System Call from Ruby

Getting output of system() calls in Ruby

I'd like to expand & clarify chaos's answer a bit.

If you surround your command with backticks, then you don't need to (explicitly) call system() at all. The backticks execute the command and return the output as a string. You can then assign the value to a variable like so:

output = `ls`
p output

printf output # escapes newline chars

How to execute a Ruby system call in the context of the current shell

These things can be tricky with RVM. Try this:

"/bin/bash -c -l 'rvm use 2.5.5 && <add your system commands here>'"

Note: I guess it would create a new shell environment. Do you really need to access your current shell?

Ruby system call with environment variables in windows

Ruby's system() invokes whatever the host's default shell is, so you need to speak that shell's language.

The default shell on Windows is cmd.exe, where environment variable FOO must be referenced as %FOO% in order to be expanded.

Thus, your code should be:

cmd = 'echo %FOO%'
system({'FOO' => '123'}, cmd)

Setting input for system() calls in ruby

Don't use system at all for this sort of thing, system is best for running an external command that you don't need to talk to.

Use Open3.open3 or Open3.open2 to open up some pipes to your external process then write to the stdin pipe just like writing to any other IO channel; if there is any output to deal with, then you can read it straight from the stdout pipe just like reading from any other input IO channel.

Get PID of system call in Ruby

If you use Process.spawn, you will spawn a process, and get its PID, without having to wait for it to finish.

See the docs for more info, and for resource limits, which might be an alternative way to achieve your goal.

System call from Ruby

Use "`" quotes:

`rake db:migrate VERSION=....`

or system

system("rake db:migrate VERSION=....")

Also you can use this notation:

%x[rake db:migrate VERSION=...]

Also see http://blog.jayfields.com/2006/06/ruby-kernel-system-exec-and-x.html

Forming sanitary shell commands or system calls in Ruby

It doesn't look like you need a shell for what you're doing. See the documentation for system here: http://ruby-doc.org/core/classes/Kernel.html#M001441

You should use the second form of system. Your example above would become:

system 'usermod', '-p', @options['shadow'], @options['username']

A nicer (IMO) way to write this is:

system *%W(usermod -p #{@options['shadow']} #{@options['username']})

The arguments this way are passed directly into the execve call, so you don't have to worry about sneaky shell tricks.

How can I pass a variable to a system() call in ruby?

Oh, happy injection. You're looking for
IO.popen.

IO.popen('grep ba', 'r+') {|f| # don't forget 'r+'
  f.puts("foo\nbar\nbaz\n") # you can also use #write
  f.close_write
  f.read # get the data from the pipe
}
# => "bar\nbaz\n"

How to call shell commands from Ruby

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 <,
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

Return output from system command in Ruby?

You could use

output = `heroku create`

See: http://ruby-doc.org/core/classes/Kernel.html