scripts in Ruby   6 Ways to Run Shell Commands in Ruby    If you only need to get the output use backticks   I needed more advanced stuff like STDOUT and STDERR so I used the Open4 gem  You have all the methods explained there

User · Answer

This explanation is based on a commented Ruby script from a friend of mine  If you want to improve the script  feel free to update it at the link   First  note that when Ruby calls out to a shell  it typically calls  bin sh  not Bash  Some Bash syntax is not supported by  bin sh on all systems   Here are ways to execute a shell script   cmd    echo  hi     Sample string that can be used    Kernel     commonly called backticks      cmd   This is like many other languages  including Bash  PHP  and Perl   Returns the result  i e  standard output  of the shell command   Docs  http   ruby-doc org core Kernel html method-i-60  value    echo  hi   value      cmd    Built-in syntax   x  cmd    Following the x character is a delimiter  which can be any character  If the delimiter is one of the characters          or  lt   the literal consists of the characters up to the matching closing delimiter  taking account of nested delimiter pairs  For all other delimiters  the literal comprises the characters up to the next occurrence of the delimiter character   String interpolation          is allowed   Returns the result  i e  standard output  of the shell command  just like the backticks   Docs  https   docs ruby-lang org en master syntax literals rdoc html label-Percent Strings  value    x  echo  hi    value    x    cmd     Kernel system  Executes the given command in a subshell    Returns true if the command was found and run successfully  false otherwise   Docs  http   ruby-doc org core Kernel html method-i-system  wasGood   system   echo  hi     wasGood   system  cmd    Kernel exec  Replaces the current process by running the given external command   Returns none  the current process is replaced and never continues   Docs  http   ruby-doc org core Kernel html method-i-exec  exec   echo  hi     exec  cmd     Note  this will never be reached because of the line above    Here s some extra advice      which is the same as  CHILD STATUS  accesses the status of the last system executed command if you use the backticks  system   or  x    You can then access the exitstatus and pid properties      exitstatus   For more reading see    http   www elctech com blog i-m-in-ur-commandline-executin-ma-commands http   blog jayfields com 2006 06 ruby-kernel-system-exec-and-x html http   tech natemurray com 2007 03 ruby-shell-commands html

User · Answer

The way I like to do this is using the  x literal  which makes it easy  and readable   to use quotes in a command  like so   directorylist    x find   -name   test rb    sort    Which  in this case  will populate file list with all test files under the current directory  which you can process as expected   directorylist each do  filename    filename chomp      work with file end

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